An alternative hypothesis on the origin of Hominins and African Apes.

The ancestors of Humans are commonly thought to have evolved on the savannas of Africa, a theory which was originally proposed by Charles Darwin. This original proposition was based upon the fact that our closest ancestors, the African Apes, lived in the forests of Africa, and that our upright walking behaviour seemed like an adaption to open grasslands, rather than any fossil evidence available at the time. This is a plausible hypothesis, and much subsequent palaeoarchaeological evidence uncovered in Africa has appeared to support it. However, a number of recent discoveries have been hard to reconcile with this scenario, leading to the emergence of alternative ideas on Human origins.

There are a number of things about Human anatomy which appear to be inconsistent with an origin on the African Savannas. We have a furless body, a layer of subcutaneous white fat, feet much flatter than other primates, run slowly, and sweat heavily when hot or exercising, leading to the loss of fluids and minerals (our water consumption needs are high compared to other Primates even when not doing this).

For a long time it was hypothesised that knuckle-walking African forest Apes migrated onto the open grasslands, where they evolved into upright Australophithecenes, and subsequently into Humans. Over time, this theory has been modified as we have come to understand more about the environments favoured by Australopithecenes, they are now seen as inhabitants of mosaic woodlands who moved from a partially bipedal lifestyle to an obligate bipedal one to facilitate crossing more open areas, although why this should be the case remains unclear.

Another puzzle is the apparent absence of fossils ascribed to either the genera Pan (Chimpanzees) or Gorilla, despite the long lineage of fossil Human-ancestors dating back into the Pliocene. The genera Australopithecus, Paranthropus, Sahelanthropus, Orrorin, and Ardipithecus, have all been classified as Hominins, more closely related to modern Humans than either Pan or Gorilla. This is in contradiction to what would be expected, as genetic studies suggest modern Chimpanzee and Gorilla populations have diverged from large ancestral populations, while Humans apparently descend from a lineage with consistently small population sizes and repeated genetic bottlenecks. It has been suggested that this has come about because the ancestors of Chimpanzees and Gorillas lived in acidic forests, where there is lower potential for remains to become fossilised. However, this environment is more favoured by Orangutans, which do have a fossil record, than it is by Chimpanzees, which do not.

'Mrs Ples' a 2.1 million-year-old skull assigned to the species Australopithecus africanus in the collection of the Ditsong National Museum of Natural History. José Braga/Didier Descouens/Wikimedia Commons.

In a review article published in the journal Academia Biology on 10 June 2026, Marc Verhaegen of the Anthropology Study Center in Putte, Belgium, Stephen Munro of the National Museum of Australia, Kathelijne Bonne of GondwanaTalks in Madrid, Spain, Frances Mansfield, an independent researcher from Volos in Greece, and Mario Vaneechoutte of the Faculty of Medicine and Health Sciences at Ghent University, present a new hypothesis on the origins of both Humans and African Apes, in which they argue that the Australopithecenes were not, in fact the ancestors of Humans but rather of modern Gorillas and Chimpanzees. 

Verhaegen et al. not that genomic studies have found that the ancestors of modern Gorillas and Chimpanzees were infected with the Endoretroviruses PtERV1 (CERV1) and PtERV2 (CERV2) between 3 and 4 million years ago (Endoretroviruses are fragments of ancient viral DNA which have been inserted into the hosts genome), but Humans and Asian Apes show no signs of any such infections in their past. Since Humans are not immune to the effects of these Viruses, the most likely explanation is that our ancestors were geographically isolated from the epidemic. It has been suggested that Human ancestors may have moved out of Africa during much of the Miocene Epoch, when the Viruses were circulating, but Verhaegen et al. suggest that a more likely scenario is that they did not arrive in Africa at all until the Early Pleistocene. Few, if any, Animals are thought to have migrated from Africa to Europe during the Pliocene, while a range of European Animals migrated into Africa, driven by the hyper-arid climate that emerged in southern Europe at this time. 

Furthermore, Verhaegen et al. consider that the Australopithecines found in Africa during the Pliocene were not closely related to modern Humans, but rather that their smaller brains, short legs, and long arms indicate that they were related, or even ancestral, to modern African Apes. The evolution of Australopithecenes has often been seen as confusing, as earlier species often have more Human-like traits, seen as 'advanced', while later forms are often more Ape-like, or 'primitive', leading to speculation about undiscovered ghost lineages connecting earlier species to Humans. 

This distinction vanishes if Australopithecines are considered to be the ancestors of African Apes. For example, the Miocene Ardipithecus ramidus, which lived about 4.4 million years ago, had small canine teeth, similar to those seen in modern Humans, whereas the Pliocene Australopithecus africanus had much larger canines, comparable to a modern Ape. Other examples are the Miocene Orrorin tugenensis, which lived about 6 million years ago, had femurs more closely resembling those of Humans that any Pliocene Australopithecene, and the Early Pliocene Australopithecus anamensis, which lived about 4.2 million years ago, had a modified talar trochlea which would have enabled it to swing its leg forward during upright locomotion, something which is absent in subsequent members of the genus. Furthermore, later Austalopithecenes such as the Early Pleistocene Australopithecus sediba, which lived about 1.8 million years ago, appear better adapted to an arboreal lifestyle than earlier members of the group.

A cast of a skull of Ardipithecus ramidus from the Miocene of Ethiopia, in the collection of the Royal Tyrrell Museum. Chris Woodrich/Wikimedia Commons.

Other than an early adaptation to bipedalism, Verhaegen et al. see Australopithecenes as consistently more Ape-like than they are Human-like, in particular noting that Australopithecus afarensis appears more Gorilla-like and Australopithecus africanus appears more Chimpanzee-like.

Verhaegen et al. do not see Homo habilis, the smallest and earliest member of the genus Homo, which appeared in the Early Pleistocene, about 2.58 million years ago, to be a true member of the genus Homo, noting that the species shares a small brain, short leg, and long arm morphology with Australopithecenes and Apes, and in particular that the morphology of the enamel–dentine junction in this species is also more Australopithecene-like. The later, but equally small, Homo naledi is also considered to be an Australopithecene. 

In Verhaegen et al.'s view, the last common ancestor of all Australopithecenes, living Apes, and Humans, would have been an upright Miocene Ape, capable of bipedal locomotion, climbing trees, and wading in shallow water. They describe a scenario in which these Apes led a lifestyle they describe as 'aquaborealism', living in forests which were at least seasonally flooded, with a lifestyle which involved wading in waters on the forest floor, climbing trees in a vertical position, and swinging beneath branches (branchiating). From this ancestral state the Hyobatids (Gibbons and Siamangs) evolved into small, fast branchiating Apes living in the tree canopy, Orangutans into larger, slower, branchiating forms, Gorillas and Chimpanzees (separately) into knuckle-walking forms, and Humans into bipedal walkers.

They give two potential scenarios from which modern African Apes could descend from Australopithecenes; either Chimpanzees descended directly from earlier, more gracile Australopithecenes, such as Australopithecus afarensis and Australopithecus africanus, while Gorillas arose from later, more robust forms such as Paranthropus boisei and Paranthropus robustus, or they went through separate but parallel evolutionary paths, running something like Australopithecus africanus-Paranthropus robustus-Chimpanzees and Australopithecus afarensis-Paranthropus boisei-Gorillas, in response to similar environmental changes.

A skull reconstruction assigned to Paranthropus robustus in the collection of the National Natural History Museum of China. Erlend Bjørtvedt/Wikimedia Commons.

Verhaegen et al. believe that the view of the common ancestor of Apes and Humans as being Ape-like, and Apes therefore as the 'primitive' state, has distorted our view of Hominid evolution for over a century. They argue that while Australopithecenes do have some Human-like traits, these are indicators that Australopithecenes are ancestral to Humans, but rather that they share some traits derived from a mutual common ancestor that frequently adopted an upright bipedal posture as an adaptation to life in flooded forests. 

The analysis presented by Verhaegen et al. suggests two distinct phases of Human evolution associated with wet environments; an initial aquaboreal phase in flooded Miocene forests, followed by a littoral (beach dwelling) phase, which may have involved frequent shallow diving to access food, which probably continued into the Early Pleistocene.

Apes differ from Old World Monkeys in a number of ways, the most obvious of which is their much larger size. This appears likely to have been a trait found in the last common ancestor of all Apes; even the relatively small Hylobatids are thought to have evolved from a larger ancestor, due to their long gestation period, which is unusual in such a small Primate, and is thought to be a hangover from a larger ancestor. Another notable trait is the absence of a tail in Apes. This has become much reduced, and forms a part of the 'pelvic cup'; a modification of the pelvis which helps to support the intestines when in an upright position. There is no comparable tail loss in any other Primate to which this can be compared, but the idea that this was an adaptation to an upright posture while engaged in vertical climbing and branchiating seems reasonable. Furthermore, if these ancestral Apes were spending a lot of time in wading in water, then a tail might have been disadvantageous, prone to heat loss, adding to the friction of the Apes when moving through the water, and prone to infections or attacks by predators. 

Verhaegen et al. note that quadruped Animals returning to the water tend to evolve in one of two ways. Those that use spinal flexation as the main means of propulsion and the tail as the driving organ, such as Whales and Sirenians, tend to lose their hind limbs, whereas those that use their limbs for propulsion, such as Seals, Bears, Penguins, and Hippos, tend to lose their tails. An exception can be seen in Animals such as Otters and Beavers, which use their tails for propulsion but make extensive use of their limbs for foraging and movement on land. Apes, which use their limbs to swim when they enter the water, have lost their tails.

The lumbar spine of Apes is stiffer and further from their dorsal surface than is the case in most Mammals, and their forelimbs (arms) are notably long, adaptations which seems favourable to climbing in an upright position and hanging below branches. They also have wide hips compared to other Primates, and a flatter pelvis, facilitating lateral leg movements, as well as a broader sternum and thorax, which pushes their scapulas into a more dorsal position, facilitating lateral and upward arm movements. 

Fossils of Miocene Apes are often found in what have been interpreted as warm, wet, forest environments, which has led to the suggestion that they may have been aquaborreal in nature, spending their time wading through flooded forests or climbing in the branches above with their arms. Such behaviour is known in extant Apes, for example Gorillas have been observed entering forest swamps to forage for Sedges, Bonobos will wade through water to obtain Waterlilies, and Orangutans have been observed wading in shallow water in Borneo.

Bipedal Gorilla, wading. Female Gorilla with a walking stick, while crossing bipedally an Elephant pool at Mbeli Bai, Republic of Congo. Thomas Breuer & Emma Stokes in Verhaegen et al. (2011).

Verhaegen et al. propose that all modern Apes derived from this initial aquaboreal Miocene Ape, with Gibbons adopting a fast branchiating motion, Orangutans a slower branchiating movement combined with knuckle walking, Gorillas and Chimpanzees separately developing a knuckle walking gait, and the ancestors of Humans going through a distinct litoral (coast dwelling) phase.

A number of lines of evidence have pointed towards Early Pleistocene members of the genus Homo may have engaged in regular wading, swimming, and even diving. Homo erectus has been observed to have a pachyosteoscletotic skeleton (i.e. unusually dense bones), something associated with slow-moving, shallow-diving Tetrapods such as Sireneans, as well as the earliest Whales and Seals. The dense skeleton can help such Animals maintain their position in the water, particularly in saltwater environments (where the body is more buoyant). The occipital bones of Homo erectus are roughly twice as thick as those of comparably sized Apes, making the interpretation of the species as a swift bipedal predator chasing down prey hard to sustain. 

Furthermore, the low positioning of the braincase, receding forehead, absence of a chin, and forward projecting face of Homo eructus appear to be an adaptation to frequent shallow water diving, and possibly floating on their backs. The forward pointing face and paranasal sinuses of Homo eructus may indicate a habit of surfacing nose first, with the nostrils above water and the heavy occipital area at the back of the head beneath the water, something which would have worked well in a back-floating position. The basicranial flexation of modern Humans holds our face in a ventral, forward facing position. In contrast, Homo erectus would have tended to look upwards, in what would be a forward position when swimming or diving (Neanderthals are somewhere between these positions). This has been suggested to be an adaptation to foraging in coastal waters, where the easy availability of coastal food sources would compensate for the lack of stability associated with a bipedal gait on land.

Several skulls of Homo erectus have been shown to have bony growths in the inner ear called aural exotoses (or surfer's ear) which is caused by chronic exposure to cold water. This has also been observed in about half of all Neanderthals. Verhaegen et al. observe that this directly contradicts the frequent claim that there is no direct palaeontological support for the coastal Ape hypothesis.

At the same time, Human-ancestors underwent both an increase in both overall body size, and relative brain size. This is a common adaptation to moving from a terrestrial to an aquatic lifestyle, seen for example in Whales and Dolphins, but is not typical when non-aquatic Mammals increase in size; for example, the largest Apes, Gorillas, do not have relatively large brains. Homo erectus underwent a significant increase in brain size, with later specimens had a brain twice as large as that of an equivalent-sized Ape. This may have benefited from the higher proportion of nutrients such as docosahexaenoic acid, folic acid, selenium, taurine, and iodine, in aquatic-derived foods, all of which are needed for brain-growth. Such disproportionate brain growth is not seen in terrestrial carnivores, and therefore seems unlikely in a Hominin chasing prey in open grassland. Notably, Australopithicenes never showed any notable increase in brain size over their two-million-year history. Verhaegen et al. take this as evidence that early Homo did not evolve from such ancestors on the African grasslands, since there seems no good reason that our brains should have started to grow rapidly while remaining in the same environment.

There is also considerable evidence that Homo erectus did consume aquatic foods. Fossils of Homo erectus from Mojokerto on Java were found in association with numerous marine Bivalve shells, while those at Trinil on the same island, were found with the shells of  freshwater Bivalves, such as Pseudodon and Elongaria, including specimens engraved with geometric patterns. Furthermore, palaeoarchaeological remains from the Koobi Fora Formation in the Turkana Basin of northern Kenya have yielded stone tools from a Oldowan technology alongside the remains of aquatic Animals including Fish, Turtles, and even Crocodiles. Early Homo specimens from a variety of locations have dental wear associated with grit and the oral processing of marine Molluscs. Archaic members of the genus Homo are known to have reached Sulawesi by one million years ago, despite this island never being connected to the Eurasian mainland, suggesting an early ability to cross open water.

A Pseudodon shell with geometric engravings from the Trinil archaeological site in East Java, dated to about 500 000 years ago. The shell was excavated by Dutch palaeoarchaeologist Eugène Dubois in the 1880s and taken to the Netherlands. It was returned to Indonesia by the Naturalis Biodiversity Center in 2025. Naturalis Biodiversity Center/Wikimedia Commons.

The genus Homo first appeared around the beginning of the Pleistocene, with the first evidence of the consumption of aquatic foodstuffs appearing about two million years ago, across the tropical and temperate regions of the Old World. This has led to the suggestion that Homo was able to spread across this area rapidly by following coasts and rivers, wading and diving for food. There is also increasing evidence for the consumption of aquatic foods by Neanderthals across their range, as well as Modern Humans across the world, and from their earliest appearance. 

Humans differentiate from the standard morphology and physiology of Primates, and indeed terrestrial Mammals in general, in several ways. We have fleshy outward lips, a small oral opening, a closed tooth row, a more globular tongue, a descended larynx, and an external nose that still today some individuals can partially close, using nasal muscles. We also have a distinct 'Cupid's bow' to our upper lips (technically the philtrum) to our upper lips, another feature which some modern individuals can use to close their nasal airway. All of these are adaptations which enable us to regulate breathing and seal our airways better, and which may have been more pronounced in Homo erectus.

Feeding on shellfish would also have required adaptations which required improved abilities to control the sucking and swallowing of food, particularly if this was done in the water. Adaptations which improved fine control over the lips, jaw, tongue, glottis, and larynx, would also have acted as pre-adaptations for the later evolution of speech. Hunting for foodstuffs underwater, particularly if we were doing this blind (by touch alone) is also likely to have improved both our manual dexterity and the sense of touch in our fingers, something also seen in Raccoons and Otters.

In Humans, the connective tissues, tendons, ligaments, and muscles of the human foot are aligned with the hallux to form a longitudinal arch. This is unlike the foot of any other Primate, and is a good adaption to walking, and acts as a shock-absorber when running. However, it is also a particularly good adaptation to swimming, particularly as our feet are relatively larger and more paddle-shaped, and makes us particularly sure-footed on wet or infirm terrain, where other Mammals often struggle. Humans are able to swim using axial undulation, a coordinated wave of motion from trunk to hips to legs, something other Primates are unable to do. Our feet can be seen as a trade-off between something useful for swimming and something useful on land. Notably, we are less efficient runners than most other Mammals, which directly contradicts the idea that we evolved to chase large prey across grasslands.

Notably, Humans have a layer of white fat beneath our skins (the adipose layer) considerably thicker than found in any other Primate, as well as an overall fat content which is also distinctively high. In a healthy male Human, between 12% and 23% of the body mass is fat, while in a female it is between 25% and 35%. In Chimpanzees and Bonobos body fat typically makes up less than 5% of the body mass of males, and less than 8% of the bodymass of females. Thus even the leanest of Humans have a significantly higher proportion of fat than healthy Chimpanzees. This suggests that Humans have undergone a significant ecological shift after our ancestors split from those of Chimpanzees, and one which decoupled the fat content of our bodies from the its use as an energy store. Such a change is again typical of aquatic and semi-aquatic Mammals, where fat has an important role both as an insulator and as a buoyancy aid. Such a layer provides no particular role during activities such as sustained running, and may be disadvantageous, as it can hamper the dissipation of heat.

Verhaegen et al. believe that plate tectonics played an important role in the evolution of Hominoids, Hominids, and Hominins, and in particular the formation of the Red Sea and the African Rift. Until about 30 million years ago (Early Oligocene), Africa and Arabia were a single island continent, separated from Eurasia by the Tethys Ocean. Over time, this continent drifted northward, the Tethys Ocean narrowed, and a series of islands and archipelagos formed between the two continents.

Between about 30 million years ago and about 20 million years ago (during the Oligocene and Early Miocene), an episode of plume volcanism beneath the Afar Triangle led to uplift, basalt volcanism, and the formation of a triple junction rift system. The three branches of this rift would go on to form the Gulf of Aden to the east, the Red Sea to the northwest, and the Ethiopian Rift to the south. At this time Eurasia and Africa-Arabia were still separated, although the Mesopotamian Seaway, which separated Arabia from what would become Mesopotamia and Persia, was becoming increasingly narrow.

Between about 20 million years ago and about 14 million years ago (Early-Middle Miocene) the Mesopotamian Seaway slowly closed, leading to the formation of a connection called the Gomphotherium Landbridge (Gomphotherium being a type of early Elephant that migrated out of Africa across this landbridge), leading to faunal exchange between Africa-Arabia to the south and Eurasia to the north. forming the first wave of the 'Great Old World Biotic Interchange', while the Mediterranean Sea and Indian Ocean became isolated from one-another. This also led to uplift and mountain formation along the Bitlis-Zagros Suture Zone, and the development of the Dead Sea Transform and Aqaba faults in response to the added tectonic stress. At this time the Red Sea had opened, and was connected to the Mediterranean by by the Gulf of Suez, but a land bridge at Bab-al-Mandeb, connecting Arabia to Africa, separated it from the Gulf of Aden.

Between about 14 million years ago and about six million years ago (Late Miocene), movement on the Dead Sea Transform Fault, combined with the mass of sediments around the Nile Delta, closed off the connection between the Red Sea and the Mediterranean at the Gulf of Suez. Following this, the Red Sea underwent a desiccation crisis, drying up and leaving vast salt deposits, albeit with occasional marine incursions.

Between about 5.9 and 5.33 million years ago (latest Miocene), the Gulf of Gibraltar also closed, cutting off the inflow of water from the Atlantic to the Mediterranean, and triggering the Messinian Salinity Crisis, in which the Mediterranean largely dried up, leaving a vast and inhospitable salt plain. 

After about 5.33 million years ago (Pliocene), the straits of Bab-al-Mandeb opened up allowing the Red Sea to flood from the Gulf of Aden, and the Strait of Gibraltar reopened, allowing the Mediterranean to refill from the Atlantic (the Zanclean Megaflood). Around this time the Gulf of Suez reconnected to the Mediterranean, cutting off Africa from the Red Sea until the beginning of the Pleistocene, when a land bridge formed across the Sinai again.

Simplified tectonic and palaeogeographic evolution of the Mediterranean Sea and the Arabian Peninsula, with key marine connections between Africa, Arabia, and Eurasia (details of European and Paratethys geographies not given). (1) Before 30 million years ago (until the Rupelian, Oligocene); (2) 30–20 million years ago (Rupelian–Burdigalian); (3) 20–14 million years ago (Burdigalian–Langhian); (4) 14–6 million years ago; (5) From about 5.9 to 5.33 million years ago: the Strait of Gibraltar closed due to plate tectonics; (6) From 5.33 million years ago onwards. Red circle in (2): Afar plume basalt eruptions; brown line in (3): uplifting mountain front at the Bitlis-Zagros Suture Zone; red line in (3) and (4): active Dead Sea Transform Fault; pale orange in (4) and (5): desiccated marine domains; blue arrows in (6): marine gateways and direction of filling of basins. Verhaegen et al. (2026).

Verhaegen et al. suggest two alternative scenarios for the emergence of the first Apes. One scenario sees a group of Early Miocene Primates living in coastal forests and islands along the Mesopotamian Seaway, which gave rise to both the Apes and the Old World Monkeys. The other sees these ancestors living in coastal forests in East Africa, which migrated northward along the Red Sea Rift as it opened, migrating into the Arabian Peninsula and then eventually into Eurasia. This latter scenario is supported by the presence of the possible Ape Morotopithecus in Uganda about 20.6 million years ago, although the exact status of this fossil is unclear.

In either scenario, Early Apes increased in size rapidly after splitting from the Old World Monkeys, at the same time developing a very broad and strong sternum in a broad thorax, a somewhat shorter lumbar spine with only five lower and more centrally placed lumbar vertebrae (indicative of a vertical body posture), an enlarged sacrum equipped with a coccyx in a broad pelvis (cup form, supporting the intestines), external tail loss, relatively longer legs (indicative of wading), and longer arms (indicative of below-branch hanging). These early Apes are thought to have lived in (probably coastal) swamp forests, hanging beneath branches, practising aquarbourism, and developing to an upright bipedal stance. 

These early Apes probably spread along the northern coast of the Tethys Ocean, with different groups splitting off and going their own way, beginning with the Gibbons. Between 22.4 and 16 million years ago the ancestors of the Orangutans had split from those of the African Apes, moving eastward towards the coastal forests of Southeast Asia, while the proto-African Apes colonised forests along the Western Tethys (what would become the Mediterranean). 

There is a surprising absence of Ape or Hominid fossils from the Middle Miocene of Africa (between about 13 and 10 million years ago). At the same time, Eurasia has an abundance of both, particularly in southern Europe and Anatolia, with many forms looking like plausible ancestors for African Apes. Verhaegen et al. cite this as support for the idea that the ancestors of modern Humans and African Apes were not in Africa during this time, but instead in southwestern Eurasia. At this time much of southern Europe was covered by  inland seas, mega-lakes, swamps, and coastal forests, and home to Dryopithecine Apes such as Dryopithecus, Pierolapithecus, Danuvius, and Rudapithecus, which are potentially ancestral to modern Gorillas, Chimpanzees, and Humans.

A partial skull of Pierolapithecus catalaunicus, a Dryopithecine Ape from the Middle Miocene of Spain. Institut Català de Paleontologia Miquel Crusafont.

During the Vallesian Crisis (between about 11.6 and 8.7 million years ago) the climate of southern Europe became much drier and the extensive forests shrank, being replaced by open grasslands. Many of the Apes there died out, while surviving forms, such as Ouranopithecus and Graecopithecus adapted to the new environment by becoming more bipedal, and foraging in mixed woodlands and river valleys. These Apes could potentially be ancestral to later Australopithecenes and African Ape (one Turkish Ape from this period, Anadoluvius turkae, has been suggested as an offshoot from the line which led to Gorillas).

The scenario envisaged by Verhaegen et al. has a group of Dryopithecine Apes taking advantage of the closure of the Mesopotamian Sea, and migrating to the swamp forests surrounding the early Red Sea. The ancestors of Orangutans must have split from these western Apes before 14 million years ago, when the Badenian Transgression would have blocked migration between Europe and East Asia. The lineage which led to Gorillas probably split off next, migrating from Europe, where taxa such as Ouranopithecus remained, while closer ancestors such as Anadoluvius turkae migrated through Anatolia, and on to Africa, where they gave rise to Gorilla-like African Apes such as Chororapithecus and Sahelanthropus, then eventually the East African Australopithecenes and modern Gorillas.

At the same time, Graecopithecus and similar species around the Mediterranean could provide plausible ancestors for Chimpanzees and Humans, explaining the Hominid-like footprints seen at Trachilos on Crete about six millions of years ago (long before the earliest such footprints in Africa). The lineage which led to Humans and Chimpanzees must also have migrated southward, at latest during the onset of the Messinian Salinity Crisis about 5.9 million years ago, which would have made the Mediterranean Basin uninhabitable. The route from Europe into Africa across the Sinai Peninsula would have been cut off during the Zanclean Megaflood (5.33 million years ago), which filled the Mediterranean and over-spilled into the Red Sea, filling that too. The final connection between Arabia and Africa was lost about 5 million years ago, when the Bab-el-Mandeb Strait opened, connecting the Red Sea to the Gulf of Aden.

Verhaegen et al. suggest that some of these Apes crossed into Africa before 5 million years ago, giving rise to the Southern African Australopithecines and eventually modern Chimpanzees. Another group remained on the southern shore of Arabia, where they were forced to turn increasingly to the littoral environment for survival, as the land became increasingly arid and hostile.

Around 2.8 million years ago the Earth's climate cooled sharply, leading sealevels to drop abruptly. At this time early Homo migrated from Arabia into Africa and Eurasia, leading to the sudden appearance of Homo erectus across the Old World. 

A reconstruction of the skull of Homo erectus in the collection of the University of Michigan Museum of Natural History. Thomas Roche/Wikimedia Commons.

Under this scenario, the genus Homo is absent from Africa until the Early Pleistocene appearance of Homo erectus/Homo eregastor, with the Pliocene Australopithecines not being ancestral to Modern Humans, but instead relatives of Chimpanzees and Gorillas. This view is broadly in-line with several recent cladistic analyses of Human origins, which suggest African Apes and Humans are derived from Eurasian Apes, and that there were multiple crossings from Eurasia into Africa. 

Verhaegen et al. take the view that East and Southern African Australopitecines largely evolved in parallel, in response to similar ecological pressures. Thus the gracile forms, Australopithecus afarensis and Australopithecus africanus, appeared in the Pliocene, when they were able to live in forest, and in particular swamp forest, environments, whereas the more robust Early Pleistocene forms, Paranthropus boisei and Paranthropus robustus, evolved in response to cooler, drier conditions, although generally sticking close to large bodies of water, such as Papyrus swamps. Eventually the modern African Apes adopted to a life on dry forest floors.

It is possible that the Southern African Australopithecines were more omnivorous, whereas the East African forms adapted to the processing of tougher plant materials, something which is reflected in the diets of modern Chimpanzees and Gorillas. 

This later arrival of Human-ancestors into Africa provides an explanation for the absence of any trace of the Endoretroviruses PtERV1 (CERV1) and PtERV2 (CERV2) in our genomes, while the ancestors of Gorillas and Chimpanzees were affected. The potential littoral phase also explains the many morphological adaptations which set Humans apart from other Apes, as well as behavioural traits such as a fondness for water and sea coasts. 

Verhaegen et al. believe that the earliest members of the genus Homo were shelfish divers, something which would have provided them with the resources for accelerated brain growth, as well as adaptions such as an external nose and pachyosteosclerotic skeleton. As supporting evidence for this, they cite the frequent occurrence of ear exostoses (surfer's ear) in early Homo, as well as tooth wear associated with a shellfish diet, the early arrival of Homo on remote islands such as Flores and Sulawesi, and the co-occurrence of their fossils and tools with shells, and even engraved shells.

See also...

Duplexium jatobensis & Anhapoa munizi: Freshwater Mussels from the Early Cretaceous of northeastern Brazil.

Freshwater Mussels, Unionida, are the only order of Bivalves found exclusively in freshwater. There are about 900 species today, found on every continent, with some species considered economically significant, due to their use as food, their ability to produce pearls, their ability to modify environments, or their impact upon Fish stocks. Unionids have a unique life cycle among Bivalves, with a larval stage which lives commensually or parasitically on the skin, fins, or gills of a Fish host.

In a paper published in the Journal of South American Earth Sciences on 20 May 2025, Débora Eliza Baumann of the Laboratório de Paleontologia de Macroinvertebrados at São Paulo State University, Luiz Ricardo Simone of the Laboratório de Malacologia at the Museu de Zoologia of the Universidade de São Paulo, Rafael Costa da Silva of the Museu de Ciéncias da Terra of the Serviço Geológico do Brasil, and Renato Pirani Ghilardi, also of the Laboratório de Paleontologia de Macroinvertebrados at São Paulo State University, describe two new species of Unionid Freshwater Mussels from the Early Cretaceous Salvador Formation of Pernambuco State, Brazil.

The Salvador Formation outcrops in the Jatobá Basin of Pernambuco State, and records a fan delta laid down within a palaeolake in the Late Berriasian age (roughly between 140 and 137 million years ago). Both new species are described from specimens collected at a site designated BL-42 within the city of Floresta, and held within the collection of the Museu de Ciéncias da Terra of the Serviço Geológico do Brasil.

Location of Jatoba Basin (08◦30′ to 9◦06′ S; 37◦06′ to 38◦30′ W) in Northeast Brazil, location where the fossils are collected (8◦41′48.60″S 38◦16′34.30″W) and stratigraphy of the Jatoba Basin. Baumann et al. (2025).

The first new species is named Duplexium jatobensis, where 'Duplexium' refers to the fact that it has two types of teeth on its hinge, and 'jatobensis' means 'from Jatobá' in reference to the Jatobá Basin where it was discovered. The species is described from four specimens, which are between 47 and 50 mm in length, 18-19 mm in width, and 6-8 mm in thickness, with three teeth on the hinge, the central one of which is distinctly larger than the other two.

Holotype, MCT.I.7202. Duplexium jatobensis; Salvador Formation (Berriasian), general morphology and muscle scars. Abbreviations: aa, anterior adductor; pa, posterior adductor; pl, pallial line; ht, heterodont teeth; tt, taxodont teeth; pp, pedal protractor; apr, anterior pedal retractor; pe, pedal elevator; S1, posterior pedal retractor? S2, posterior adductor? or S1 + S2, slow and fast components of the posterior adductor? Scale bar is 10 mm. Baumann et al. (2025).

The second new species is named Anhapoa munizzi, where 'Anhapoa' derives from 'Anhapoā', which means 'canine tooth' in the Tupi-Guarani indigenous language of Brazil, in reference to a well-pronounced cardinal tooth on the hinge of the right valve of the shell. Anhapoa munizzi is described from five specimens, which range from 43 to 52 mm in length, 23 to 27 mm in width, and from 8 to 14 mm in thickness. The hinge of the right valve has a large cardinal tooth between two pits, and small lateral teeth.

Holotype, MCT.I.7147, Anhapoa munizi, Salvador Formation (Berriasian), general morphology and muscle scars. Abbreviations: aa, anterior adductor; pa, posterior adductor; pl, pallial line; ht, heterodont teeth; tt, taxodont teeth; pp, pedal protractor; apr, anterior pedal retractor; pe, pedal elevator, S1, posterior pedal retractor? S2, posterior adductor? or S1 + S2, slow and fast components of the posterior adductor? Scale bar is 10 mm. Baumann et al. (2025).

Both Duplexium jatobensis and Anhapoa munizi lack any form of ornamentation, have smooth beak, lack a prodissoconch, have a heterodont hinge and an additional taxodont hinge, have an elevator muscle scar, and lack a pallial sinus, all traits consistent with assignment to the Family Iridinidae within the Superfamily Etherioidea, one of the two major subdivisions of the Unionida.

The oldest members of the Family Iridinidae described to date come from the Middle Cretaceous of Africa, with some possible Iridinid fossils from the End Cretaceous of South America. Modern Iridinids are restricted to Africa, leading malacologists toconclude that the genus originated there, after the continents of Africa and South America split apart, between 140 and 120 million years ago. The Salvador Formation of the Jatobá Basin, however, was laid down during the earliest part of this rifting process, when Africa and South America were still joined, raising the possibility that the Iridinidae originated in South America, but died out there during the End Cretaceous Extinction.

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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.

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