Understanding the climate of southeast Australia during the Eemian Interglacial.

The Eemian or peak of the Last Interglacial (Marine Isotope Stage 5e, roughly 129 000–116 000 years Before Present) is the most recent geologic period when global temperatures were similar to present, but in response to orbital forcing rather than greenhouse gas loading of the atmosphere. While this makes the Eemian an imperfect analogue for near-future climate due to anthropogenic global warming, the latitudinal temperature distribution was similar to the present. Eemian global mean temperature was 0-2°C warmer than present, mean global sea surface temperatures were indistinguishable from current sea surface temperatures, though sea level was around 6–9 m higher from meltwater inflows from the Greenland and Antarctic ice sheets. Therefore, understanding the climate of the Eemian may provide valuable insight to future climate and its variability.

While often thought of as a period of relative climate stability, climate variability during the Eemian was likely greater than in the Holocene. This has been attributed to meltwater outflows disturbing the Atlantic meridional overturning circulation and a general global cooling trend toward glacial inception. The resulting changes in sea surface salinity and temperature are believed to have driven regional changes in atmospheric circulation leading to periods of widespread aridity across Europe, and onset of abrupt cold periods as sea surface temperatures cooled by several degrees Celsius.

Our understanding of Southern Hemisphere Eemian climate is limited as proxy records are rare. The climate of Australia is globally relevant, as it responds to key global teleconnection forcing, including El Niño Southern Oscillation, Pacific Decadal Oscillation, and the Indian Ocean Dipole. It is also relevant as the severity of recent droughts and bushfires have been linked to increasing temperatures caused by anthropogenic greenhouse gas emissions, thereby stimulating interest in understanding past warm climate conditions such as during Marine Isotope Stage 5e.

Numerical modelling studies suggest reduced seasonal contrast in Marine Isotope Stage 5e temperatures across Australia with warmer winter temperatures, most notably in the north and northwest of the continent by +3°C to +5°C. Austral summer (December–February) temperatures are believed to have been cooler by −1°C to −2°C with modelling studies suggesting that the Australian monsoon was weaker and drier than present with January-April precipitation anomalies of more than − 3 mm per day. A weaker Marine Isotope Stage 5e monsoon aligns with evidence of a variable Lake Eyre hydrology with temporally isolated small inflow events These were likely caused by irregular southward penetration of the Intertropical Convergence Zone and decaying tropical cyclone(s) over the headwaters of the Lake Eyre Basin; conditions similar to those of today that deliver inflows to Lake Eyre.

Winter temperatures in southern Australia during Marine Isotope Stage 5e are believed to have been warmer than present, reflecting warmer sea surface temperatures in the Southern Ocean. This would have reduced the meridional temperature gradient between the sub-tropics and mid/high latitudes with less frequent precipitation bearing cold fronts and extratropical depressions at a time concurrent with strengthening of interannual variability of sea surface temperatures in the eastern tropical Pacific associated with El Niño Southern Oscillation. Slightly cooler Marine Isotope Stage 5e sea surface temperatures to the northwest of Australia relative to present may have contributed further to reduced rainfall over southeast Australia similar to the impact of present day positive Indian Ocean Dipole events. Combined with changes to Atlantic meridional overturning circulation and associated sea surface temperatures, which have been shown under present day conditions to teleconnect to the Pacific Ocean, the climate of Marine Isotope Stage 5e in Australia is likely to have been variable, but in general drier than present. In particular, in southeast Australia where under the current climate positive Indian Ocean Dipole and El Niño Southern Oscillation result in reduced rainfall. However, a dearth of high temporal resolution paleoclimate records from Marine Isotope Stage 5e have until present not been available to test this thesis and the possible links to climate state forcings such as teleconnections and solar variability.

In a paper published in the journal Scientific Reports on 22 October 2020, Hamish McGowan of the Atmospheric Observations Research Group at the University of Queensland, Micheline Campbell and John Nikolaus Callow of the School of Agriculture and Environment at the University of Western Australia, Andrew Lowry, also of the Atmospheric Observations Research Group at the University of Queensland, and Henri Wong of the Australian Nuclear Science and Technology Organisation, present a near annual resolution reconstruction of climate developed from a speleothem that spans the Eemian (Marine Isotope Stage 5e) from 117 500 to 123 500 years before present, the most recent period in the Earth’s history when temperatures were similar to those of today.

In Australia, speleothems offer the greatest potential to develop high temporal resolution terrestrial palaeoenvironmental records. Stalagmite GC001 was removed from a small chamber approximately 60 m into the Grotto Cave, Yarrangobilly Caves, New South Wales, Australia in 2012. The caves are located at the northern end of the Snowy Mountains, at an elevation where the water balance changes from an energy (demand) to supply (precipitation) limited system. The caves were formed through karstification of the Yarrangobilly Limestone, a massive Silurian limestone formation with an extent of about 1.4 km by 14 km and a maximum depth of roughly 450 m. Past work has concluded that reduced rainfall invokes prior calcite precipitation at Yarrangobilly and hence up (down) trend in magnesium/calcium ratios implies increased (decreased) drip water contact time under drier (wetter) conditions.

 

Location map of the Yarrangobilly Caves (a) and aerial oblique perspective of the entrance to the Grotto Cave (b) 300 mm long cross-section of stalagmite GC001 from which samples were extracted for uranium series dating and geochemical analysis (c). The small core perpendicular to the growth axis is the result of in-situ sampling of GC001 for age determination prior to removal from the Grotto Cave (c). McGowan et al. (2020).

The present day climate of the Yarrangobilly area is influenced by the northern extension of the mid-latitude westerly winds. It experiences cool to cold montane temperatures through winter, while in summer warm to hot and dry conditions dominate under the influence of the subtropical ridge. Major Southern Hemisphere ocean–atmosphere teleconnections including the El Niño Southern Oscillation, Southern Annular Mode, Pacific Decadal Oscillation, and Indian Ocean Dipole all affect the regional climate state. Rain bearing weather systems that affect Yarrangobilly Caves may originate from the tropics and the southern mid-latitudes. Mean annual precipitation is approximately 1147 mm, with a mean annual temperature of 12°C. The most effective precipitation occurs during the cool, wet winter and while snow may fall, the site is below the seasonal snowline and no snowpack remains through winter. Temperature logging from where GC001 was collected in the Grotto Cave found mean internal cave temperature was 8.8°C during the study (August 2014 to February 2015).

The magnesium time series for GC001 shows clear cycles of higher/lower concentrations with a marked increase occurring around 120 800 years before present. This period of elevated magnesium concentrations prevailed until around 118 500 years before present. It then remained mostly stable until a sharp decrease at approximately 117 850 years before present. This period of higher magnesium concentrations (drier) in GC001 aligns with a period of increased depletion of the proportion of oxygen¹⁸ in a speleothem from northern Borneo indicating wetter conditions there, indicative of a more northerly position of the Intertropical Convergence Zone. It also correlates with periods of increased dust deposition recorded at Dome C, Antarctica. Australia is a known source of interglacial dust to the Antarctic. Accordingly, McGowan et al. infer drier conditions in southeast Australia as indicated by higher magnesium concentrations from 120 800 to 118 500 years before present in GC001 sustained regional mega-drought(s) across this period with associated wind erosion contributing to increased dust flux at Dome C, Antarctica such as between 119 050 to 120 300 years before presnt. Slightly higher dust flux values from 117 500 to 117 800 years before presnt, may have also originated from southeast Australia but do not correlate with elevated magnesium concentrations in GC001, or any notable signal in the proportion of oxygen¹⁸ record from northern Borneo.

 

Time series of magnesium, strontium and barium concentrations from GC001 (a)–(c); proportion of oxygen¹⁸ from a speleothem collected at Whiterock Cave, northern Borneo (d); dust concentrations from Dome C, Antarctic (e), and temperature departure from the average of the past 1000 years Dome C, Antarctic (f). Pink shading indicates periods of intermediate magnesium concentrations and dust flux (a), (e); yellow shading highest magnesium concentrations and dust flux (a), (e) and blue shading (d) corresponding period of depleted proportion of oxygen¹⁸ indicating wetter conditions in Western Pacific. Vertical dashed blue lines constrain the period interpreted as a mega-drought(s). McGown et al. (2020).

The strontium time series displays some periods of higher/lower concentrations that are concurrent with variability in the magnesium record, such as around 121 000 years before present. However, unlike the magnesium record, the strontium time series exhibits an overall trend of decreasing concentrations with the period between approximately 120 800 years to 118 500 years before present, remaining relatively constant at approximately 25 parts per million, before decreasing more quickly. This is similar to the barium concentrations time series which is strongly correlated with strontium reflecting a reduced growth rate (less drip water) under the drier conditions known to have minimal impact on magnesium concentrations. Barium concentrations do display a more consistent decreasing trend that flattens between 120 800 and 119 100 years before present, before decreasing further after 118 500 years before present.The decreasing trends in the strontium and barium concentrations correlate with the Marine Isotope Stage 5e temperature anomaly reconstruction for Dome C, Antarctic and Marine Isotope Stage 5e sea surface temperatures reconstructions from northwest of Australia and east of New Zealand. Accordingly, the magnesium, strontium and barium concentrations in GC001 collectively record the onset of a prolonged drier period from around 120 750 years before present to 118 500 years before present that was associated with increased atmospheric dust and a cooling in atmospheric and ocean temperatures from Marine Isotope Stage 5e maximums.

Normalised Lomb-Scargle periodograms for magnesium, strontium and barium were calculated to test for the influence of teleconnection and/or solar cycle forcing on the Marine Isotope Stage 5e climate of southeast Australia. The periodograms show common peaks in spectral density around 200 years which align with the de Vries solar cycle (about 205 years). Spectral peaks are also found from 1147 years (magnesium) to 1268 years (strontium) and are within the range of the roughly 1000 year Eddy solar cycle. The strontium and barium records have peaks at about 66.8 and 88.6 years, which match with the Pacific Decadal Oscillation and Gleissberg solar cycle respectively and display coherent spectral peaks at 301.3 years, 454.8 years, 602.7 years and at 3443.8 years. The 301.3 years and 454.8 years cycles are close to the 300 year and 470 year cycles found in sediments from Jeju Island, South Korea. These have been attributed to solar forcing along with the 3443.8 year cycle, which is aligned with the 3300 year Holocene cycles in the proportion of oxygen¹⁸ record from Greenland Ice Sheet Project 2 core. The magnesium record displays a strong 709 year cycle that may be a harmonic of the 1400 year cycle found in glacial and interglacial periods in magnesium/calcium derived sea surface temperatures records from the South China Sea. Accordingly, the geochemistry of GC001 displays cyclic variability indicative of climate variability forced by both teleconnections and solar cycles recorded globally in geologic archives.

 

Periodograms of magnesium, strontium and barium with 0.9 (red dashed line) and 1.0 probability thresholds shown. Selected dominant periods (annotated) are rounded to the nearest decade. McGowan et al. (2020).

Wavelet transforms for magnesium, strontium and barium show regions of significant periodicity (95% confidence level). All three elements display significant 4 to 8 year cycles that McGowan et al. interpret as El Niño Southern Oscillation and around 20 to 30 years, and 50 to 65 years, which are indicative of the Pacific Decadal Oscillation. While caution is required in attributing shorter-duration cycles given laser ablation sampling resolution, the magnesium record shows clear gaps in El Niño Southern Oscillation (4–8 year) cycles from approximately 121 500 years before present to around 120 500 years before present. These coincide with a change in magnesium concentrations from about 140 to about 200 parts per million indicating a transition to drier conditions, while the period from 118 500 to 117 500 years before present overlaps with the return to magnesium concentrations, about 140 parts per million suggesting increased moisture availability. Pacific Decadal Oscillation like cycles are most common from around 119 800 years before present to 118 500 years before present.

 

Wavelet plots of magnesium (a), strontium (b), and barium (c). The thick black lines represent 95% confidence levels. The lightly shaded region toward the bottom is the Cone of Influence and values in the light region should be considered with caution. McGowan et al. (2020).

In the strontium wavelet transform diagram, the El Niño Southern Oscillation signals are most frequent from the start of the record to about 122 100 years before present and are concurrent with the occurrence of the multi-decadal Pacific Decadal Oscillation like cycle. This longer decadal cyclicity is also evident from 121 500 years before present to 118 500 years before present with evidence of a separate 20 to 30 year cycle and longer 55 to 75 year cycle. The barium wavelet plot shows also a dominant short duration El Niño Southern Oscillation signal in the early part of the record that progressively becomes less frequent. The Pacific Decadal Oscillation period cycle is not strong and only occurs above the 95% confidence level occasionally from 123 500 years before present to around 122 200 years before present and again from around 121 000 years before present to 119 500 years before present.

Understanding the causes and frequency of climate variability is essential to inform prediction of future climate. Proxy of climate variability dating from Marine Isotope Stage 5e, the most recent warm period with temperatures similar to the present, offer potential to develop this understanding. The magnesium Marine Isotope Stage 5e record from stalagmite GC001 shows multi-centennial periods of increased Mg concentration indicating drier climatic conditions in southeast Australia, notably from 118 500 to 120 750 years before present. This period coincides with increased dust flux recorded in Antarctic ice. The strontium and barium concentrations show evidence also of this climate variability with trends of decreasing concentrations correlated with cooling air temperatures and sea surface temperatures. Collectively, the magnesium, strontium and barium records from GC001 with supporting regional paleoclimate records suggest that southeast Australia during Marine Isotope Stage 5e experienced multi-centennial periods of reduced precipitation, temperature variability and increased atmospheric dust. These conditions such as from 118 900 to 120 400 years before present are indicative of mega-droughts. Superimposed on these multi-centennial periods of drier climatic conditions are higher frequency interannual to interdecadal teleconnections and solar forced cycles of further variability.

A drier climate during Marine Isotope Stage 5e such as from 118 900 to 120 400 years before present would align with results from numerical modelling which found a generally weaker summer monsoon and warmer winter conditions. A weaker monsoon aided by cooler sea surface temperatures northwest of Australia at this time would lessen moisture transport into southeast Australia via meridional conveyors such as northwest cloud bands, while warmer temperatures during winter would have increased evaporation, intensifying dry conditions that spanned centuries as evident in our record. Numerical modelling of future climate at +1.5°C indicates that the Australian monsoon may weaken, resulting in an increase in the frequency of dry days. Accordingly, the convergence of our paleoclimate record from stalagmite GC001 and published numerical modelling strongly suggest that the climate of southeast Australia will likely become drier throughout the twenty-first century with increased risk of multi-centennial duration dry periods widely referred to as mega-droughts.

The magnesium, strontium and barium records from stalagmite GC001 all display cycles with periodicity of well documented solar and teleconnection forcings of climate. Dominant solar cycles including the Gleissberg solar cycle (88 years), de Vries solar cycle (about 205 years) and possible Eddy solar cycle (about 1000 years) are evident in the geochemistry of GC001. Increases (and decreases) in total solar irradiance through such cycles result in direct heating (and cooling) of the troposphere and Earth’s surface. Indirectly, changes in total solar irradiance influence galactic cosmic ray flux affecting cloud microphysics and cloud cover. Variation in total solar irradiance also causes change in ultraviolet radiation flux, which for a 0.1% increase in total solar irradiance, increases by 4 to 8%. Concurrently this will increase ozone production in the mid and upper stratosphere through the photolysis of oxygen. Corresponding increases in ultraviolet absorption by stratospheric ozone leads to heating and change in the thermal stratification of the atmosphere.

Numerical modelling has shown that enhanced ultraviolet heating of the atmosphere corresponding to total solar irradiance maxima results in a stratospheric zonal wind anomaly that is most pronounced in the Southern Hemisphere during mid to late winter. Dynamical links between the stratosphere and troposphere caused by such ultraviolet forcing lead to change in atmospheric circulation, including jet-stream behaviour, and therefore tropospheric synoptic circulation and ocean dynamics. McGowan et al. therefore postulate that periods of increased total solar irradiance contribute to drier conditions (increased magnesium in GC001) over southeast Australia in response to increased zonal (westerly) flow and a corresponding more southern track of mid-latitude winter storm systems. This occurs in response to a stronger thermal wind component between the tropics/subtropics and the mid to high latitudes causing a response in atmospheric circulation similar to a more positive Southern Annular Mode. The associated decline in precipitation is recorded clearly in stalagmite GC001 by elevated magnesium concentrations and stable to slight decreases in strontium and barium concentrations due to reduced speleothem growth, i.e. reduced drip rate during Marine Isotope Stage 5e.

GC001 records 4 to 8 years El Niño Southern Oscillation-like cycles along with multi-decadal cyclicity indicating Pacific Decadal Oscillation type influences, two teleconnections which have been shown to have the greatest impact on the modern climate and in particular rainfall in southeast Australia. It is therefore reasonable to suggest that these Pacific Ocean teleconnections have been robust and long-lived drivers of interannual and multi-decadal hydroclimate variability in southeast Australia with warm/cool El Niño Southern Oscillation and Pacific Decadal Oscillation phases causing dry/wet periods.

Evidence of El Niño Southern Oscillation cycles in GC001 become no longer statistically significant from around 118 200 yrs before present. This is about 500 years before rapid decline in magnesium concentrations, which suggests reduction in drip water residence times and onset of a wetter climate. This change in magnesium and to a lesser amount in strontium and barium correspond to a rapid increase in sea level of around 5 to 6 m at 118 100 years before present caused by ice sheet melt. McGowan et al. suggest that this dramatic rise in sea level possibly disrupted El Niño Southern Oscillation and Pacific Decadal Oscillation teleconnections and their impact on the climate of southeast Australia at this time.

Collectively, the Marine Isotope Stage 5e climate record developed from GC001 indicates that southeast Australia will likely continue to experience interannual to interdecadal wet–dry cycles driven by teleconnections and solar variability at least until current global warming exceeds Eemian temperatures, possibly within the next decade. However, McGowan et al.'s record also shows that in a warm interglacial climate such as today or near future, there is risk of multi-centennial periods of less effective precipitation (mega-droughts), initiated by natural variability. Should such prolonged periods of drier conditions occur again, then they may be reinforced by anthropogenic global warming, thereby increasing their severity. As a result, McGowan et al. stress the need for further research into the Eemian climate of Australia and the Southern Hemisphere to resolve the causes of prolonged dry periods during Marine Isotope Stage 5e and to determine their spatial impacts. This will allow new insights to our future climate and the risks it may bring such as drought and associated bushfires.

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Investigating the origins of sedentism and plant cultivation in northeast China.

The transition to a sedentary way of life and the domestication of plants is arguably the most significant ‘revolution’ in human history. This transition gradually resulted in a dramatic increase of population size and density, in craft specialisation and the division of labour and the initiation of social dynamics and the accumulation of resources that are linked to the development of socio-political stratification. The processes, mechanisms and possible drivers of a transition from one subsistence mode to another have long been at the forefront of anthropological and archaeological research. Three hypotheses have been proposed to explain such transitions. The first suggests that stress, induced either by deteriorating environmental conditions (the ‘oasis hypothesis’) or by population increase led to an overexploitation of the environment and forced innovation. A current example is the optimal foraging theory and the diet breadth model, which predict that a transition to a delayed returns system (i.e. long-term tending resources) will occur during times of scarce resources, promoting a border dietary range. The second hypothesis argues that transitions occur at times of enhanced resources. A current example is the cultural niche construction theory, which claims that organisms shape their environment during stable or affluent times, these allow experimentation with locally available resources, which improve crop yield and eventually lead to domestication. The third hypothesis argues that transitions occur as a result of socioeconomic competition and are not affected by external forcings.

In a paper published in the journal PLoS One on 18 July 2019, Gideon Shelach-Lavi of the Department of Asian Studies at The Hebrew University of Jerusalem, and the Center for Frontier Archaeology at Jilin University, Mingyu Teng, also of the Center for Frontier Archaeology at Jilin University, Yonaton Goldsmith of the Department of Geological and Planetary Sciences at the California Institute of Technology, and the Institute of Earth Sciences at The Hebrew University of Jerusalem, Ido Wachtel of the Department of Archaeology at The Hebrew University of Jerusalem, Chris Stevens of the Institute of Archaeology at University College London, Ofer Marder of the Department of Bible Studies at Ben-Gurion University, Xiongfei Wan of the Institute of Archaeology and Cultural Relics, Xiaohong Wu of the School of Archaeology and Museology at Peking University, Dongdong Tu, again of the Department of Asian Studies ar The Hebrew University of Jerusalem, Roi Shavit, also of the Department of Bible Studies at Ben-Gurion University, Pratigya Polissar of the Lamont Doherty Earth Observatory, Hai Xu of the Institute of Surface-Earth System Science at Tianjin University, and Dorian Fuller, also of the Institute of Archaeology at University College London, present evidence for the timing, duration and environmental conditions of the onset of sedentism and plant cultivation in northern China, the second oldest center of domestication in the Old World.

Shelach-Levi et al. address the first two hypotheses by examining whether the onset of sedentism and cultivation occurred at a time of stress or affluent conditions and whether these environmental conditions were stable or fluctuating. They assume that the limiting environmental parameter is rainfall and thus we focus on the timing of rainfall changes and human settlement patterns at the onset of sedentism and cultivation in north China. They acknowledge the possibility that temperature might have also played a role in restricting Millet growth, however, as the magnitude, timing and seasonal structure of temperature change in north China in uncertain, they focus on rainfall. The combination of high-resolution archaeological and palaeoclimatic data enable to test the theoretical frameworks of the onset of sedentism, domestication and population increase.

North China is one of a few centres in the world where complex agricultural systems emerged independently. The native staple grains are two types of Millet (Foxtail and Broomcorn), however, the date, process and the palaeoclimatic context of their domestication are debated. Shelach-Levi et al. focus on northeast China because currently the earliest undisputed examples of domesticated Millet grains and the most extensive evidence for early Millet consumption have been found in this region, at sites of the Xinglongwa culture.

Early research in north China began in the 1970s and focused on the middle reaches of the Yellow River. It identified remains of early Neolithic sedentary societies of the 7th and 6th millennia BC, which were sorted into a series of ‘cultures’, most notably Cishan and Peiligang. These findings focused attention to this area – which classical Chinese tradition identifies as ‘the cradle of Chinese civilization’, the birthplace of agriculture from whence it spread to other regions in north and central China. By the late 1980s, it became clear that contemporaneous sedentary societies also existed in other regions of north China, including the Houli culture in the lower Yellow River area and the Xinglongwa culture in Northeast China. In recent years even earlier phases of the 'Neolithic' period were identified in the different sub-regions of North China, such as the Xiaohexi phase in the northeast, but those are not well studied and are poorly dated. While substantial archaeological knowledge of sedentary prehistoric societies has been presented, the earlier phases that preceded the sedentary cultures and the transition process from mobile to sedentary societies remains mostly obscure. To address these developments and their context during the important phase of the transition to agriculture, Shelach-Levi et al. integrate novel data from the excavations of two early Neolithic sites, the results of two high-resolution archaeological surveys in northeast China and novel palaeoclimatic data.

Over the past 20 years, four high-resolution, detailed, large-scale regional archaeological surveys have been conducted in northeast China. Shelach-Levi et al. focus on the surveys conducted around the modern cities of Chifeng, in Inner Mongolia, and Fuxin, in Liaoning Province, because these are the only surveys that recovered evidence of the earlier phases of sedentism. These surveys were designed to systematically recover data that enable population estimates and the reconstruction of local socio-political trajectories. The full area was systematically surveyed on foot (at a 50 and 20 m resolution), the locations and densities of pottery shards and stone tools were mapped, counted and statistically analyzed. A total pottery amount was calculated for each period by integrating the area of the occupied territory, the density of artifacts and the length of each period. These results provide a direct reconstruction of population trajectory in these two regions and present a qualitative demographic history of northeast China. The earliest sites found in both regions are those attributed to the Xiaohexi phase and the Xinglongwa culture, which represents the earliest phase of sedentism in this region. The combined results of the two surveys demonstrate a rapid increase in population and the complexity of village society following the initial stage of sedentism.

Map of the survey areas and the palaeoclimate records discussed. Shelach-Levi et al. (2019).

To evaluate the timing and socio-economic processes involved in the transition to sedentism, Shelach-Levi et al. excavated two single-occupation sites discovered during our survey in the Fuxin area. Those sites capture the onset and earliest phases of sedentism.

At the earlier site, 12D56 (also named Jiajiagou west site 贾家沟西遗址), the excavation exposed a single-occupation, irregular-oval structure of ~3.5 m in diameter, which contained ash, pottery and stone tools. Based on seven ¹⁴C ages, the site was occupied between 7900 and 7750 years ago (5900-5750 BC). This is the first time that a Xiaohexi phase site is radiocarbon dated. Previous research, based on ceramic typology, suggest dates in the 7th and even 8th millennium BC, but according to Shelach-Levi et al.'s findings the date of this phase cannot be earlier than 6,000 BC. The shape of the structure we excavated is also different from the supposedly rectangular structures reported for some Xiaohexi sites. At some of those sites the rectangular structures may belong to a later Xinglongwa occupation and in others the report is not detailed enough to examine the shape of the structures. Thus, it is possible that in other sites as well, structures were oval or that both oval and rectangular shape structure existed during this period.

The 12D56 site. Plans of the excavation site showing the structures excavated (solid lines are places where the perimeter of the structure was excavated, dashed lines represent extrapolation of the structure parameter and are provided as context reference, shading is pottery density (shard/m²). Shelach-Levi et al. (2019).

About three hundred potshards were excavated from this site. There are are crude, soft and crumbly, similar in style and quality to those known from other Xiaohexi sites. Most shards are unadorned, but a few pots are decorated with appliqué and narrow bands of diagonal incisions. 330 stone artifacts were also recovered, including ground and chipped stone tools. Of the ground stone tools, almost 80% are spades. Although no use-wear analysis was done, the fact that those artifacts, which are not known from earlier periods in the region, are so dominate suggest that they are associated with new set of activities, probably related to the clearance of woods and the cultivation of the land Similar artifacts are known form other site dated to the Xiaohexi phase.

Artifacts excavated at site 12D56. Stone artifacts (upper half) and pot shards (lower half). Shelach-Levi et al. (2019).

Floral remains from the site consist of a predominance of wild fruits and nuts and a small amount of wild Panicum Millet (Panicum miliaceum ruderale). The two seeds that were recovered are small in size and are representative of what is expected of grains from early cultivated plants in the initial process of domestication.

At the later site, 12D16 (also named Tachiyingzi site 塔尺营子遗址), Shelach-Levi et al. excavated an area of ~40 m² out of a much larger site. Based on ten ¹⁴C ages, the site was occupied between 7.5–7.4  thousand years ago (5500-5400 BC). The area excavated contained a rectangular structure measuring about 7.5 × 6.5 m and remains of at least one additional structure. The shape of the structure is identical to those known from the nearby site of Chahi (查海), where a large area of the site was exposed revealing a densely occupied Xinglongwa period village.

The 12D16 site. Plans of the excavation site showing the structures excavated (solid lines are places where the perimeter of the structure was excavated, dashed lines represent extrapolation of the structure parameter and are provided as context reference, shading is pottery density (shard/m²). Shelach-Levi et al. (2019).

The structure in this area was severely burned during or after its abandonment and remains found on its floor indicate that it was supported and covered with large oak beams. On its floor Shelach-Levi et al. recovered 22 complete ceramic vessels of typical Xinglongwa culture. The quality of ceramic production is high and most are highly decorated with incised motifs. The stone artifacts are more abundant and diverse than those of site 12D56 (1003 such artifacts were found at site 12D16), and show evidence for highly standardised pressure bladelets produced on-site, as a household routine. The ground stone assemblage consists of grinding stones, polished axes and adzes, spades, pounders and hammer-stones.

Artifacts excavated at site 12D16. Stone artifacts (upper half) and ceramic vessels (lower half). Shelach-Levi et al. (2019).

Floral remains include a few millet seeds but are predominantly composed of wild fruits and nuts. Sixteen millet grains are identified as Common Millet (Panicum miliaceum). Their size is small, on average 1.25–1.70 mm in length and 1.05–1.40 mm in width, closer to the wild type Panicum (Panicum miliaceum ruderale) than to modern domesticated Broomcorn. These grains seem to represent plants that are not yet fully domesticated, but represent an early phase in a sequence of size change expected to take 2–3 thousand years.

Remains of Common Millet (Panicum miliaceum) from site 12D16. Shelach-Levi et al. (2019).

To evaluate northeast China’s palaeo-hydroclimate during the transition to domestication and sedentism, Shelach-Levi et al. integrated data from Dali lake-level to provide an estimate of rainfall change, and Lianhua Cave oxygen isotopes to provide continuous, high-resolution absolute dating. In addition, they analysed the isotopic contents of soil alkanes from both archaeological sites, which primarily records the isotopic composition of rainfall. Alkanes are organic compounds produced in leaf waxes incorporating oxygen and hydrogen derived from water. These can be preserved in in soils and give a record of the ratios of different isotopes of the elements within the local water when the plant was growing. When air masses hit areas of high ground and are uplifted, they cool and preferentially precipitate moisture with the heavier isotopes of oxygen and hydrogen, thus the ratio of these isotopes in the preserved alkanes in the soil gives a measure of how high up and how far from the source water the soil was when it formed. The palaeoclimate records show similar hydroclimate patterns. Between 9000 and 8100 years ago (7000-6100 BC) relatively dry conditions prevailed in northeast China. Between 8100 and 7900 years ago (6100-5900 BC) a sharp substantial increase in rainfall amount occurred. The Lianhua Cave record shows a 2.5‰ (2.5 parts per thousand) isotope change at 8100 years ago (6100 BC), which is only comparable in magnitude to the transition from the Younger Dryas to the Early Holocene at 11 500 years ago (9500 BC). The soil alkanes are more depleted in heavy isotopes at the later 12D16 site than the the earlier 12D56 site and is thus in agreement with the other records. The relatively wet conditions prevailed for  about 400 years. A decrease in rainfall amount occurred between 7550 and 7400 years ago (5550-5400 BC). This transition appears in both records, though the timing of the rainfall decline differs by about 150 years.

The onset of sedentism, cultivation and the early phase of domestication in northeast China is an important test case for studying the theoretical frameworks for the transition to agriculture. The transition to sedentism in the Xiaohexi phase is defined by investment in permanent structures, unknown in this region in prior periods, the dramatic increase in the production of pottery found at all sites in relatively large quantities, and the presence of large grinding stones. The empirical coincidence of the onset of sedentism and cultivation about 7900 years ago (5900 BC) with the transition to a significantly wetter climate between 8100 and 7900 years ago (6100-5900 BC) suggests that affluent conditions played an important role in facilitating the onset of these processes in northeast China. Shelach-Levi et al.'s results suggest that in northeast China, fundamental and multi-scale changes occurred not due to necessity or stress but rather under stress-free affluent conditions and thus support the affluency hypothesis. Such plentiful resources allowed Human communities to settle down in one place and enhance their interaction with their immediate environment, without the need to migrate in search of food.

The Xinglongwa culture thrived throughout this affluent period and declined within a century from the end of the wetter condition. This strengthens the notion that affluent climatic conditions were an important factor in the ability of this culture to thrive.

Sedentism and plant cultivation initiated simultaneously, but sedentism and the development of a complex village society matured much faster. The initial phase of sedentism, in the Xiaohexi phase is typified by small sites with makeshift irregular-shaped huts, which suggest low investment in construction and perhaps short life-use. This phase rapidly transitioned into the full-fledged Xinglongwa culture in which large scale-villages containing up to 40 domestic structures, represent an increase in community size that coincided with a rapid increase of regional population levels. Rectangular domestic structures, like the one excavated at site 12D16, and community-wide projects, such as the ditch that surrounded the Chahai site, suggest substantial investment, and communal construction efforts.

Simultaneous processes, such as the intensification of craft production and improvement of ceramic and stone tool technologies, suggest a development of craft specialisation and economic intensification. Similar developments are known from other parts of north China, suggesting that the processes described by Shelach-Levi et al. were shared by many contemporaneous societies.

The process of plant domestication progressed at a much slower pace. Elsewhere, researchers argue for an earlier date of Millet domestication and use in other regions of North China. However, because they use different types of data it is difficult to compare their results to those of Shelach-Levi et al. According to their study, evidence for its initiation (i.e. collection of plants that later will be domesticated and probably the cultivation of wild plants) is found at site 12D56, even after 500 years it was not yet completed at site 12D16. Moreover, the percentage of domesticated foods seems to have remained limited even during the height of the Xinglongwa period. Shelach-Levi et al.'s results disagree with earlier studies that found Human bone carbon isotopes from the Xinglongwa period suggestive of a grain (possibly Millet) dominated diet. Future research will be needed to resolve these contradictions. Recent evidence from other parts of North China suggest that like the process Shelach-Levi et al. describe for the Fuxin area, there too the trajectory of domestication and transition to agriculture was relatively long. In the initial phases, addressed by Shelach-Levi et al., it was not domestication per-se which was important but the cultivation of the land (indicated by the large number of spades found at site 12D56) and the harvesting and consumption of durable food resources such as seeds (of domesticated and wild plants), nuts (such as Acorn and Walnut) and wild fruits (Apricot, Amur Cork). Those resources were collected in large quantities, stored for long periods, and thus enable a year-long occupation of the same site and the stable support of larger communities. Based on the data they collected, Shelach-Levi et al. suggest that the suite of traits that found in the earliest phase sedentism, expansion into new types of food resources and perhaps cultivation, were the driving force behind domestication and not vice-versa.

The archaeological, botanical and palaeoclimatic data presented above help clarify the context of the transition to a sedentary way of life, plant cultivation and initial stages of plant domestication in Northeast China. The multi-dimensional nature of Shelach-Levi et al.'s data allow them to suggest that these processes occurred during a period of comparative affluence that enabled population increase and experimentation with new resources and technologies. Our botanical findings support the view that the domestication process took a long time after Humans started cultivating plants, as has been demonstrated for several other crops in other regions, and that not all the plants that were initially cultivated or intensively collected were botanically transformed and became domesticated. These observations, which may not be true for all cases of independent transitions to sedentary agricultural societies, are crucial for a global view of the evolution of Human society. They suggest that at least in some cases, fundamental and multi-scale changes occurred not due to necessity or under stress but rather because of opportunities to experiment and expand under stress-free affluent conditions.

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Giraffe tracks from the Late Pleistocene of Western Cape Province, South Africa.

Giraffes, Giraffa camelopardalis, are iconic African Mammals associated with rich savanna grasslands. Because of their highly distinctive morphology and specific environmental requirements, these animals have the potential to serve as good environmental indicators in the fossil record. Unfortunately, dead animals exposed in the open in savanna environments are unlikely to enter the fossil record, as skeletons are rapidly destroyed by a combination of scavenging and weathering, with almost all fossils found in these environments coming from rock shelters, enclosed spaces where Humans, Hyenas or other predators took prey animals to prevent them being stolen. However the large size of Giraffes means that few predators will attempt to tackle them, generally only Human populations with advanced hunting technologies, and that even when killed these animals are almost impossible to transport to a rock shelter for safe consumption.

In a paper published in the South African Journal of Science on 30 January 2018, Charles Helm of the Peace Region Palaeontology Research Centre and the Centre for Coastal Palaeoscience at Nelson Mandela University, Hayley Cawthra, also of the Centre for Coastal Palaeoscience at Nelson Mandela University, and of Geophysics Competency at the Council for Geoscience, Richard Cowling and Jan De Vynck, again of the Centre for Coastal Palaeoscience at Nelson Mandela University, Curtis Marean, once again of the Centre for Coastal Palaeoscience at Nelson Mandela University, and of the Institute of Human Origins at the Arizona State University Richard McCrea also of the Peace Region Palaeontology Research Centre, and Renee Rust of the Evolutionary Studies Institute at the University of the Witwatersrand, describe a series of preserved Giraffe footprints from a coastal cliff east of Still Bay, in Western Cape Province.

The footprints are preserved in a detached block of aeolian sandstone (i.e. sands that were laid down by the action of the wind rather than water) roughly 3.9 by 7.9 m, which is estimated to be between 133 100 and 114 500 years old, and probably between 126 000 and 121 000 years old. A total of twelve tracks are preserved, with the best preserved tracks being found at the southern end of the block. The block is located in the uppermost part of the tidal zone, making it vulnerable to erosion both by wave action, and by Nodilittorina africana knysnaensis, a coastal Gastropod found in the area which burrows into soft rocks. The slab is believed to have become exposed at some time since 2011, when the area was last surveyed, and is in danger of sliding further into the sea, or being damaged irreparably by the action of the sea and Gastropods. However it is two large to be removed from the location where it is currently perched, making long term preservation impossible. Helm et al.  suggest that instead it may be possible to make moulds of the tracks.

Giraffe tracks on rock surface perched above the Indian Ocean. Helm et al. (2018).

The large size and distinctive nature of the tracks makes it almost certain that the track-maker was a Giraffe, though Helm et al. did consider the possibility of other track-makers. Southern Africa is home to a number of Bovid species, which are often hard to identify from their tracks, so that such markings are generally referred to simply as 'large Bovid', 'medium Bovid' or 'small Bovid', however even the tracks of even the largest of these animals are difficult to mistake for those of the much larger Giraffe. 

One possible extinct track maker is the Sivathere, or Short-necked Giraffe, Sivatherium maurusium, a large animal closely related to the Giraffe, and therefore presumably capable of making similar sized tracks (no tracks directly attributed to a Sivathere have ever been found). However Helm et al. reject the idea of a Sivathere as the track-maker for a number of reasons. Firstly, while the exact date of extinction of the Sivathere is unclear, the youngest known fossils of the species date to about 400 000 years ago, making it likely that they had vanished by the time the Still Bay tracks were laid down. Secondly, all known Sivathere remains are found much further to the north, with the most southerly specimens known coming from around Bloemfontein, making it unlikely that one made tracks at Still Bay (although Giraffes are not known from much further south). Finally, while similar in general proportion to Giraffes, Sivatheres were much more robust, had had a wider gait, which means that their tracks were unlikely to have closely resembled those of Giraffes.

Detail of four Giraffe tracks. The best preserved adult track is left of centre, and the best preserved purported juvenile track appears below it, above the top right corner of the 10 cm scale bar. Helm et al. (2018).

Giraffes are known in Western Cape Province, but are not usually found south of the Orange River, with the most southerly recorded sighting being made at Spoeg River in Namaqualand, by Danish explorer Pieter Van Meerhoff in 1663; this is about 700 km to the north of Still Bay, and in a completely different climatic zone, dominated by Vachellia karroo woodlands (thornscrub) on deep sandy soils. 

Representations of Giraffes have been found in rock art paintings in the Hex River Valley, about 250 km to the northwest of Still Bay, which indicates that the animals were familiar to the people there about 2000-1600 years ago when these images were made. However the rock art makers are believed to have been nomadic in nature, ranging over a wide area of the Western Cape in response to the changing seasons, and there is no reason to believe that they actually observed Giraffes in the Hex River Valley. 

There are also some fossil Giraffe remains in the South African Museum in Cape Town which were originally identified as having come from Darling’, about 350 km to the northwest of Still Bay and 450 km south of Namaqualand, but precise details of how and where these specimens were obtained is lacking, and they are now thought unlikely to have come originally from Darling'.

Tracings of giraffe rock art from Buffelshoekkloof, Kanetvlei, Hex River Valley. Helm et al. (2018).

The Still Bay area today has a wet temperate climate and coastal fynboss vegetation dominated by Proteas and Wild Olives, quite unsuitable for Giraffes. However this climate is known to have varied considerably during the Pleistocene, as has the position of the coastline in the area, which would have retreated away from the area during periods of glacial maximum, including the period when the tracks were laid down. 

Glacial periods are generally associated with cooler, drier climates, even in areas unaffected directly by glaciation. However the Late Pleistocene fauna of the Palaeo-Agulhas Plain, as represented by fossil assemblages from Nelson Bay Cave and Klasies River, includes many species found further north in the Cape today, such as Grysbok, Raphicerus melanotis, Grey Duiker, Sylvicapra grimmia, and Bushbuck, Tragelaphus sylvaticus (these assemblages contain predominantly small animals as they are primarily Human hunting remains), suggestive of a warmer climate, possibly with a seasonal climate with large grazers visiting the area during a local wet season, presumably driven more by the loss of a maritime influence on the climate than by global cooling. 

The presence of Giraffes in this environment adds further support to the idea of a subtropical grassland biome extending southwards into an area with a temperate climate today during periods of glacial cooling, as these animals are found exclusively in subtropical grasslands in Southern Africa today.

(a) Locality of the study area in the context of the south coast of South Africa. The continental shelf which borders this coastline reaches the broadest extent in South Africa, and the 120 m isobath lies up to 100 km offshore. The relatively shallow and low-gradient Agulhas Bank sweeps from Cape Agulhas in the southwest to Robberg in the east. (b) The dominant geological units at the study site, which are formations of the Bredasdorp Group and were deposited in the Neogene and Pleistocene. Helm et al. (2018).

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Could there have been life on ancient Venus?

The discovery of extensive sedimentary structures on Mars has led to considerable speculation about the possibility of ancient oceans and even life on that planet, despite considerable doubts about the possibility of a planet as small as Mars ever being able to retain a dense enough atmosphere to maintain a climate stable over long periods of time, and models of Solar evolution that suggest all the planets in the Solar System are likely to have received far less heat from the Sun billions of years ago than is the case today. However there is another rocky planet in the Solar System which is closer in size to Earth and therefore able to retain a denser atmosphere, and which, being closer to the Sun would be less negatively effected by lower Solar radiation emissions in the past. Venus today is covered by a thick and inhospitable atmosphere, the result of a runaway greenhouse effect, but a past Venus with a cooler climate could potentially have hosted extensive liquid water and potentially even life.

In a paper published on the arXiv database at Cornell University Library on 2 August 2016 and submitted for publication in the journal Geophysical Research Letters, Michael Way of the NASA Goddard Institute for Space Studies and the Department of Astronomy & Space Physics at Uppsala University, Anthony Del Genio and Nancy Kiang, also of the NASA Goddard Institute for Space Studies, Linda Sohl, again of the NASA Goddard Institute for Space Studies and of the Center for Climate Systems Research at Columbia University, David Grinspoon of the Planetary Science Institute, Igor Aleinov, also of the NASA Goddard Institute for Space Studies and of the Center for Climate Systems Research at Columbia University, Maxwell Kelley, once again of the NASA Goddard Institute for Space Studies and Thomas Clune of the Global Modeling and Assimilation Office at the NASA Goddard Space Flight Center, discuss the possibility of a habitable climate on early Venus, and present the results of a series of climatic models based upon known and plausible parameters for the planet's geography and atmosphere.

Modern Venus has a far denser atmosphere than the Earth, with more than twice as much nitrogen and many times as much carbon dioxide, a powerful greenhouse gas that contributes to the searing temperatures on the surface of the planet. However there is no reason to believe that the planet has always had an atmosphere as dense at this.

Venus also rotates extremely slowly and in a retrograde direction (i.e. the planet spins in the opposite direction to its orbit and the other planets of the Solar System), completing one rotation every 243 Earth Days; longer than the 225 Earth day Venusian year. This leads to a Venusian day (i.e. the time a point on the surface of Venus takes to complete one rotation relative to the Sun) of 116 Earth days. It has long been thought that this slow rotation speed has been brought about by breaking caused by atmospheric tides, suggesting that a younger Venus with a less dense atmosphere may have had a longer rotation period. However recent models have suggested that even an atmospheric density similar to that of Earth would have had a considerable breaking effect on Venus.

The nature of the topography of ancient Venus is unknown; the entire surface if the planet having been reworked by a period of intense vulcanism hundreds of millions (but not billions) of years ago. The surface of the planet is divided into areas of highlands and lowlands that superficially resemble the continents and ocean basins of Earth, however the surface of Venus is much flatter than Earth and an ocean of equivalent volume to that of the Earth's would lead to only 10% of the surface of Venus being exposed above the sea.

Way et al. constructed four models of the climate of ancient Venus, each starting with a Nitrogen atmosphere as dense as the (nitrogen plus oxygen) atmosphere of Earth today, plus carbon dioxide and methane levels equivalent to that of the modern Earth, and an ocean averaging 310 m in depth across the planet (i.e. some areas much deeper but some exposed). None of the models started out with any ice caps at the poles or on mountains, but these were allowed to evolve.

The first model was given a topography similar to that of modern Venus and was assumed to be receiving levels of Solar radiation equivalent to those predicted for Venus 2.9 billion years ago (about 40% higher than that recieved by Earth today), plus a rotation rate equivalent to that of modern Venus. This resulted in the surface of the planet evolving an average surface temperature of 11°C, with temperatures ranging from -22°C to 36°C. This is a much narrower range of temperatures than experienced on modern Earth, largely due to an absence of polar ice packs and more efficient heat distribution in the atmosphere of the slowly rotating planet.

 Surface temperature time average (over 1/6th of a Venusian solar day) of the surface air temperature (SAT) for the paleo-Venus world with modern day rotation rate and a solar spectrum estimated for 2.9Gya. Way et al. (2016).

This world developed a thick cloud cover on its Sunward side, at times approaching 100% cloud cover. Updraught in the tropics led to the formation of a shield of cirrus anvil clouds similar to that seen on the Intertropical Convergence Zone on Earth, but much more persistent, due to the slower rotation of Venus. The model planet was able to radiate considerable heat into space from its night side, unlike modern Venus where a thick carbon dioxide atmosphere creates a runaway greenhouse effect, leading to sufficient cooling on the night side of the planet for snowfall to occur. Most of this snow would be expected to disappear as the surface moves back into daylight, but a permanent ice-cap evolved on the highlands of the Ishtar Terra (which is located close to the northern pole), with permanent snow as deep as 5 m being found above in areas above 5000 m.

 High level clouds (white) for the paleo-Venus world with modern day rotation rate and a solar spectrum estimated for 2.9Gya. Way et al. (2016).

Very high levels of rainfall were observed in the Venusian tropics, with rainfall levels always exceeding 8 mm/day within 20° of the equator, and in places reaching tens of mm/day, wetter than anything seen on Earth. This led to a prediction of very high levels of erosion and potential development of sedimentary structures in the tropical highland areas of Aphrodite Terra and Beta Regio, deposits that could potentially be detected by future missions to Venus, even space-based missions reliant on remote sensing.

Surface level winds for the paleo-Venus world with modern day rotation rate and a solar spectrum estimated for 2.9Gya. Way et al. (2016).

The second model also adopted the topography and rotation rate of modern Venus, but was given levels of Solar radiation equivalent to that expected to have been received by Venus about 715 million years ago (equivalent to the Early Cryogenian on Earth, around the time when the first simple animals were beginning to appear); about 70% higher than that received by the Earth today and only 6% lower than that received by Venus today. This world developed an average surface temperature of 15°C, with surface temperatures ranging from -17°C to 35°C; slightly warmer than the previous model, but with less difference between the extremes and still quite hospitable by modern Earth standards.

Surface temperature time average (over 1/6th of a Venusian solar day) of the surface air temperature (SAT) for the paleo-Venus world with modern day rotation rate and a solar spectrum estimated for 0.715 Gya. Way et al. (2016).

This model also developed an significant ice cap on Ishtar Terra, and a wet climate likely to lead to the evolution of high erosion and the development of sedimentary structures in Aphrodite Terra and Beta Regio.

The third simulation retained the solar radiation for Venus 2.9 billion years ago and the rotation rate of modern Venus, but was given an Earth-like topography. This model evolved an average surface temperature of 23°C and a range of surface temperatures from -13°C to 46°C, considerably warmer than either of the previous models.

Surface temperature time average (over 1/6th of a Venusian solar day) of the surface air temperature (SAT) for the paleo-Venus world with modern day rotation rate and a solar spectrum estimated for 2.9Gya, but using a modern day Earth topography. Way et al. (2016).

This planet has considerably more water present in the tropics, leading to a higher rate of evaporation, and a significant water vapour driven greenhouse effect (though not as high as the carbon dioxide driven greenhouse effect on modern Venus). Nevertheless this planet is still cool enough for snow to fall and settle on its night side, though in this case all of the snow would be expected to be lost again as the surface moves back into sunlight.

The final simulation retained to topography of modern Venus and Solar radiation levels expected on the planet 2.9 billion years ago, but sped up the rotation of the planet till a Venusian day was only 16 times as long as an Earth day. This led to a considerably warmer planet with an average surface temperature of 56°C, and temperatures ranging from 27°C to 84°C in the surface.

Surface temperature time average (over 1/68h of a Venusian solar day) of the surface air temperature (SAT) for the paleo-Venus world witha 2.9Gya solar spectrum but contrary to the other 3 simulations has a sidereal day 16 times that of modern Earth. Way et al. (2016).

This world develops a considerable greenhouse effect as the factor rotation period prevents significant cooling on the night-side of the planet, leading to lower air circulation (less wind). Consequently snowfall does not occur in this model, and the greenhouse effect begins to reach the level at which water could potentially be lost from the atmosphere into space, leading to a permanent drying of the climate.

Surface level winds for the paleo-Venus world witha 2.9Gya solar spectrum but contrary to the other 3 simulations has a sidereal day 16 times that of modern Earth. Way et al. (2016).

All of this is, of course, highly speculative, as we have little idea about the geology of, or water levels present on, early Venus. Modern Venus is known to have no equivalent of the plate tectonics seen on Earth, but ancient Venus could potentially have had something similar, or a quite different tectonic system that we have not thought of. Similarly the planet may have always been dry, or may have previously had far more water than the Earth and lost it due to the planet's runaway greenhouse effect.

Nevertheless these simulations provide an interesting insight into the possible evolution of our solar System, and provide us with some insight into where to look on Venus for information about the planet's past. They are also informative in relation to our search for Earth-like world's in other stellar systems, in that Venus is considerably inside the inner limit of what is considered the habitable zone of our Solar System (i.e. the zone in which an Earth-like planet might be expected to have liquid on the surface, and which might consequently be considered to potentially support life), and yet these simulations have shown that under a range of conditions Venus could have maintained a mild climate and extensive oceans, simply because of its low rotation rate.

See also...

Multicellular Eukaryotic organisms from the 1.56-billion-year-old Gaoyuzhuang Formation of North China.                                       Sediments across the Earth contain numerous macrofossils (fossils of big things that can be found with the naked eye, as opposed to...

The possibility of an Earth-mass planet in the habitable zone of the Kepler-68 system.      The Kepler Space Telescope has located many multi-planet systems since its inception, which combined with discoveries made by other planet-hunting...

Reconstructing cloud cover for ancient Earth, with a view to spotting a new one.             Four the past four billion years or so the continents have ambled back and forth across the face of...

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