Tag Archives: Genetic bottlenecks

Human evolution and revolution in Africa

Toba eruption: a hindrance or a spur to humans?

English: Erosional dissection of an ash deposi...

Ash-fall blanket at Mount Pinatubo volcano in the Philippines. (credit: Wikipedia)

The eruption that created the 100 by 30 km Toba caldera 74 ka ago was the largest recorded volcanic event during the two million years of the genus Homo’s evolution. It ejected an estimated 800 cubic kilometres of ash to blanket the land surface thousands of kilometres away. By analogy with the known effects of stratospheric ash and sulfate aerosols from  the much smaller 1991 eruption of the Mount Pinatubo in the Philippines, which  reduced mean global temperature by 0.5 °C, Toba might be expected to have had an even larger cooling effect, perhaps by as much as 10° C. Such a scenario has led palaeoanthropologists to suggest that there would have been major effects on humans migrating across Eurasia at the time, such as dramatic population reduction and maybe a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations. Yet, not only are stone tools found below the Toba Ash in Sumatra, but also in South India and immediately above it too. Yet analysis of Toba’s environmental effects recorded by sediments on the bed of Lake Malawi in southern African reveal little if any sign of a global ‘volcanic winter’.

A letter in Nature, published online on 15 March 2018 (Smith, E.J. and 15 others 2018. Humans thrived in South Africa through the Toba eruption about 74,000 years ago. Nature, v. 555; doi:10.1038/nature25967) decisively refutes any retardation of human cultural progress, in southern Africa at least, and suggests the opposite. Smith and colleagues from South Africa, Australia and the US found ash dated at around 74 ka 9,000 km away from Toba in sedimentary sequences that contain anatomically modern human remains and artefacts at the coastal Vleesbaai and Pinnacle Point sites in Cape Province South Africa. To check on the likelihood of a fortuitous coincidence of another eruption having shed the ash, the team compared detailed geochemical analyses of ash samples with those from other volcanoes and bona fide Toba samples, with a clear confirmation of provenance. In the archaeological record, rather than any sign of a cultural setback, the intensity of use of the sites increased after the ash-fall event, accompanied by significant technological innovations. Perhaps the Pinnacle Point community was lucky and also responded in the spirit of the adage ‘necessity is the mother of invention’. Discovery of the Toba ash at other ancient human sites would resolve the issue.

Hominin cultural revolution 320,000 years ago

As regards stone tools, the Olorgesailie Basin in southern Kenya is about as good as it gets; a long-used ‘factory’ that covers a time span from about 1. 2 to 0.03 Ma. In places, large areas of the surface underlain by sedimentary strata, some of which have become major tourist attractions, are liberally strewn with tools and debitage from their manufacture. The area is ideal for stone-tool makers: being within the East African Rift it contains outcrops of many hard, fine-grained volcanic rocks and cherts formed by hot springs interleaved with its dominant fill of lake and riverine sediments. There are two dominant sedimentary units: the Olorgesailie Formation (1.2 to 0.49 Ma) overlain by the Oltulelei  Formation (0.32 to 0.05 Ma), the time gap between the two marking an extended period of regional erosion. Despite the rich tool assemblages, hominin remains have yet to be unearthed from the sediments, although there are plenty of bones from potential prey mammals. Olorgesailie was  good place to live, especially as the Rift would have channelled migrating herds predictably between its steep-sided flanks.

Acheulean biface tools strewn on a bedding surface in the Olorgesailie Basin, Kenya (credit: mmercedes_78 https://www.flickr.com/photos/27325832@N06/3833105587)

The older formation has yielded biface tools of the Acheulean technology from bottom to top. The earliest Acheulean tools in Africa date back to about 1.7 Ma and have been attributed to Homo ergaster/erectus, although examples in Europe are associated with H. antecessor and H. heidelbergensis – the technology was active for about 1.4 Ma. The Acheulean method involved striking flakes from large blocks of rock to result in a symmetrical, pear-shaped core that served as a multipurpose tool.   The oldest strata in the Oltulelei  Formation contain exclusively tools that are very different , having been made by a significantly more complicated procedure and covering a wide variety of designs with different uses. This Levallois technique focused on thin flakes produced from cores after careful preparation, which enabled similar tools to be made repeatedly rather than relying on chance fracturing. Precise dating of the oldest of these assemblages gives an age of 320 ka (Deino, A.L. et al. 2018. Chronology of the Acheulean to Middle Stone Age* transition in eastern Africa. Science, v. 359 online; DOI: 10.1126/science.aao2216). The makers clearly were able to visualize the finished product within the original lump of raw stone, but in a more nuanced way than did the makers of Acheulean biface tools. The first-described Levallois tools were associated with European Neanderthals.

English: Levallois flake obtained by the prefe...

Producing a flake by the Levallois technique (credit: Wikipedia)

Sometime in the 500 to 320 ka interval removed by erosion a major shift in technology and almost certainly cognition took place. Not only was this a technological revolution, but the Levallois tools are found in association with a variety of pigments, such as ochres, which show signs of having been worked, presumably for decoration of some kind. Also, the tool makers seemed to have a clear preference for specific rocks – black, glassy obsidian and cherts of white and green hues from sources 25 to 90 km from the tool-making sites (Brooks, A.S. and 14 others 2018. Long-distance stone transport and pigment use in the earliest Middle Stone Age. Science, v. 359 online; DOI: 10.1126/science.aao2646). The sheer volume of tools at each site and the evidence for long-distance transport of the raw materials have prompted the authors to hazard a guess at some kind of trade, or at least cooperative intergroup interaction. Together with the use of pigment, probably for body ornamentation, this suggests individual and perhaps group identity within a kind of social network.

Of course, the big question is: Who made the leap? That’s a hard one in the absence of human remains associated with the tool-making factories at Olorgesailie . The authors of both papers argue for the earliest modern humans. But, to me, this seems like an assumption based on the age of the transition rather than any convincing evidence. The original Levallois tools from northern France were found in association with skeletal remains of Neanderthals but much later in the Pleistocene. An age of 320 ka does place the Olorgesailie tools in the same ballpark as early AMH fossils from Morocco, later than the genetically derived date of separation of Neanderthals and AMH. However, the 180 ka time gap in which the technological revolution took place gives some room for so-called African ‘archaic modern humans’ (not subdivided as are similar fossils from Europe) are known from Zimbabwe, Tanzania and Ethiopia. If the Neanderthals were using the Levallois technique in Europe there is every reason to suspect that they, or their possible forebears H. heidelbergensis, may have brought it with them from Africa.

The Olorgesailie tools figure in a third paper in the same volume of Science, but one with less shaky grounds (Potts, R. and 14 others 2018. Environmental dynamics during the onset of the Middle Stone Age in eastern Africa. Science, v. 359 online; DOI: 10.1126/science.aao2200). The wet-dry cycle of the Pleistocene, related to global warming and cooling in interglacial and glacial episodes respectively, had become more marked after the 500 to 320 ka period of tectonically induced erosion. In itself, this would have resulted in more marked shifts in the ecosystems of the basin – perhaps a case of necessity being the mother of invention. Yet, the evidence base for changing climate cycles in Africa is not from local lake-sediment stratigraphy, micropalaeontology or geochemistry, but from the modelled variation of insolation based on Milankovich’s hypothesis.

*Note:  The Middle Stone Age in Africa does not correlate with the Mesolithic of Europe, but is a legacy of the development of archaeology in Africa. It corresponds to the European Middle Palaeolithic.

See also:  Gibbons, A. Complex behaviour arose at dawn of humans. Science, v. 359, p. 1201-1202 (with video)

Amazonian forest through the last glacial maximum

Accelerated evolution may occur when a small population of a species – whose genetic variability is therefore limited – becomes isolated from all other members. This is one explanation for the rise of new species, as in the Galapagos archipelago. Creation of such genetic bottlenecks encourages rapid genetic drift away from the main population. It has been suggested to explain sudden behavioural shifts in anatomically modern humans over the last hundred thousand years or so, partly through rapid and long-distance migrations and partly through a variety of environmental catastrophes, such as the huge Toba eruption around 74 ka. Another example has been proposed for the teemingly diverse flora and fauna of the Amazon Basin, particularly among its ~7500 species of butterflies, which has been ascribed to shrinkage of the Amazonian rain forest to isolated patches that became refuges from dry conditions during the last glacial maximum.

Top: Arid ice age climate Middle: Atlantic Per...

Potential forest cover inferred from global climate models for the last glacial maximum (top) the Holocene thermal maximum and at present.. (credit: Wikipedia)

A great deal of evidence suggests that during glacial maxima global climate became considerably drier than that in interglacials, low-latitude deserts and savannah grasslands expanding at the expense of humid forest. Yet the emerging complexity of how climate change proceeds from place to place suggests that evidence such continental drying from one well-documented region, such as tropical Africa, cannot be applied to another without confirming data. Amazonia has been the subject of long-standing controversy about such ecological changes and formation of isolated forest ‘islands’ in the absence of definitive palaeoclimate data from the region itself. A multinational team has now published data on climatic humidity changes over the last 45 ka in what is now an area of dense forest but also receives lower rainfall than most of Amazonia; i.e. where rolling back forest to savannah would have been most likely to occur during the last glacial maximum (Wang, X. et al. 2017. Hydroclimate changes across the Amazon lowlands over the past 45,000 years. Nature, v. 541, p. 204-207; doi:10.1038/nature20787).

Their study area is tropical karst, stalagmites from one of whose caves have yielded detailed oxygen-isotope time series. Using the U/Th dating technique has given the data a time resolution of decades covering the global climatic decline into the last glacial maximum and its recovery to modern times. The relative abundance of oxygen isotopes (expressed by δ18O) in the calcium carbonate layers that make up the stalagmites is proportional to that of the rainwater that carried calcium and carbonate ions dissolved from the limestones. The rainwater δ18O itself depended on the balance between rainfall and evaporation, higher values indicating reduced precipitation. Relative proportions of carbon isotopes in the stalagmites, expressed by δ13C, record the balance of trees and grasses, which have different carbon-isotope signatures. Rainfall in the area did indeed fall during the run-up to the last glacial maximum, to about 60% of that at present, then to rise to ~142% in the mid-Holocene (6 ka). Yet δ13C in the stalagmites remained throughout comparable with those in the Holocene layers, its low values being incompatible with any marked expansion of grasses.

English: View of Amazon basin forest north of ...

Amazonian rain forest north of Manaus, Brazil. (credit: Wikipedia)

One important factor in converting rain forest to grass-dominated savannah is fire induced by climatic drying. Tree mortality and loss of cover accelerates drying out of the forest floor in a vicious circle towards grassland, expressed today by human influences in much of Amazonia. Fires in Amazonia must therefore have been rare during the last ice age; indeed sediment cores from the Amazon delta do not reveal any significant charcoal ‘spike’.

See also: Bush, M.B 2017. The resilience of Amazonian forests. Nature, v. 541, p. 167-168; doi:10.1038/541167a