Assessing the importance of plate tectonic for the development of complex life.

Over the past three decades, research has shown that the overwhelming majority of stars in our galaxy have planets. Logically, a proportion of those planets will have the conditions for life, and life will actually arise on some proportion of the planets which can support it. Furthermore, a small proportion of the planets with life are likely to produce intelligent life, capable of radio communication and other activities which we might be able to detect. This works out at a very small proportion of stars hosting civilizations, but a very small proportion of hundreds of billions still works out at a very large number. We have, nevertheless, never detected any sign of intelligent life (of life at all) beyond our own planet, leaving us, as far as we know the only civilization in the universe, something referred to as the Fermi Paradox. A number of possible explanations have been offered for this paradox, most of which revolve around the idea that while life is probably quite common in the galaxy, complex life is probably much rarer. A variety of reasons why this might be the case have also been proposed, with a link to the other phenomenon also as far as well know unique to our planet, plate tectonics, frequently being proposed.

In a paper published in the journal Scientific Reports on 12 April 2024, Robert Stern of the Department of Sustainable Earth Systems Science at the University of Texas at Dallas, and Taras Gerya of the Department of Earth Sciences at ETH-Zurich, re-examine the biological and geological history of the Earth, looking for connections between key stages in the development of life on Earth and changes in the Earth's geological activity.

Life is generally accepted to have first appeared on Earth more than 3800 million years ago, but the first Animals did not appear until less than 1000 million years ago. Various possible explanations have been proposed for this, including lower oxygen levels on the early Earth and a possible lack of key biological innovations until more recent times. The fact that Animals, Plants, and even multicellular Algae did not appear until quite late in the Neoproterozoic suggests that some profound change in the Earth or its biology led to this development, and Stern and Gerva suggest that this change was a shift from single lid to plate tectonics, and the subsequent profound shifts in the Earth's atmospheric oxygen levels.

The timing of the onset of plate tectonics is still a matter of debate among Earth Scientists, with two broad camps, one which argues that plate tectonics began during the Archean, while the other argues that the current plate tectonic regime began in the Neoproterozoic (although there may-or-may-not have been earlier phases of plate tectonics on Earth). The processes of seafloor spreading, subduction, and continental collision lead to the formation of a distinctive set of minerals, rocks, and rock assemblages, some of which appear to be absent before the onset of the Neoproterozoic, although it has been argued that these differences are due to higher mantle temperatures on the early Earth. Similarly, it is possible to track the movements of continents over the Neoproterozoic and Phanerozoic through the traces of magnetic fields left in some types of rocks when they form, but in rocks older than about 1200 million years this method is not reliable, leaving us uncertain about the movement of the continents that long ago. 

In previous work, Robert Stern divided indicators of plate tectonics into three groups, (1) indicators of seafloor spreading and subduction initiation, (2) subduction indicators, and (3) plate collision indicators. All of these indicators are present in rocks from Neoproterozoic and Phanerozoic times, but their presence in older rocks is far less clear. 

In order for plate tectonics to operate, it is necessary for oceanic lithosphere to be subducted at plate margins, and it is likely that this oceanic lithosphere was not sufficiently strong and dense enough for this to happen in a coherent way until the upper mantle had cooled to between 100°C and 150°C above current temperatures, again something which is predicted to have happened in the Neoproterozoic.

If plate tectonics did indeed start, then it is unlikely to have started all over the world at once, but instead should have started in one place, then taken hundreds of millions of years to spread around the globe. This would result in a gradual increase in the presence of Stern's three indicators for the presence of plate tectonics, something which can again be seen in the Neoproterozoic rock record. The first ophiolites, which are indicators of subduction initiation, appear about 870 million years ago, with the first indicators of actual subduction zones appearing about 750 million years ago, and the first indicators of continental collision appearing about 600 million years ago. 

As far as we understand, an active silicate body can have either a single lid or plate tectonic system. This being the case, for plate tectonics to have initiated in the Neoproterozoic, the Earth must have had a single lid system in the Mesoproterozoic. Stern's previous work also identified three potential indicators for single lid systems. These are (1) an elevated thermal regime, (2) an abundance of unusual dry magmas such as A-type granites and anorthosites, and (3) a lack of new continental margins. The Mesoproterozoic shows an absence of Stern's indicators for plate tectonics, but is rich in the three indicators for single lid tectonics. Curiously, indicators for plate tectonics do appear to be present in the Palaeoproterozoic.

Evolution of Earth’s tectonic regime over the past 1.6 billion years; (a) single lid tectonic indicators, (b) plate tectonic indicators cumulative plot, (c) simplified climate history, (d) Simplified biological evolution. Stern & Gerva (2024).

There is a similar shift in the formation of mineral deposits between the Mesoproterozoic and Neoproterozoic, with orogenic gold and porphyry copper deposits, all associated with an active tectonic regime, being common in Neoproterozoic strata but absent in the Mesoproterozoic, while iron oxide copper gold  deposits and iron-titanium-vanadium-phosphorus deposits, both of which are associated with anorthosites are common in the Mesozoic but rare in the Neoproterozoic.

Finally, the palaeomagnetic record does not show any significant movement of continental landmasses during the Mesoproterozoic, while this is common in younger rocks. Particularly noteworthy is the supercontinent of Nuna (or Colombia), which assembled during the Palaeoproterozoic, then appears to have persisted relatively unchanged throughout the Mesoproterozoic.

Plate tectonics requires a global mosaic of plates, which it could be reasonably expected would take hundreds of millions of years to form a a single lid system. Theoretically, following the formation of an initial subduction zone with associated transform systems and divergent plate boundary margins, the system could slowly propagate to form a global mosaic. The rate at which such a system could form would be governed by the rate at which new subduction zones can form and lengthen. In Cretaceous and younger rocks, subductive trenches appear to be able to lengthen at rates of between 100 and 600 km per million years, which would require between 92 and 550 million years to develop a global network of about 55 000 km of convergent margins.

The last 1.6 billion years of Earth’s tectonic history. Stern & Gerva (2024).

The Neoproterozoic is also known to have had major carbon isotope excursions (changes in the proportions of different carbon isotopes laid down in sedimentary deposits) as well as several glacial interludes, major disruptions to the Earth's environment which we associate with plate tectonics. The most notable of these is the Neoproterozoic Snowball Earth, a prolonged phase of near-global glaciation, which may have been triggered by a huge increase in volcanic activity or true polar wander.  The first major carbon isotope event in the Neoproterozoic is the Bitter Springs Event, at about 811 million years before the present, while the youngest is the Shuram Event, about 570 million years ago. If these events bracket a change from a single lid to a plate tectonic system, then that change took about 241 million years, with the Neoproterozoic Snowball Earth, which started at about 720 million years ago and ended about 580 million years ago, in the middle. If the beginning and end of the Snowball Earth mark the transition period then it is shorter, at about 140 million years. The fact that both the carbon isotope excursions and the glaciation events were sporadic suggests that this process was not smooth, bur occurred in a series of episodes. The Palaeoproterozoic is also noted for several isotope excursions, as well as glacial events, and the Great Oxidation Event, during which the Earth first developed an oxygenated atmosphere. All of these events occurred between about 2.50 and 2.05 billion years ago, with a proposed interval of plate tectonics between 2.05 and 1.80 billion years ago, giving an apparent different relationship between these geochemical events and plate tectonics. This Palaeoproterozoic tectonic interval appears to have ended with the formation of the Supercontinent of Nuna, although these ancient events are not well understood and would merit significant further investigation.

Life first appeared on Earth more than 3.8 billion years ago, but appears to have remained fairly simple for the next three billion years, with all terrestrial ecosystems dominated by Prokaryotes (Bacteria and Archaea), simple organisms which lack cell nuclei organelles. All complex multicellular life on Earth is Eukaryotic (i.e. has cells with nuclei and organelles), so Eukaryotic life had to appear before multicellular life-forms. The fossil record shows what appear to be Eukaryotic single-celled organisms dating back to at least the Palaeoproterozoic, suggesting a link between the emergence of the first Eukaryotes and the Great Oxygenation Event, the first case of co-evolution between the evolution of the atmosphere and Eukaryotic life.

The Mesoproterozoic lacks any such major events, making it impossible to split it into any subdivisions, and making its beginning and end points somewhat arbitrary. The interval between 1800 and 800 million years ago (roughly the Mesoproterozoic) has been referred to as the 'Boring Billion' because of this lack of events, with oxygen levels staying roughly constant, geobiological systems apparently remaining unchanged, and constant carbon isotope ratios throughout. The period, which lasted for about 20% of Earth's history, also saw stability in the proportion of sulphur molybdenum, chromium, and strontium isotopes trapped in sediments throughout, and a prolonged low nutrient system.

In contrast, the Neoproterozoic is a period of both climatic instability and rapid biological evolution, during which the Snowball Earth occurred, as well as major shifts in the carbon cycle, the ocean's oxygen content, a major diversification in microscopic Eukaryotes, and the appearance of Metazoans. The era can be split into three periods based upon clear geological differences, the longer and somewhat uneventful Tonian, between 1000 and 720 million years before the present, the Snowball Earth Cryogenian, and the Ediacaran, which saw the first widespread Metazoan fossils. Molecular clocks suggest that the first multicellular organisms appeared during the Tonian, the bilaterian body plan appeared in the Cryogenian, and that the majority of Metazoan phyla diversified during the Late Ediacaran, between 560 and 540 million years ago. All known Animal phyla are believed to have arisen during the Neoproterozoic.

Five conditions are thought to have been needed for this biological shift to have occurred; an increased nutrient supply, increased oxygen levels in both the atmosphere and oceans, an improved climate, an increased rate of habitat formation and destruction, and a sustained evolutionary pressure caused by such shifting environments.

Summary diagram showing how plate tectonics stimulates life and evolution whereas a single lid tectonic style retards life and evolution. Stern & Gerva (2024).

Nutrients are essential for life, and in particular organic carbon (i.e. compounds with bio-available carbon - we cannot, for example, eat diamonds), ammonium (which provides bio-available nitrogen), ferrous iron (again, bio-available iron) and phosphates (bio-available phosphorus). Phosphorus, in particular, plays vital role in biogeochemistry and is considered a global limiting nutrient. A shortage of phosphorus is thought to have been one of the major restrictions on the Mesoproterozoic biosphere. Phosphorus typically becomes available through the erosion of rocks, and its subsequent delivery to the oceans via rivers. This makes it likely that rock weathering was much reduced during the Mesoproterozoic. In the modern world, fresh rocks are constantly exposed at the surface due to tectonic processes, providing new sources of phosphorus and other nutrients, while soil formation covers rocks, inhibiting this supply. The Earth has gone through phases of enhanced nutrient supply associated with major uplift events, such as the Pan-African Orogeny, the Transgondwanan Supermountain Orogeny, and the Circum-Gondwanan Orogens, which all occurred on convergent plate boundaries associated with tectonic transitions. These events greatly increased the rate of erosion, and therefore the delivery of phosphorus into the oceans, with the microbial enhancement of carbon and sulphate acid weathering being an important part of this delivery process. Rising oxygen levels in the atmosphere would have increased the role of microbes in weathering, which in turn would have increased the rates at which organic carbon was buried and phosphorus was delivered to the oceans, resulting in depleted phosphorus depletion in palaeosols (preserved terrestrial soils), something observed during both the Neoproterozoic Oxygenation Event and the Palaeoproterozoic Great Oxygenation Event.

Further evidence for a major onset of uplift, erosion, and weathering during the Ediacaran can be seen in a rise in the proportion of the isotope strontium⁸⁷ within marine sediments. Strontium⁸⁷ is radiogenic, formed by the decay of rubidium within rocks, and can enter the water column either by the erosion of rocks in which this decay has occurred, or by the erosion of older marine sediments. The proportion of this isotope began to rise during the Tonian, and continued to do so throughout the Neoproterozoic, with a significant increase during the Ediacaran, and the highest values recorded in Earth's rock record being found in the Early Palaeozoic. This Neoproterozoic increase in the proportion of strontium⁸⁷ is thought to have been associated with the Pan-African uplifts and the formation of the Transgondwanan Supermountains. These events were caused by continental collisions, with no similar events having apparently happened during the Mesoproterozoic. Thus the low strontium⁸⁷ levels seen in the Mesoproterozoic are another line of evidence supporting a phase of single lid tectonics during this era. The production of phosphorus, iron, and other nutrients by erosion broke the Mesoproterozoic nutrient drought, stimulating biological evolution. 

Free oxygen levels in both the atmosphere and oceans are likely to have been caused by a proliferation of photosynthetic Cyanobacteria, Prokaryotes which have been around since at least the Palaeoproterozoic, combined with a more efficient burial of organic carbon (which will tend to react with free oxygen). This increased oxygen availability enabled the evolution of larger more complicated organisms, such as Animals, something impossible under the low-oxygen conditions of the Mesoproterozoic. Larger, more complicated Animals need higher oxygen levels than smaller, simpler ones, with oxygen levels during the Cambrian thought to have been much lower than today, but Mesoproterozoic oxygen levels are thought to have been lower still, incapable of supporting even simple Animal life. A range of isotopic proxies indicate a significant oxygenation event during the Neoproterozoic, leading to oxygen levels capable of supporting Animal life in most marine ecosystems by the end of the Cryogenian. 

The most likely explanation for this increase in oxygen is that an increase in nutrient supply led to a boom in phytoplankton growth, converting more carbon dioxide into organic matter. This would have allowed the development of more sophisticated Algae with increased photosynthetic abilities, something thought to have happened in the Late Cryogenian. This in turn further boosted oxygen production, as well as transforming the base of the food chain and providing novel food sources for the first Animals. An alternative explanation is increased weathering on land, leading to more nutrients flowing into the oceans, provoking a surge in Cyanobacterial and Algal production, which caused oxygen levels to rise. The common element to all hypothesis is that more phytoplankton were dying and being buried, increasing the  amount of organic carbon sequestered at the same time as sedimentation rates increased in the new rift basins and continental margins of the changing world.

A stable climate is important for Metazoan life. Prokaryotes can thrive at temperatures between 0°C and about 120°C, but most Animal life needs a temperature between about 5°C and about 35°C. Single lid and plate tectonics will provide different climatic regimes. Under a plate tectonic system, the regular release of volcanic gasses can have either a warming or cooling effect; notably mid-ocean ridges produce large amounts of carbon dioxide, tending to warm the climate, while volcanoes on convergent margins produce lots of sulphur dioxide, tending to cool the environment. 

The presence of oceans on the Earth's surface tends to modulate the overall temperature, due to the thermal inertia of water (it takes a lot more energy to warm water than air). This means that the Earth has a more temperate climate when a higher proportion of its surface is covered by water, and a harsher climate when the proportion of the surface covered by water is lower. This means that during a plate tectonic regime, the climate will go through cycles, with a warm greenhouse phase typically arising about 100 million years after a continental breakup event as the oceans widen. It is unclear how the depth and extent of the oceans would have varied under a single lid tectonic system, but it is likely that any change would have been considerably less significant than under a plate tectonic regime.

The process of weathering silicate rocks uses carbon dioxide. This means that the continual exposure and weathering of new silicate rocks, as happens under a plate tectonic system, will consume more carbon dioxide, leading to a reduction in the proportion of this greenhouse gas in the atmosphere, cooling the climate. Thus the enhanced erosion and weathering under a plate tectonic regime will not only release more nutrients, leading to more photosynthesis in the oceans and a subsequent rise in the burial of organic carbon in marine sediments, it also directly removes carbon dioxide from the atmosphere. Under a single lid system, the amount of uplift occurring should be close to zero, leading to a much lower nutrient flux and a lower exposure of silicate rocks to weathering by carbon dioxide.

Plate tectonics also removes large amounts of marine carbonate rocks and buried organic carbon from the Earth's surface systems as they are drawn down into the Earth at subduction zones, further reducing the amount of carbon dioxide in the atmosphere, and leading to further cooling. 

The carbon cycle on planets with single lid tectonic cycles is not well understood, and that of the Mesoproterozoic Earth less so. Two planets in the modern Solar System have single lid tectonic systems, Venus and Mars, and both of these have atmospheres which are more than 95% carbon dioxide, suggesting a poor ability to cycle this gas. However, models of the early Earth suggest that it might have been possible to recycle carbon dioxide reasonably efficiently if volcanic activity was sufficiently high, through the weathering, burial, sinking and delamination of carbonated crust. This fits with the observation that the Mesoproterozoic Earth had a relatively warm climate without any glacial phases, despite the Sun being 5-20% dimmer than today, presumably due to the contribution of greenhouse gasses.

The constant formation and then destruction of new ecosystems is a feature of an active plate tectonic system. This is also something required for the efficient evolution of biological organisms, but is unclear to what extent this would happen under a single lid tectonic system, possibly leading to a system of biological stasis.

The continuous environmental change of a plate tectonic system should present a constant need for biological innovation, with constantly changing nutrient fluxes, topographies, climates, and habitats. This is particularly true along active plate margins, in shallow marine ecosystems which appear to have been hotspots for biological innovation throughout the Earth's recent history. In these environments plate tectonics produces shifting habitats with abundant nutrient and sediment supplies, as well as strong currents and tides, which will tend to distribute these nutrients. 

The switch a plate tectonic system appears to have stimulated the rapid diversification of life, something which may not have been possible at all under a single lid system. The most dramatic environmental shifts encountered under a single lid system are likely to have been mantle plumes, which would cause global warming when they first appeared, due to the production of carbon dioxide, followed by a period of cooling as weathering of basalts leads to carbon oxide levels lowering again. Under this system the oceans would also likely suffer from conditions of anoxia, acidification, and toxic metal-input.

Without the driving force of plate tectonics, the evolution of biological life appears to be an extremely slow process. Under a plate tectonic regime, the evolution and demise of new organisms, groups of organisms, and whole global ecosystems, appears to follow the opening and closing of oceans. This makes it extremely unlikely that complex life would have arisen on Earth without the development of a plate tectonic system.

We know that life appeared in Earth's oceans more than 3.8 billion years ago, and remained within the oceans for more than 3 billion years. Despite this, it is generally accepted that the presence of dry land on Earth was needed for both the origin and evolution of life, since without this all nutrients would eventually be lost from surface systems. It is possible that the first life originated in ancient palaeosols (or more accurately, regalith), but seawater appears to have been a vital environment for much of the history of life, providing a nutrient bath in which primitive organisms could absorb nutrients through their cell membranes, as well as protection from the Sun's harmful ultraviolet radiation. All complex life on Earth is Eukaryotic, and it is generally accepted that Eukaryotic cells first evolved in the sea, where water would provide structural support for these larger cells until they evolved it themselves. This structural support would also have been needed for the first Metazoans, which appear to have been the soft-bodied organisms recorded in the Ediacaran Biotas.

Stern and Gerva reason that while primitive life must evolve in the sea, advanced civilizations need to evolve on dry land. Changing terrestrial ecosystems provide even more varied habitats than the oceans, providing an additional stimulus for biological evolution, and areas around the margins of continental plates tend to produce particularly varied habitats, as can be seen in the circum-Mediterranean, Mesoamerica, Madagascar and Southeast Asia today. The harsher terrestrial environment also stimulates life to produce specialist water retention and gas exchange structures, reproduction by internal fertilization, and movement systems which do not rely on the support of water, all of which lead organisms to become increasingly sophisticated and complex. 

This biological complexity is one of the prerequisites to developing a system for organisms to transfer experiences and knowledge to one-another, which in turn has the potential to lead to abstract thinking, and the development of language, technology, and science.  In particular, an advanced civilization would require the organisms building it to develop a familiarity with both fire and electricity, something more-or-less impossible if they are restricted to water. Thus the development of an advanced civilization on a planet would require the presence of a plate tectonic system, which Stern and Gerva suggest should be added to the Drake Equation.

The Drake Equation, as envisaged by astrobiologist Frank Drake, proposed that the number of potentially detectable  advanced civilizations in the Galaxy would be equal to the average rate of star formation, multiplied by the fraction of stars which host planets, multiplied by the fraction of planets which hold the conditions for life, multiplied by the fraction of planets which hold the conditions for life which actuallt develop life, multiplied by the proportion of planets with life which develop civilizations, multiplied by the proportion of civilizations which produce detectable signals (such as radiowaves etc.), multiplied by the lifetime of such civilizations. 

Based upon this, Drake made an 'educated guess' that between 200 and 50 000 000 detectable civilizations might exist in the Galaxy, with subsequent estimates by other scientists producing figures from below a hundred to several million. Stern and Gerva suggest that the proportion of planets with life that go on to develop civilizations should be considerably lower than in most estimates, due to the additional requirement for these planets to develop plate tectonic systems which operate for several hundred million years.

See also...

A possible asteroid shower at the onset of the Cryogenian Period.

Understanding meteoroid bombardment of the Earth system is an issue of both great scientific interest and practical importance because impacts are potentially hazardous to the Earth. Since the 541 million year ago Cambrian biodiversity explosion, mass extinction events have occurred at least five times (the so-called Big five events), and extra-terrestrial impacts are considered a potential cause of some of them (e.g., Late Triassic and Cretaceous-Palaeogene extinctions), competing with the flood basalt eruption-related hypotheses. After the first discovery of fossil L-chondrites in Ordovician limestones in Sweden, abundant L-chondrites (low iron stony meteorites), meteorite-tracing chromite grains and iridium enrichment have been found in Sweden, England, Scotland, China, and Russia in rocks whose stratigraphic ages are 470-480 million years. Moreover, several large terrestrial craters in the Northern Hemisphere have been found to have radiometric ages of approximately 430-470 million years. Further, approximately two-thirds of ordinary L-chondrites are known to be heavily shocked and degassed, with Argon³⁹-Argon⁴⁰ ages near 470 million years (Argon-Argon dating relies on determining the ratio of radioactive Argon⁴⁰ to non-radioactive Argon³⁹ within minerals from igneous or metamorphic rock to determine how long ago the mineral cooled sufficiently to crystallise). Therefore, it is generally considered that the L-chondrite parent body suffered a major impact approximately 470 million years ago and was catastrophically disrupted, causing a very large meteoroid shower on Earth for several million years. A recent study suggested that the extraordinary amounts of dust during an interval of over 2 million years cooled the Earth and triggered Ordovician icehouse conditions, sea-level fall, and major faunal turnovers related to the Great Ordovician Biodiversification Event. However, to date, other ancient meteoroid impacts and their relations to environmental changes have not been well understood because of erosion and/or resurfacing processes on Earth.

In a paper published in the journal Nature Communications on 21 July 2020, Kentaro Terada of the Department of Earth and Space Science at Osaka University, Tomokatsu Morota of the Department of Earth and Planetary Science at the University of Tokyo, and the Department of Earth and Planetary Sciences at Nagoya University, and Mami Kato, also of the Department of Earth and Planetary Sciences at Nagoya University, and of the Meisei Electric Co, present the results of a study in which they investigate the lunar crater record in order to reveal ancient meteoroid impacts on Earth, because there is less weathering and erosion on the Moon.

The lunar orbiter, Kaguya provides a new insight that disruption of asteroid had occurred and formed the several craters larger than 20 km simultaneously on the Moon approximately 800 million years ago. Based on crater scaling laws and collision probabilities with the Earth and Moon, at least 40-50 million megatonnes of meteoroids, approximately 30–60 times more than the Chicxulub impact, must have struck the Earth, immediately before the Cryogenian, which was an era of great environmental and biological changes.

Crater size-frequency distribution measurement is a well-established technique to derive relative and absolute ages of planetary surfaces; thus, the density of 0.1–1 km-diameter craters in the ejecta of a large crater (more than 20 km) potentially gives the formation age of the large crater itself. In this study, we investigate the formation age distribution of 59 lunar craters with fresh morphology and diameters larger than approximately 20 km using the software tool craterstats. Terada et al. select and investigate the regions where there is no pond (impact melt region) to avoid the target property effects that may cause craters formed in impact melts to be smaller than those in ejecta.

 

The locations of the 59 investigated lunar craters with fresh morphologies and diameters larger than approximately 20 km are shown. The craters with ages the same as that of Copernicus  are indicated by red circles. Terada et al. (2020).

First, Terada et al. estimate the formation ages of individual craters using the conventional constant flux model over 3 billion years. Eight of 59 craters, including Copernicus, are concentrated at approximately 660 million years, and the weighted mean is 658 million years. The spatial distribution of these craters seems to be slightly concentrated in the equatorial plane, but there is no significant difference between the far and near sides.

 

Mosaics of the Terrain Camera images of individual craters shown in simple cylindrical map projection. Terada et al. (2020).

To evaluate the probability of the observed concentration of crater ages, Terada et al. performed a simple test using a Monte Carlo simulation (a technique used to understand the impact of risk and uncertainty in financial, project management, cost, and other forecasting models). They assumed that craters are created with uniform probability within an age range from 3.0 billion years to 0 billion years and compute the ages of the 59 craters using a uniformly distributed pseudorandom number. The procedure is iterated 100 000 times. The results show that the possibility that seven of the 59 craters formed at the same time (for 50 million years from 630 to 680 million years ago) by chance is 0.69%, where the 54S161E crater (747 million years old) is masked because it is an obvious outlier with large uncertainties (if the 4S161E crater is included, the possibility that eight of the 59 craters formed during a 100 million year interval by chance is 7%). From these considerations, Terada et al. conclude that sporadic meteorite bombardment occurred across the whole Moon, possibly due to the disruption of asteroids, analogous to the Ordovician meteorite shower.

Theoretically, the mass of an impactor can be estimated from the density of the impactor, the density of the crust, the velocity of the impactor and the diameter of the crater. Assuming a density of near-Earth asteroids (1.29 grams per centimetre cubed for C-type asteroid Ryugu, 1.9 grams per centimetre cubed for S-type asteroid Itokawa, and 2.7 grams per centimetre cubed for S-type asteroid Eros) and a relative velocity of 20 km per second of Earth-crossing asteroids to the Moon, the masses and sizes of the impactors for eight lunar craters formed at 660 million years ago are calibrated. As a result, the total mass of the asteroid shower on the Moon is estimated to be 1.3-1.6 million megatonnes, corresponding to an impactor 10–13 km in diameter.

To date, the lunar impact history has been well investigated based on lunar impact glasses collected by the Apollo/Luna missions and/or lunar meteorites. The age of Copernicus crater is generally taken as about 800 million years based on both crater chronology and the radiometric dating of 12033 brecciated soil, which is considered to consist of ejecta from Copernicus crater. The discrepancy in crater age between 800 and 660 million years ago in Terada et al.'s study is due to the difference in the selected area to be counted. Terada et al. obtained an age of 797 million years, whereas an earlier study reported ages of 678 million years for the area observed by KAGUYA CE1 and 678 million years for the Copernicus ray. In addition, Terada et al. investigated other areas around Copernicus crater (floor, ejecta area and melt region near central peak), giving about 660 million years. All of these results mean that there is no discrepancy in counting and calibration between previous work, and Terada et al.'s study. Terada et al also realize that the selected area with an age of 800 million years is close to the centre of Copernicus crater and tends to be affected by secondary craters, so a counting method yielding a younger age of 660 million years is correct for Copernicus crater. Note that the most important of the new findings is that eight craters, including Copernicus, show identical relative ages based on a constant flux model.

 

The terrain camera images and their cumulative size-frequency distributions from the floor, ejecta area and melt region near the central peak of Copernicus crater. Terada et al. (2020).

On the other hand, the absolute age of Copernicus crater is considered to be 800 million years based on the radiometric ages of 12033 brecciated soil collected from the ejecta of Copernicus crater. In addition, a 2015 study reported that Argon⁴⁰/Argon³⁹ data for impact spherules from Apollo 12, 14, 16, and 17 samples show an 800 million years ago spike, similar to that of the 12033 breccia, and concluded that there must have been a transient increase in the global lunar impact flux at 800 million years ago other than Copernicus crater, in the context of diverse compositional ranges and sample locations of impact glass spherules. Such geochemical observations of simultaneous global lunar impacts recorded in Apollo samples well match the coincidence of (at least) eight crater formations derived from our observations, of which the probability is 0.69%. From these considerations, Terada et al. infer that these two observations must be related to each other and newly propose a constant with a spike model of about 800 million years ago instead of a conventional constant model.

This scenario in which sporadic asteroid showers did not occur at 660 million years ago but at 800 million years ago is also supported by recent Lunar Reconnaissance Orbiter observations and/or numerical simulations of the asteroid families. Based on the temperature of large impact ejecta with crater sizes larger than 10 km in diameter, a recent study concluded that there is no evidence of a sporadic peak at approximately 660 million years ago, although that study found that the production rate of lunar craters with diameters of at least 10 km was 2–3 times higher over the last 290 million years. However, the age of 800 million years is very close to the limit of resolution for that study, so there is no contradiction with the 800 million years ago spike model. Furthermore, numerical simulation of the orbits of asteroid families also provides crucial chronological information about the impact flux to the inner solar system. A recent investigation of the dynamics of the asteroid family and the best available Yarkovsky measurements (the measurment of the mechanism by which asteroids are torn apart by their own spin) suggest that the breakup of the parent bodies of Agnia Family of asteroids (669–1003 million years ago) and/or Hansa Family of asteroids (763–950 million years ago) was related to the sporadic asteroid shower at 660 million years ago. However, it is known that these families are not sizable enough and/or not well enough positioned to produce the sporadic asteroid shower, including Copernicus crater with a diameter of 93 km, for which impactor is expected to be 10 km in diameter. Moreover, the Agnia Family is located near the 5:2 resonance, where the probability of a projectile hitting the Moon is very low. The Hansa Family also has difficulty producing an impactor of 10 km for Copernicus because it is located at high inclinations near the 3:1 and 8:3 resonances. However, the Eulalia Family of asteroids, whose age is 830 million years old, could potentially have produced an impact spike at about 800 million years ago. A recent study suggested that when the parent body of Eulalia was disrupted, a large share of the sizable family was directly injected into the 3:1 resonance at low inclinations. This disruption certainly could have produced an impact spike on terrestrial planets and/or their satellites inside the asteroid belt. Interestingly, the Eulalia family is a carbonaceous chondrite family and is considered to be the parent body of near-Earth C-type asteroids, such as Bennu and Ryugu. Such an asteroid shower must have contaminated the lunar surface with volatile elements. This scenario is harmonized with (i) the observation of H₂O in Copernicus crater that may reflect retention of volatiles from hydrous impactors; (ii) the scenario that may have been formed by a cometary nucleus, 4 km in diameter based on geochemistry of the 12033 breccia; and (iii) recent KAGUYA remote-sensing observation of persistent positively charged carbon ions emitted from the whole Moon, which is significantly larger than influxes due to solar wind and/or current micrometeoroid accretion and suggests that the lunar surface might have been contaminated by volatile-rich impactors in the past.

It is obvious that the break-up of large asteroids increases not only the large (over 20 km) crater production rate but also the small (0.1–1 km) crater production rate. From these considerations, we propose the new simplest model: a constant flux with a spike between 830 and 800 million years ago for small craters (0.1–1 km). The basic idea is that the crater counting age of Copernicus crater must be identical to the radiometric age of 800 million years and that the fluxes before and after the sporadic spike at about 800 million years were constant. Although the duration time of the spike is slightly uncertain, Terada et al assume that this duration was 30 million years (from 830 to 800 million years ago), based on the break-up age of Eulalia and the radiometric age of Copernicus crater and/or the deviation of eight clustered ages (658 million years ago) for a constant model.The constant flux of the new spike model is 75% (663 million years/800 million years) of the conventional constant flux model, and the flux between 800 million years and 830 million years is 23 times higher than that in other eras to ensure that the total crater production over 3 billion years is identical for both models.

As a result, the modified age distribution shows that 16 of the 59 craters coincide with that of Copernicus crater within the analytical certainties although the large (over 20 km) crater production rate and the small (0.1–1 km) crater production rate might be coupled in this model. However, the estimated total masses are not significantly changed (1.3–1.6 million megatonnes for the constant flux model and 1.8–2.3 million megatonnes for the 800 million year spike model) because Copernicus crater is dominant among the eight coincident craters (by the constant model) and the 17 coincident craters (by the spike model). Therefore, the latter discussion on the total mass estimation of the impactor is not affected by the choice of a flux model with/without the spike. Moreover, it should also be noted that the slopes of the lines below 300 million years in both models are gentler than those of other eras, which is quite consistent with the previous study that the production rate of lunar craters (over 10 km) has been 2–3 times higher over the last approximetely 290 million years, which is derived from an independent approach based on the temperature of large impact ejecta.

Recent observations by the Chandrayaan-1 and LADEE lunar orbiters suggests that an active water cycle exists on the Moon and that hydrated soil is present under the desiccated soil layer of several centimetres over the Moon surface. One 2017 study observes that the Copernicus crater exhibits high water content that may reflect the retention of volatiles from hydrous impactors according to the Moon Mineralogy Mapper. In addition, Terada et al recently found that positive carbon ions are persistently emitted from the whole of the Moon as detected by the lunar orbiter KAGUYA and that this flux is significantly larger than the influx estimated from solar wind and/or current micrometeoroid accretion. These observations suggest that volatile elements are ubiquitous over the lunar surface and that the Moon is currently in the process of losing volatiles (water, carbon, etc.), although when and how the surface of the Moon attained and/or retained such volatiles has been enigmatic. Assuming a CI chondrite-like chemical composition (a few percent by weight of carbon and H₂O), Terada et al 's scenario predicts that a C-type asteroid shower at 800 million years ago must have supplied about 100 000 megatonnes of carbon and H₂O to the lunar surface. This new paradigm undoubtedly should place new constraints on the history of lunar volatiles.

Since the Earth–Moon system has been co-evolving over 4.5 billion years, this new finding provides crucial insight into the Earth–Moon system because asteroid showers must have occurred not only on the Moon but also on the Earth. Based on the probability ratio of collisions with the Earth and the Moon of 23:1, Terada et al. conclude that a mass of 40-50 million megatonnes (corresponding to a diameter of about 30–40 km and about 30–60 times greater in mass than the Chicxulub asteroid impactor must have collided successively on the Earth at about 800 million years ago, i.e. immediately before the Cryogenian (720–635 million years ago), which was an era of great environmental and biological changes. To date, however, no direct geological evidence of a large-scale impact in the Neoproterozoic has been found. Moreover, remarkable iridium concentrations as well as other platinum-group elements anomalies, such as those at the Cretaceous-Palaeocene boundary, have not been found, although only the Marinoan Glaciation (650–635 million years ago) is characterized by increased concentrations of iridium. The straightforward interpretation is that the following large-scale Neoproterozoic glaciations, the socalled Snowball Earth (that is, the Kaigas-Sturtian glaciation from 730 to 700 million years ago and the Marinoan Glaciation) and/or their deglaciation processes, might have erased a significant part of the earlier geological and/or geochemical history.

To date, the impact history and subsequent effects on the environment in the Neoproterozoic and Cryogenic have not been understood because terrestrial craters are not well preserved due to erosion. One recent study discussed the mechanical effects of one impact of an asteroid 5–10 km in diameter on the Snowball Earth environment, suggesting that the products of impact (mainly water vapour) could be quickly distributed over a substantial part of the globe, influencing the global circulation (e.g. facilitating cloud formation), because one impact cratering event (shock waves and impact crater formation) might produce much dust that entered the atmosphere and might have caused albedo changes. Another study noted that the Ordovician meteorite shower should have triggered the mid-Ordovician ice age based on the sizes of the remaining terrestrial craters. Thus, large asteroid showers should influence the global ecosphere in some ways, although mechanisms are not well realised because of the unknown characteristics of the dust, e.g. size, albedo, mineralogy, and chemical composition.

Interestingly, another study found that the average phosphorus content of late Tonian samples is more than four times greater than that of pre-Cryogenian samples and noted that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic Eon after 800 million years ago (until 635 million years ago). Terada et al.'s new finding suggests that about 100 000 megatonnes of extra-terrestrial phosphorus should have accreted across the Earth assuming CI chondrite composition (0.1 percent by wieght phosphorus) at 800 million years ago, which is one order of magnitude higher than the total phosphorus amount of the modern sea (assuming that the volume of modern seas is 1350 million km³ and the concentration of phosphorus is about 3 μg/litre). In general, large-scale changes in marine biogeochemical cycles are undoubtedly forced by tectonic and magmatic processes and chemical weathering of the continental crust, but Terada et al.'s new finding suggests that the flux of extra terrestrial bioavailable elements might also have influenced marine biogeochemical cycles, marine redox states, severe perturbations to Earth’s climate system, and the emergence of Animals. Thus, lunar crater chronology provides new insight into external forcing from asteroids that might have driven ecosystems towards larger and increasingly complex organisms after 800 million years ago, although further quantitative discussion will be required.

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Finding a connection between the formation of the oldest known Himalaya-style mega-mountains and the appearance of Metazoan Animals.

It was extremely fortunate for the stability of life that despite the planet active dynamics over geologic time, surface temperatures were always sufficiently moderate to maintain liquid water in the oceans and, therefore, sustain the continuous biological evolution since at least early Archean time. The first oceans were accumulated via mantle degassing and/or cometary bombardment much earlier, during the Hadean Eon. Within this realm, life appeared on Earth at least by about 3.8 billion yeats ago, the age of supracrustal rocks of southwest Greenland bearing biogenic carbon isotopes of graphite, or possibly as early as 4.1 billion years ago, the age of similar evidence more recently reported from a Western Australian zircon grain. For the following three billion years or so, microorganisms were the only life forms in the primitive oceans, and their fossil evidence attests to a very slow evolutionary rate of morphological change. The first organisms were Prokaryotes, such as Bacteria and Archaea, which continue microscopic and morphologically simple, yet ubiquitous and extremely important, even today. Later, during Proterozoic time, environmental pressures and opportunities led to evolutionary developments that eventually produced the Eukarya, initially and for a very long time solely microscopic, which have lived side-by side with Prokaryotes ever since.

During a relatively long span of geologic time from the Middle Neoproterozoic to Late Cambrian (roughly750–500 million years ago), important innovations in the fossil record register the birth and proliferation of more complex macroscopic forms of life, especially in the Ediacaran Period (635–541 million years ago), while the geologic record reveals rapid changes in continental configurations related to the Gondwana supercontinent formation, extreme climatic variations, including the most severe glacial episodes in Earth history and the oxygenation of oceans and atmosphere, all of which surely exerted great selection pressures on the evolutionary processes responsible for these innovations. In the final part of this time-frame, during the Tommotian–Botomian interval (535–513 million years ago), the 'Cambrian Explosion' of life took place, as evidenced by the variety and amount of shells, carapaces, and other fossil remains representing all known modern phyla conserved worldwide in Cambrian strata.

The planet Earth started hot, and has continuously cooled since then. Plate tectonics is the main mechanism by which the planet loses its internal heat, with geologic evidence indicating that it started functioning gradually in Archean times. Lithosphere subduction is the plate tectonics hallmark, and in 2008 Michael Brown of the University of Maryland, after analysing Earth’s metamorphic record over geologic time, was able to define a 'Proterozoic plate tectonic regime' characterized by oceanic lithosphere subduction. Continental lithosphere remained stable, however, with large areas of granitic-type crust providing the nuclei of major continental masses and several supercontinents during this period. Meanwhile, as Earth cooled slowly during the Archean and most of the Proterozoic, life remained microscopic.

However, in the Neoproterozoic, important modifications occurred both in the planet dynamics and in the life and biosphere complexities. Due to continuous heat loss, the Earth cooled to a point in which, relatively quickly, basaltic-type oceanic lithosphere could become negatively buoyant. As a result, basalts could be transformed into even denser eclogites in deep subduction slabs. This greater negative buoyancy led to the 'slab-pull type' driving the force and appearance of 'modern-type subduction zones' and 'subduction-to-collision' orogenic belts that would become widespread in the Phanerozoic. For the first time in Earth’s history, ultra-high pressure metamorphic terrains bearing coesite or diamond appeared on Earth, indicating subduction of continental crust to depths greater than 100 km, followed by rapid exhumation. The oldest ultra-high pressure eclogites described so far are Ediacaran in age as shown by robust uranium-lead zircon ages of about 610 million years (as zircon forms it can incorporate a variety of different elements into its crystal matrix, including uranium but not lead; this is useful as over time uranium decays to form lead, so any lead in a zircon mineral must be the result of the decay of uranium). It has been further demonstrated that these eclogites are located along the Transbrasiliano-Kandi megashear, which is a huge tectonic lineament, more than 5000 km long, linking South America and Africa, interpreted as the possible site of a collisional suture associated with the West Gondwana Orogeny that produced Earth’s first Himalayan-type mega-mountains.

In a paper published in the Brazilian Journal of Geology on 22 May 2020, Umberto Cordani, Thomas Fairchild, Carlos Ganade, Marly Babinski, and Juliana de Moraes Leme, of the Universidade de São Paulo, suggest that the practically simultaneous appearance of the first very large high-relief mountain chains on the Earth’s surface, similar in magnitude to the Himalayas, and the radiation of macroscopic Metazoans in the Ediacaran are probably intimately related. Other authors have alluded to the influence of Gondwana mountain upon Ediacaran evolution, but Cordani et al. claim to have identified just when and where it began.

The Neoproterozoic (1000–541 million years ago) was a time of important modifications on the planet dynamics, including especially the onset of modern-type plate tectonics that prevails until the present. Such a regime is characterized by subduction-to-collision orogenic belts and disappearance of large oceanic domains. The Neoproterozoic hosted a specific and long-term episode involving the planet lithosphere, when continental masses of all sizes underwent an extended fragmentation period and new assembly with the disruption of the supercontinent Rodinia into many fragments, starting around 900 million years ago, and Gondwana amalgamation that ended around 500 million years ago. Moreover, at the end of the Neoproterozoic, in the Ediacaran, with Gondwana assembly in full progress, marked changes occurred in relief, climate, oceans and atmosphere, while complex forms of life, including the first macroscopic Metazoans appeared, proliferated, and spread quickly over the Earth.

Rodinia was assembled in the late Mesoproterozoic by a series of orogenic pulses usually included in the Grenvillian Orogenic Cycle, roughly between 1300 and 1000 million years ago, the last of which the Rigolet Orogeny dated to between 1010 and 980 million years ago. Rodinia was a long-lived supercontinent centered around Laurentia and comprised the existing continental masses, which by 1000 million years ago occupied a significant part of the planet surface. It remained tectonically stable for more than 200 million years, but at about 750 million years ago a series of breakup events produced rift-related basins, including large passive margins on both sides of Laurentia.

Fragmentation terminated around 600 million years ago, when Laurentia rifted away from Amazonia, thereby opening the Iapetus Ocean. By that time, Gondwana had become almost completely assembled out of various cratonic fragments that had previously rifted away from Rodinia.

Gondwana was made up of several older continental masses, such as Amazonia, West Africa, Sahara, São Francisco Congo, Kalahari, Arabia, India, Madagascar, Australia, Antarctica and a few minor fragments, but it remained for a long time independent of Laurentia, Siberia, and China. Its amalgamation began about 650 million years ago and was complete at about 500 million years agp. About 200 million years later, the latter three continental masses were reunited with Gondwana to form the Pangea supercontinent. In the transition from Rodinia to Gondwana, two very large oceans were consumed by means of subduction-to-collision roughly between 900 and 500 million years ago, with closure of the Goiás-Pharusian Ocean in the West, and the Mozambique Ocean in the East. The former is now the West Gondwana Orogeny, and the latter is the site of the East African Orogen.

Reconstruction of the former supercontinent Gondwana showing the distribution of ancient cratonic nuclei within modern continents and their position regarding important orogenetic features (broken lines), with emphasis on the West Gondwana and East African orogens. Cordani et al. (2020).

The Gondwana supercontinent was formed as a succession of three first-order orogenic events that belong to the Brasiliano–Pan-African orogenic cycle, each occupying a specific space and time. This began with the convergence between the São Francisco-Congo and Amazonia-West Africa-Rio de la Plata cratonic masses that closed the Goiás-Pharusian Ocean and produced the West Gondwana Orogeny (650–600 million years ago). The other two orogenic episodes were responsible for the closure of the Mozambique Ocean and, consequently, the formation of the East African Orogen, which is the world’s largest Neoproterozoic to Cambrian orogenic complex. This involved a collision between Congo and India-Madagascar during the East African Orogen (600–550 million years ago), followed by collision of the rest of Gondwana and Australia-Antarctica in the Kuunga Orogeny (550–500 million years ago).

It has been argued that deeply subducted rocks, specifically the eclogites with coesite exposed in the West Gondwana Orogeny, comprise the earliest evidence of large-scale deep continental subduction on Earth. This linear belt, more than 5000 km long and including the oldest known ultra-high pressure eclogites in the world, cuts across South America and extends into present-day North-West Africa. It represents a series of sutures associated with final convergence between the conjoined Amazonian and West African cratons, on one side, and the Saharan and Congo-São Francisco cratons, on the other, close to the present site of the Transbrasiliano-Kandi mega-shear zone.

The long-term convergent plate motion and evolution of eastward subduction within the Goiás-Pharusian Ocean started at about 900 million years ago. Successive 'soft collisions' occurred between about 800 and 650 million years ago and culminated in terminal continental collision roughly between 650 and 600 million years ago, when the Pharusian-Araguaia-Paraguay belts were closed. The Goiás magmatic arc is a major area of soft collision and accretion of the Amazonian Craton margin made up essentially by Neoproterozoic granitoids with juvenile signatures that indicate they represent the roots of a series of intra-oceanic island arcs produced by consumption of oceanic lithosphere. The predominantly calc-alkaline chemistry of these magmatic rocks indicates persistent, subduction-related and active margin processes.

Graphic representation of the history of West Gondwana assembly and the formation and denudation of Earth’s oldest known Himalaya-style mega-mountains. Cordani et al. (2020).

Along the West Gondwana Orogeny, ages of ultra-high pressure coesite-bearing eclogitic rocks marking the suture zones between colliding cratons in Mali, Togo and northeast Brazil were obtained using coupled uranium-thorium-lead and rare earth element zircon analyses together with geothermal-barometric constraints. The deep continental collision timing was determined from uranium-lead sensitive high-resolution ion microprobe measurements on overgrowth rims of zircon in the eclogites, as follows: 611.3 million years ago for Mali, 608.7 million years ago for Togo and 616.0 million years ago for northeast Brazil. Within experimental error, these results indicate Ediacaran age for the ultra-high pressure metamorphism and simultaneous subduction of at least three fragments of continental margin along the West Gondwana Orogeny. Moreover, pressure-tempreature determinations indicate subduction of this continental margin to depths conducive to ultra-high pressure metamorphic conditions over a distance of at least 2500 km within the West Gondwana Orogeny, which is comparable to the extent of the present Himalayas. Such extreme metamorphism occurs along sutures due to very high temperatures and pressures and the internal rearrangement of material within subducted continental lithosphere, such as at depths of greater than 100 km, followed by rapid exhumation. 

All presently known locations of ultra-high pressure terrains exposing eclogites that contain coesite or diamond and their respective peak metamorphic ages in millions of years. Most of them are Phanerozoic in age and commonly related to major orogenetic cycles (i.e., Caledonian, Variscan, Alpine-Himalayan etc.). The youngest, at 4-million-years-old, is from Papua, New Guinea. The oldest are those related to the West Gondwana Orogen in Brazil and northwest Africa, with ages greater than 600 million years. The occurrence marked 'Minas Gerais, 630 million years' is still under suspicion due to its uncertain location and lack of direct age dating of the sample. Cordani et al. (2020).

The East African Orogen, in turn, is a collage of continental cratonic nuclei, such as the Saharan, Congo, Kalahari, India, Madagascar, and Australia-Antarctica. Its northern part, the Arabian-Nubian Shield, represents an oceanic domain formed by island arc terranes with juvenile crust. South of the Arabian-Nubian Shield lies the Mozambique Belt, the main orogenic component of the East African Orogen, with a complicated tectonic history involving continent-continent collisions among several cratonic masses with continental crust that are now located in Kenya, Tanzania, Madagascar, India, Sri Lanka, Zambia, Malawi, Mozambique, and Antarctica. Along the collisional sutures of the Southern part of the East African Orogen, high mountains also resulted from crustal duplication produced by convergence and compressional orogenic deformation of continental crust.

The oldest ultra-high pressure terrains, in which eclogites with coesite or diamond and their respective peak metamorphic ages have been identified are those related to the West Gondwana Orogeny in Brazil and northwest Africa with ages greater than 600 million years. All others are Phanerozoic in age, some of them related to the Caledonian and Variscan belt in Greenland and Europe, and others related to the closure of the Paleo-Tethys and Neo-Tethys oceans to form Eurasia, when the East Gondwana sector of Pangea began to break up, disperse, and amalgamate. Particularly noteworthy are the Meso-Cenozoic ultra-high pressure terrains linked to the Alpine-Himalayan collisional chain and the very recent age, 4 million years, of the youngest known eclogites with coesite, from Papua New Guinea. Geothermal-barometric evidence indicates that coesite rocks can be exhumed from depths exceeding 100 km, which means that crustal material produced by subduction of felsic continental crust less dense than the mantle may be subject to rapid isostatic rebound and exhumation to give rise to significant topographic relief, as in the Alps and Himalayas. Hence, these arguments indicate that the West Gondwana Orogeny registers the oldest known continent-continent collision, resulting in Himalayan-style mountain-building in Earth history. The West Gondwana Orogeny mega-mountains were higher than any previous Neoproterozoic mountain belts and, therefore, a source of vast amounts of erosional sediments, part of which is preserved in the Ediacaran foreland basins and epicontinental seas of Gurma, Taoudeni, and Volta in Africa and Parecis, Paraguay, and Bambui in South America.

Furthermore, within the same Neoproterozoic time frame, at least three great Neoproterozoic glaciations took place, namely, the Sturtian (roughly 720–660 million years ago) and Marinoan (roughly 650–636 million years ago) glaciations, both considered as global in extent, or nearly so, and the more restricted Gaskiers glaciation (582  million years ago). The Snowball Earth hypothesis alleges that in each of these episodes the Earth was covered, or nearly so, by ice at least for a few million years. All continents contain evidence of this phase of Earth history, although the number of glaciations, their duration, and extent are still the subject of debate. Similarly, the importance of these drastic climatic episodes upon microscopic life is unquestioned, although not much is known about just how they provoked or affected evolutionary changes.

It has been suggested that the increased rate of sediment accumulation resulting from erosion of the high relief produced during the amalgamation of Gondwana could have influenced life radiation on Earth. This was supported other studies which argued that massive sediment supply related to Gondwana assembly would have provided abundant nutrients for Algae and Cyanobacteria, and, consequently, a major stimulus for oxygen production by photosynthetic. The subsequent increase of O₂ in the atmosphere and oceans would have been an important factor in the radiation of Late Neoproterozoic, and especially Ediacaran, life. In our view, the full implications of this innovation within the context of Rodinia breakup and Gondwana amalgamation have not still been unfathomed. However, the appearance of very high, open-air, ice-free mountains between about 620 and 570 million years ago in the aftermath of drastic Snowball Earth scenarios must have exerted a dramatic influence on climate, weathering, erosion, nutrient fluxes and carbon burial, with important consequences for the Earth system and especially as triggers for the evolution of Ediacaran life.

Uplift and denudation of Earth’s first very high Himalayan-style mountains along the Transbrasiliano-Kandi alignment began at about 615 to 610 million years ago and putatively continued for the following 40 to 50 million years during a very significant moment on Earth, and especially biosphere, history. Earlier to this event, life was pre-eminently microscopic, yet by the time it was over, macroscopic organisms, albeit many of them of enigmatic biological affinities, appeared and began to diversify en route to the crown group Metazoans that we recognise today.

Claims of the most ancient fossil evidence of life include carbon isotope signatures, microbialites, and microfossils with ages extending back to around 4 billion years ago, near the base of the known geological record. All, however, have been subject to considerable contention. Consequently, present consensus places the oldest reliable evidence of life on Earth from rocks about 3.45 billion years ago in age from the Warrawoona Group of Northwest Australia, in the form of putative stromatolites, microfossils and associated carbon and sulphur isotope data. This means that, independently of the biogenicity of older fossils, Prokaryotic life proliferated and rose to dominance in benthic and planktic environments by mid-Archean and continued that dominance, certainly in benthic settings, even after the rise of unicellular Eukaryotes (Microalgae and other Protists) in the later Palaeoproterozoic. In fact, it was these ancient Prokaryotes, specifically the Cyanobacteria, that furnished oxygen to the atmosphere via photosynthesis beginning no later than 2.7 billion years ago and were responsible for the Great Oxygenation Event in the early Palaeoproterozoic around 2.4 billion years ago. A postulate of modern biology is that stable oxygen in the atmosphere provided an environmental stimulus that favored the appearance of unicellular Eukaryotes, practically all of which are obligate aerobes that depend upon oxygen, later in the Palaeoproterozoic, by at least 1.7–1.9 billion years ago.

Consensual geological evidence for the next great innovation in biological history, multicellular but still microscopic Eukaryotes, dates from the mid-Mesoproterozoic, about 500 million years later. It consists of permineralized microscopic filamentous Algae about 1200-million-years-old, which are very similar, in fact, to modern crown-group Bangiophycean Red Algae. The origin of multicellularity certainly must have originated much earlier. Indeed, decimeter-scale organic compressions have been found in the Gaoyuzhuang Formation of North China just above putative evidence for a global oceanic oxygenation event. If their interpretation as Macroalgae is correct, macroscopic multicellular Eukaryotic Algae may have originated a little after 1600 million years ago. As such, this coincides with increased abundance and diversity of Eukaryotic Microalgae observed in other stratigraphic units in China, Siberia, and Australia. 

After the appearance of Macroscopic Algae, the dominance of marine Prokaryotes, especially in benthic settings, began to falter, first, through competition for space and light and, nearly a billion years later, beginning about 600 million years ago, as a consequence of the radiation of complex sessile and vagile macroscopic multicellular Eukaryotes, including not only heterotrophic and autotrophic Metazoans, but also other diverse organisms apparently unrelated to modern crown groups, known as Vendobionts. Moreover, phylogenomic analyses have demonstrated that by 730 million years ago, in middle Neoproterozoic time, just prior to the near-global Cryogenian 'Snowball Earth' glaciations, many groups of Amoeboid heterotrophic Protists among unicellular Eukaryotes had already attained crown-group status. This is evidenced by late Tonian (800–720 million years ago) vase-shaped microfossils, representative of at least two very separate groups of Testate Amoebae, as well as by other scale-bearing Protists (from about 811 million years ago onwards). Younger agglutinated Protists interpreted as Foraminifera are reported from the mid-Cryogenian, but evidence of Metazoan body fossils older than 600 million years is limited to controversial tiny Sponges and supposedly corroborative coeval or older Sponge biomarkers.

Surprisingly large Vendobionts nearly 580 million years old effectively mark the advent of continuous geological record of macroscopic Metazoan and other Animal-like body fossils. However, two exceptionally preserved fossil biotas (Konservat-Lagerstätten) in post-glacial (i.e. post-Marinoan) formations in China provide permissive evidence of a probably older Ediacaran record. The well-known Weng’an Biota from phosphorites in the upper half of Doushantuo Formation could be as old as about 600 million year, then the Lantian biota, from stratigraphically deeper pelites of the Lantian Formation, would have to be older than this. Precise ages of these assemblages have yet to be established.

Main occurrences of macroscopic Ediacaran metazoans. The classic localities of the soft-bodied Avalon biota occur in Newfoundland and the UK. Those of the White Sea biota in northwest Russia and Norway; and those of the Nama biota, in Namibia and Australia. The localities of the older Weng’an and Lantian biotas of South China are indicated by W and L, respectively. Cordani et al. (2020).

The Weng’an Biota consists of phosphatised and silicified microfossils so well preserved three-dimensionally as to allow claims (not all without dispute) that they may represent Animal embryos in various ontogenetic stages, Bilaterian Animals, Red and Green Algae, Acritarchs, and a Ctenophore. In addition, two-dimensional carbonaceous compressions within the formation have been attributed to Macroalgae. A tuff bed overlying the fossil-bearing units has been dated to 609 million years ago, and Acritarchs in the formation are similar to immediately post-Marinoan (i.e. less than 635 million years old) forms elsewhere make the Weng’an assemblage a serious candidate for housing the oldest fossil evidence of embryonic, larval and adult Eumetazoans, which are even older than the classical soft-bodied macroscopic Avalon, White Sea, and Nama Ediacaran biotas. However, other researchers have reviewed the evidence and arguments regarding biological affinities of the Weng’an fossils and concluded that, although some might represent animals, none can yet be confidently identified as stem- or crown-group Metazoans.

Representative fossils of the Weng’an biota (A)–(F) and Lantian biota (G)–(I) of South China. These biotas are older than the middle to late Ediacaran soft-bodied Avalon, White Sea and Nama macrobiotas. (A)–(F) Scanning electron microscope images of selected fossils of the Weng’an biota preserved in three dimensions. (A)–(C) Embryo-like fossil Tianzhushania exhibiting various stages of division, from a few cells (A) to many hundreds of cells (C). (D) Spiralicellula. (E) Archaeophycus, a putative Red Alga. (F) Eocyathispongia, suggested to be a Sponge. (G)–(I) Representative fossils of the Lantian biota of South China preserved as compressions. (G) The Macroalga Flabellophyton, with a fan-shaped or conical thallus. Scale is 5 mm. (H) Lantianella, a putative Scyphozoan Cnidarian, approximately 36 mm long. (I) Xiuningella, a Scalidophoran introvert(?), approximately 18 mm long. Scale bar in (A) 265 μm, (B) 200 μm, (C) 280 μm, (D) 380 μm, (E) 255 μm, (F) 415 μm. Cordani et al. (2020).

The Lantian Biota consists of carbonaceous compression fossils preserved in place in carbonaceous pelites that were deposited below storm-wave base, yet still within the photic zone, judging from the alga-like morphologies of some of the fossils. The biota represents about 15 morphospecies exhibiting complex morphological differentiation and reaching up to several centimeters in maximum dimension. The fossiliferous member of Lantian Formation may be correlated with strata in the Doushantuo Formation older than that containing the Weng’an biota, thus making the Lantian biota the earliest known well-preserved assemblage of varied macroscopic multicellular Eukaryotes. Although many probably represent Macroalgae, few may arguably be Cnidarians or Worms. however, once again, this requires corroboration. The Lantian Formation is assuredly Ediacaran in age and possibly as old as 635 million years; however, from a conservative point of view, all that can be said is that they are most likely older than 551 million years.

The so-called Vendobionts, represented by dozens of taxa of fossilized soft-bodied, but firm macroscopic organisms, appear at about 580 million years ago and continue as the predominant elements of Ediacaran biotas worldwide till the end of the period at 541 million years ago. Three distinct, bio-stratigraphically useful assemblages are recognised, each drawing its name from its most representative area of occurrence. The oldest, the Avalon assemblage (579 to about 560 million years ago), known from Newfoundland and England, has been characterized by modular organisms built from repetitively branched ('fractal') units comprising the Rangeomorpha. Potential macroscopic sponges have also been recognised. The widespread White Sea assemblage (roughly 560 to roughly 550 million years ago) is more than three times more diverse in genera than the Avalon assemblage. It also includes diverse trace fossils, indicating increased eco-space occupation and behavioral complexity. The vagile Mollusk-like soft-bodied Kimberella, that first appears in this  assemblage is considered by some to be a total-group Bilaterian (the group which includes all extant Animals except Sponges, Cnidarians, Ctenophores and Placozoans), as is Dickinsonia. The Nama assemblage is the youngest, at about 550 to 541 million years old, and dominated by soft-bodied Erniettomorpha. However, it also includes the oldest biomineralized macroscopic fossils and fossils such as Corumbella and Paraconularia which some palaeontologists consider to be crowngroup Metazoans (Scyphozoa). Holes in some biomineralised exoskeletons of the emblematic Nama fossil Cloudina have been interpreted as evidence of the predation that seems to have been fundamentally important in the subsequent radiation of biomineralising Metazoans marking the beginning of the Phanerozoic Eon and characterising the Cambrian Explosion of marine Invertebrates.

Representative taxa of middle to late Ediacaran macrobiotas. (A) Soft-bodied, frondose members of the Avalon macrobiota of Newfoundland, Canada, buried in place with holdfasts, stipes and fronds. Scale  bar is 4 cm. (B) Ediacaran Arborea with associated trace fossil; Flinders Ranges, South Australia. Smooth central part is 12.5 cm long. (C) Ediacaran crown-group Metazoan (Scyphozoan Cnidarian) Corumbella, an organic-walled tubular fossil from Corumbá, Brazil (total length of larger individual, 20 mm). (D) Probable Bilaterian Eumetazoan Dickinsonia from the Flinders Range, Australia (length, 14 mm). (E) Plausible stem-group Ctenophoran Eoandromeda from South China (diameter about 14 mm). Cordani et al. (2020).

In light of the incompleteness regarding the early record of macroscopic fossils, evolutionary biologists have developed models, molecular clocks, that attempt to establish the divergence timing of evolutionary lineages in the Metazoa, from stem groups to crown groups, based on the comparative analysis of amino acid sequences in proteins common to the biological groups under consideration. Molecular clocks depend upon estimates of molecular substitution rates, detection and correction of heterogeneities in these rates, choice of calibration points in the fossil record, choice of calibration strategy, and proper consideration of uncertainties in these parameters. They date from the 1980s and figure importantly in the current understanding of early Metazoan evolution in the Neoproterozoic. The crucial point in the configuration is the choice of calibration points from the geological record of body fossils, trace fossils, and biomarkers (geologically stable organic compounds diagenetically derived solely from known biological precursors) for the nodes that mark major phylogenomic divergences, such as the appearance of the Eumetazoa, Bilateria, Deuterostomia/
Protostomia, and so on.

Molecular clock data for main events in the early Animal evolution together with relevant fossil evidence, Neoproterozoic oxygenation history, and record of Neoproterozoic glaciations. Cordani et al. (2020).

Much of the molecular genetic toolkit required for Animal development originated deeply in eukaryote evolutionary history. Hence, the appearance of complex multicellular Animals in the geologic record may have been limited or triggered by such factors as biosphere oxygenation, scarcity of trace metal micronutrients, pulse of continental weathering in nutrient flows to the oceans, and environmental restrictions imposed by extreme Cryogenian and Ediacaran icehouse scenarios, among others. Uncertainties regarding the nature of ancient fossils together with violations of the molecular clock models are such that it is not possible to accurately pinpoint early divergence events in metazoan evolution from the available fossil data and molecular clock models.

However, critical reexamination of the fossil evidence reveals a much less bleak picture for the early Metazoan evolution. The most widely cited evidence for Metazoans in rocks predating accepted Ediacaran body fossils is the biomarker 24-isopropylcholesterol, attributed to Demosponges, in rocks about 635-million-years-old from Oman and Cryogenian Sponge bioclasts in the Trezona Formation of Australia, yet they, too, are subject to debate. Nevertheless, these and other key findings provide a pattern of fossil evidence that is consistent with the molecular clock model. Together, they suggest that the Metazoa originated no later than 635 million years ago; and the divergence and initial diversification of Bilaterians likely occurred prior to about 560 million years ago. A closer fit between the fossil record and molecular clocks is hampered by problems of preservation and identification of biological affinities of these and older possible animal fossils, given that they must include stem-group organisms with unfamiliar and commonly poorly preserved character sets. All would agree, however, that the major diversification of the Metazoa that sets the stage for Phanerozoic animal evolution was indeed an Ediacaran phenomenon.

At the end of the 'boring billion' years in the history of life (between about 1800 and 800 million years ago), a succession of Earth-changing events involving crustal, atmospheric and hydrosphere dynamics and chemistry impinged itself upon life systems. This new complex and dynamic environment effectively transformed the long-reigning microbe-dominated ecosystems into a rapidly evolving macroscopic biosphere capable of ever more complex interactions and presenting greater physical presence within the Earth system.

Primary among the events affecting the biosphere was the oxygen increase in the atmosphere and oceans beginning in mid-Neoproterozoic and perhaps reaching 40% of the present atmospheric level by 550 million years ago. It is now a consensus that physical and chemical processes of carbon recycling, acting during the latter half of the Neoproterozoic, beginning about 800 million years ago, were largely responsible for elevating the oxygen level in the atmosphere and oceans to evolutionarily significant thresholds over the remainder of the terminal Neoproterozoic. Whether this phenomenon, dubbed the Neoproterozoic Oxygenation Event, was episodic or continuous is still under debate, but most researchers accept it as a fundamental factor in the expansion of multicellular eukaryotes. Indeed, by the very early Palaeozoic, increased oxygen availability allowed the introduction of complex, active and multicellular macroscopic Eukaryotes into the biosphere. This is a level of complexity that it sustains to the present day.

Of the various reasoning lines offered as evidence for this event, the most important in the present context are those that demonstrate a continuous increase in strontium⁸⁷/strontium⁸⁶ values in seawater and generally high relative Carbon¹³ values in carbonates in the Neoproterozoic after about 800 million years ago. These tendencies may be explained, respectively, by the increasing rates of continental weathering as responsible for radiogenic strontium input into the oceans and by relatively high sustained rates of organic matter burial (derived from mostly unicellular Prokaryotic and Eukaryotic micro-phytoplankton). The sequestration of organic carbon by burial, together with the withdrawal of atmospheric CO₂ by continental weathering, would have: liberated O₂ that otherwise would have been consumed in the oxidation of the sequestered organic matter; increased the flow of nutrients to photoautrophs in the oceans (favoring high levels of primary production); and reduced the participation of CO₂ in the greenhouse effect, leading to a cooler atmosphere.

Just as the earlier Paleoproterozoic Great Oxidation Event between 2.4 and 2.0 billion years ago, favored the emergence and radiation of the unicellular Eukaryotes, not only capable of O₂-powered metabolism but, in fact, also dependent upon oxygen, the Neoproterozoic Oxygenation Event, more than a billion years later, elevated oxygen levels in the atmosphere and oceans sufficiently to allow heterotrophic Eukaryotes to exploit pluricellular body plans on a macroscopic scale. This was something that putative decimetric Eukaryotic photoautotrophs (Macroalgae) from China had apparently achieved, under less oxygenic conditions, by about 1500 million years ago. They, however, could produce oxygen to their own system through photosynthesis; and their increased size was likely advantageous in terms of photosynthetic area and competition for space on the sea floor. The soft-bodied macroscopic forms that appeared in the Ediacaran much later are interpreted as heterotrophic pluricellular organisms, dependent upon higher levels of ambient oxygen. Initially, these included the enigmatic Vendobionts as well as difficult-to-classify stemgroup Metazoans, but by 560 million years ago Eumetazoans were also included. How these animals lived has not been always clear, and some may even have been sessile osmotrophs, but several Bilaterians are now recognized, such as Kimberella and Dickinsonia. Clearly, from what we know of how Eukaryotes function and reproduce, the Neoproterozoic Oxygenation Event afforded unprecedented opportunities for Eukaryote evolution at macroscopic pluricellular levels and a new level of ecospace opportunities to exploit novel body plans, physiologies, and growth and feeding strategies.

The appearance of multicellular Eukaryotes, even prior to attaining macroscopic size, could have also influenced the atmosphere oxygenation and ventilation of the oceans. For example, the advent of fecal pellets and greater body size in Eukaryotic organisms would have increased sinking rates of organic carbon, thus shortening both residence time in the water column and exposure to microbial decomposers. This would have facilitated carbon incorporation (sequestration) within sediments, thereby favoring oxygen accumulation. By the same token, Neoproterozoic colonisation of the surface of the continents by microorganisms could have promoted CO₂ drawdown from the atmosphere and more efficient weathering of silicates and micronutrient delivery to the oceans. Finally, in the latter half of the Ediacaran, the appearance of infaunal organisms capable of intensely utilising the substrate prompted nothing less than a 'revolution' in redox conditions in near-surface sediments and ecospace exploitation.

Three extreme palaeoclimatic changes, the Sturtian, Marinoan and Gaskiers glacial events, the severest ever registered in the geologic record, also occurred concomitantly with the breakup of Rodinia, Gondwana amalgamation, and early Metazoan evolution. The Snowball Earth hypothesis put forward to explain these events asserts that the Earth was totally covered, or nearly so, by ice, several times between 720 and 580 million years ago. Rocks on all continents record one or more of these glaciations, yet the precise age and temporal equivalence of glacial events have been difficult to be established because their most emblematic sedimentary signature, diamictites, cannot be directly dated, unless intercalated by contemporaneous beds of volcanic materials, which is rare. Much more commonly, maximum depositional ages have been deduced from uranium-lead ages of the youngest detrital zircons within the diamictites, whereas minimum ages have been furnished by lead-lead age-determinations for immediately post-glacial cap carbonates, when present.

Current evidence indicates that the Sturtian event started at about 717 million years ago. The best available ages for Sturtian rocks have been obtained in ash beds intercalated within diamictites from Oman, which yielded uranium-lead zircon ages of 723 million years and 711.5 million years. Ash beds of the Mount Harper Group from Canada, dated by the chemical abrasion, isotope dilution, thermal ionisation mass spectrometer method, yielded a precise age of 716.33 million years. Rhenium-osmium ages of about 659 million years, from the youngest cap carbonates associated with this glaciation suggest that Sturtian glacial events took place over nearly 60 million years, from about 717 to 660 million years ago. Some authors argue that Sturtian glaciation encompasses several shorter glacial episodes. In Brazil, a lead-lead isochron age of 740 million years, obtained on cap carbonates from Sete Lagoas Formation is consistent with their correlation with the Sturtian event.

The best age for the Marinoan glacial event is 636 million years, as indicated by the uranium-lead age of 635.5 million years from zircons from ash beds interlayered in diamictites of the Ghaub Formation, Namibia. A nearly identical age of 636.3 million years was obtained on zircon grains from an ash bed in Nantuo Formation, China. Post-Marinoan cap carbonates have yielded very similar ages, for example a chemical abrasion, isotope dilution, thermal ionisation mass spectrometer age of 635.2 million year was obtained from zircons from an ash bed intercalated within cap carbonates overlying the Nantuo tillite on Yangtze Platform, China. Lead-lead isochrons on post-Marinoan cap carbonates at the base of the Araras Group in Brazil yielded ages of about 633 million years.

Finally, diamictites representing the much more restricted mid-Ediacaran Gaskiers Glaciation in Eastern Canada contain many ash beds in the type section that have been precisely dated within the interval from 581 to 579 millione yeats ago. It is noteworthy that the same region is the site of the classical localities of the soft-bodied Avalon fossil macrobiota.

The extreme climates associated with the initiation, maintenance, and termination of these worldwide glacial events must have had a significant effect on the biosphere, but not much is known about how they provoked or affected subsequent evolutionary changes, either as a bottleneck and/or as a trigger for adaptive innovations. As evident from the fossil record, microscopic Prokaryotic and Eukaryotic lineages, as well as putative macroscopic Algae and possibly Sponges, obviously survived the snowball scenarios of the Cryogenian (720 to 635 million years ago). A pronounced spike in marine phosphorous, an essential nutrient, during this period suggests that massive amounts of nutrients (including phosphorus) may have been delivered to the seas as a product of glacial erosion during deglaciation following individual glacial episodes at this time. Together with concomitant eustatic sea level rise, this certainly must have favored colonisation of benthic and planktonic habitats in the vast shallow seas that spread over low-lying, glacially planed continental margins. Their resultant increase in primary production, oxygenation, and habitat diversification undoubtedly transformed the biosphere and presented new evolutionary options. As a possible example of this, we see relatively complex and diverse Eukaryotic biotas for the first time at Lantian and Weng’an in the interval between the Marinoan and Gaskiers glaciations (635 to 580 million years ago). Moreover, typical, large soft-bodied elements of the Ediacaran macrobiota, the oldest known Vendobionts, date from just after the Gaskiers event at 579 million years ago in rocks from the classical region for that event. Shortly thereafter, Bilaterians, as inferred from trace fossils, appeared between 570 and 566 million years ago and bona fide Bilaterian Metazoan fossils showed up at 558 million years ago.

How and when the Earth moved beyond the ‘boring billion’, in phase with extreme glaciations, increased oxygenation, and ultimately the emergence of animals, remain among the crucial questions in the history of Earth-life co-evolution. In Cordani et al.'s view, 'How' remains an open question currently within the domains of molecular and developmental biology, organic chemistry, and geochemistry, but 'When', on the other hand, is an issue that they address using biological, climatic, and tectonic lines of evidence. Although recognizing the relevance of the oxygen rise in the oceans and climatic extremes in this discussion, they argue specifically that the main trigger for the emergence (and maintenance) of modern ecosystems dominated by macroscopic organisms was tectonic, related to the first appearance of high surface relief involving Himalayantype mega-mountains within the West Gondwana Orogeny.

The Transbrasiliano-Kandi lineament, stretching 5000 km from Brazil to Africa and marking the site of the West Gondwana Orogeny, records the oldest evidence of a continent-continent collision capable of producing very high mountain chains. Such mountains were probably comparable to the present Himalayas and may have been at least as long-lived. The central Tibetan plateau, for example, has maintained elevations greater than 5000 m, with many peaks surpassing 8000 m, since the Eocene. By analogy, then, mega-mountains in Gondwana may also have sustained similar high relief for at least 40 million years after the original collision responsible for their formation.. 

Therefore, if the West Gondwana Orogeny originated near 610 million years ago in association with high Himalayan-like mountains, it could have persisted at least until 580 million years ago. During this time and through the period of erosion until 540 million years ago, as estimated from the age of several post-collisional granitoids, they have most certainly served as a major source of sediments and nutrients for contemporaneous seas. Just as the Tibetan plateau exerts a profound influence on modern climate, sedimentation, tectonics and biology, the same was probably true for the West Gondwana Orogeny during the Ediacaran as well. The chronology of these events corresponds closely with important events in the evolution of macroscopic Eukaryotes.

Several places in the West Gondwana Orogeny expose low or ultra-high pressure metamorphic rocks typical of the diagnostic prototypes that characterise deep continental subduction, such as observed in modern plate tectonics. Ultra-high pressure rocks in Mali dated at about 620 million years ago thus comprise the earliest evidence not only of large-scale deep-continental subduction, but consequently also of Himalayan-type mountains. Hence, the uplift and subsequent erosion of these mountains in the Late Ediacaran provided massive amounts of sediments and nutrients throughout the most important phase in the emergence and early diversification of megascopic Metazoans on Earth.

Thus, Cordani et al. consider that the West Gondwana Orogeny, harboring as it does the oldest evidence of Himalayan-style relief resulting from continent–continent collisions, is one of the features that changed the way evolution proceeded on Earth in the Ediacaran. The distribution and ages of all known localities of known ultra-high pressure metamorphic rocks shows that none of them is older than those of the West Gondwana Orogeny. Ultra-high pressure metamorphic rocks indicating deep subduction occur within collisional belts associated with globally important tectonic cycles throughout the Phanerozoic, as in the Caledonian, Hercynian, and Alpine cycles. The high mountain ranges they represent are, in fact, a characteristic of Phanerozoic times. The implication is that they have furnished nutrients to the seas through weathering and erosion sufficient enough to sustain intense Eukaryotic evolution and permit an increasingly complex exploitation of ecospace within the biosphere throughout the Phanerozoic. 

Cordani et al. expect that similar work on the East African Orogen will add further support for the relevance of Gondwana amalgamation as an important influence upon the acceleration of biological evolution at the Neoproterozoic end. For instance, they expect that investigation of high-pressure metamorphic rocks already known in Tanzania and Mozambique and related to the collisions of India, Madagascar and Australia-Antarctica with the Central African Block, may well confirm the suspicion that mega-mountains were likewise formed in the Southern part of the East African Orogen. It will also be important to test the hypothesis presented by Cordani et al. by means of source-to-sink investigations of delivery rates of nutrients and their bioavailability in basins fed by the erosion of mega-mountains. Regarding the West Gondwana Orogeny, answers to these questions may be found in basins related to the dissection of high mountains associated, for example, with the Voltaian Basin in Ghana or the Parecis Basin in Brazil. As in all of Geology, time will tell.

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