Tag Archives: Human migrations

Humans and mass extinction

It is often said that the biosphere is currently undergoing species losses that may rival those of the ‘Big Five’ mass extinction, with the rate of new extinctions being estimated at about 100 times the background rate during geological time. Scientifically, this is probably a dodgy assumption for palaeobiologists simply do not have the evidence to suggest what such a ‘normal’ rate might be. The fossil record is notoriously incomplete for a whole variety of reasons largely to do with both preservation and fossil collection strategies. For instance, as today, some genera may have been very common and widespread in past times, whereas others rare and restricted to small ecological niches. The record of life is prone to huge errors so that only huge, global shifts in diversity, such as mass extinctions, can be viewed with statistical rigour; and then only with caveats. For sure, the rapid demise of species today is cause for alarm and dismay, and more taxa – mainly of smaller and more restricted groups – probably have escaped identification, and will continue to do so. In the context of growing human impacts on ecosystems across the globe extinction is an increasingly emotive topic, as witness the clamour among some geoscientists for adding a new Anthropocene Epoch to the to the Stratigraphic Column. Does that require renaming the Holocene, beginning 11,700 years ago at the end of the last Ice Age, during which agriculture began? Should its start be assigned to some event during recorded history, such as the European invasion of the Americas after 1493, the beginning of the Industrial Revolution or the explosion of the first thermonuclear weapons in the 1940s and 50s? Or did humans begin significantly to affect the biosphere once their spread from Africa started after about 130 ka ago, i.e. in the late Pleistocene? That argument may well run and run: it is foremost a scientific issue, to which rules apply. A cogent example is that of the fate of megafaunas on the major continents except Antarctica as humans migrated far and wide.

The demise of the large flightless birds of Madagascar and New Zealand form a well known case as they almost certainly followed first colonisation by humans around 200 BC and 1300 CE respectively. The megafaunas of the much larger continents of Australia and the Americas have been deemed to have been more than decimated in the same way after about 65 ka and 15 ka respectively. There are no longer giant armadillos and ground sloths in South America, mammoths ceased to roam North America, and giant wombats, marsupial predators and kangaroos only remain as bones, to name but a few. It has been argued that their extinctions stemmed from the first human migrants literally eating their way through vast terrains. Yet the vast herds of Africa seem not to have been affected in the same way, until much more recently as population grew and modern projectile weapons became widely available. That has been suggested to have resulted from co-evolution of humans and megafauna over two million years, together with instinctive caution among large African beasts, whereas the ‘naivety’ of their counterparts in the Americas and Australia doomed them to extinction. Of course, it is likely that things were a great deal more complicated in every case, as argued in a review of Late Pleistocene megafaunal extinctions by Gilbert Price of the University of Queensland, and colleagues from Australia, the US and Denmark (Price, G.J. et al. 2018. Big data little help in megafauna mysteries. Nature, v. 558, p. 23-25;  doi:10.1038/d41586-018-05330-7).

The gist of Price and colleagues’ critique of meta-analyses of data – 32 since 1997 – concerning allegedly human-induced extinctions is that much of the pertinent data is either low quality or poorly understood. For starters, much of the dating is questionable, either using inaccurate and outdated methods or based on inference. For instance, fossils of some alleged victim, e.g.  Australian land crocodiles (Quinkana) and giant wombats (Ramsayia), have never been dated. Moreover, dates of the last known fossils are used when they may have remained extant until more recently: wooly Eurasian mammoths were long supposed not to have survived the last glacial maximum, yet recently mammoth bones from Wrangel island were found to be as young as the second millennium BCE. In 2010 spores of the fungus Sporormiella, in sediment cores, which grows only on digested plant matter in herbivore dung, was used as a proxy for the former presence or absence of large herbivore herds. Its decline in sediments after 13 ka in North America happened to coincide roughly with the start of the North American Clovis hunter culture, which was used to show that extinctions of large herbivores were linked to human predation. Yet such fungi also live on excrement of many animals both large and small, and its preservation is affected by changes in climate and water flow. To properly link declines and extinctions in human prey animals requires concrete evidence of predation, such as cut marks on identifiable bones within middens associated with human habitation, such as hearths.

When emotion, ambition and bandwagon tendencies become associated with science, objectivity sometimes gets compromised.

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Early human dispersal through Asia

When first mooted, the Out of Africa model for the spread of anatomically modern humans (AMH) centred on a single exodus from African to Eurasia, which researchers broadly agreed to have occurred about 60 thousand years ago. That was when an advance of continental glaciers and sea level fall narrowed to manageable proportions the obstacle presented by the Red Sea. The only archaeological drawback was that AMH had occupied the Levant at around 110 ka. That was formerly considered to have been a temporary occupation corralled by hyperarid conditions immediately to the east and a mountain barrier to the north, with the Mediterranean Sea to the west. Yet, during humid periods there was every chance that the eastern barrier would occasionally have been permeable. Plumping for the 60 ka exit model was a conservative view stifled by a lack of high-quality dates for scattered suggestions of an Asian AMH presence, such as occurrences of stone tools resembling those of early moderns and even rarer, incomplete and often ambiguous skeletal remains. The ‘modern-looking’ tools that occurred both above and below the 74 ka Toba ash deposit in southern India were disposed of as ‘advanced’ tools of earlier migrants; probably Homo erectus. In retrospect, the established fact of earlier occupation of Eurasia by such ‘primitive’ African migrants, as long ago as ~1.8 Ma in the case of Homo fossils in Georgia, should have encouraged the view that culturally better-endowed AMH would have had less problem in diffusing eastwards once they found an escape route from Africa.

Whatever, the flurry during the last couple of decades of more skeletal and archaeological remains of AMH in Asia, genetic evidence for their interbreeding in the west and east with earlier human groups and, principally, improvements in dating ancient sites suggests a more complex geographic flow. Christopher Bae of the University of Hawaii and colleagues based in the UK, Germany and the US have reviewed this growing wealth of new data to put forward various scenarios for Out of Africa dispersal through Asia (Bae, C.J. et al. 2017. On the origin of modern humans: Asian perspectives. Science, v. 358, p. 1269 (summary); online full paper DOI: 10.1126/science.aai9067). They highlight growing evidence for at least one pre-60 ka dispersal, and probably several, to reach the Levant, Arabia, India, China, Laos, Indonesia, the Philippines and Australia before that date. This tallies with Neanderthal and Denisovan DNA segments within the genomes of living Eurasians that indicate interbreeding before 60 ka.

Bae and colleagues also assemble data that bear on where AMH managed to move out of Africa. They resolve the dispute between routes around the northern shores of the Red Sea and crossing the southern Straits of Bab el Mandab by concluding ‘why not both’. Where the migrations went to is currently suggested by the distribution of sites that reveal either pre- or post-60 ka occupancy. The earlier dispersals may have been dominated by following coastlines along the Mediterranean in North Africa to the Levant and via Bab el Mandab across the Persian Gulf, along the northern Indian Ocean littoral to south-east and east Asia. The later, more ‘adventurous’ movements using both routes led to Europe and deeper into continental Asia and thence to its north east. The review conveniently covers in seven pages much the same geoarchaeological and anthropological ground as Earth-Pages has visited bit-by-bit as it has unfolded since 2000. Clearly, great swathes of Asia have not been explored by palaoanthropologists. As in most geographic sciences there is a tendency to follow up known sites year after year – often decade after decade – to ensure publishable results, and that will consume lots of economic and human resources. It is more risky to try and fill in the gaps, but that basic field work is urgently needed to supply new material.

Did Out of Africa begin earlier?

It is widely thought that anatomically modern humans (AMH) began to diffuse out of Africa during the climatic cooling that followed the last interglacial episode. Periods of build-up of ice sheets, or stadials, also saw falls in sea level, which would have left shallow seas dry and easily crossed. The weight of evidence seems to point towards the narrowing of the Red Sea at the Straits of Bab el Mandab between modern Eritrea and the Yemen. Because the Red Sea spreading axis goes onshore through the Afar region of Ethiopia further north, the Straits today are shallow. Between about 70 and 60 ka, during a major stadial, much of the Bab el Mandab would have been dry. Dating of the earliest AMH remains in Asia and Australasia seems to suggest that the move out of Africa probably began around that time. But, of course, that presupposes the AMH fossils being the oldest in existence, although some would claim that genetic evidence also supports a 70-60 ka migration. Yet, AMH human remains dated at around 100 ka have been found in the Middle East on a route that would also lead out of Africa, but for the major problem of crossing deserts of modern Syria and Iraq. The supposed desert barrier has led many to suggest that the earlier venture into the Levant met a dead end. Should AMH fossils older than 70 ka turn up in Eurasia or Australasia then a single migration becomes open to doubt.

Mitochondrial DNA-based chart of large human m...

Chart of large human migrations based on variations in mitochondrial DNA in living humans(Numbers are millennia before present.) (credit: Wikipedia)

It appears that challenge to what has become palaeoanthropological orthodoxy has emerged (Bae, C.J. et al. 2014. Modern human teeth from Late Pleistocene Luna Cave (Guangxi, China). Quaternary International, In Press). Scientists from the US, China and Australia found two molar teeth within calcite flowstone in Lunadong (‘dong’ means ‘cave’). That speleothem is amenable to uranium-series dating, and has yielded ages between 70 and 127 ka. That antiquity does open up the possibility of earlier migration, perhaps during the interglacial that ended at about 115 ka when sea levels would have stood about as high as it does nowadays (in fact it was only after about 80 ka that it stood low enough to make a move across the Bab el Mandab plausible). If that were the case, the migration route would have more likely been through the Middle East, perhaps along the Jordan valley and thence to the east. Had there been greater rainfall over what is now desert then there would have been no insurmountable barrier to colonisation of Asia.

These teeth are not the only evidence for earlier entry of AMH into east Asia; a date of 66 ka for a modern human toe bone was recently reported from the Philippines. Yet many experts remain unconvinced by teeth alone, especially from east Asia where earlier humans had evolved since first colonisation as early as 1.8 Ma ago. There are other pre-70 ka east Asian bones with more convincing AMH provenance, however.

There is another approach to the issue of earlier Out of Africa migration; one resting on theoretical modelling of the observed genetic and morphological variation among living Eurasians, especially the decreasing diversity proceeding eastwards (Reyes-Centeno, H. et al. 2014. Genomic and cranial phenotype data support multiple modern human dispersals from Africa and a southern route into Asia. Proceedings of the National Academy of Sciences, v. 111, p. 7248-7253. doi: 10.1073/pnas.1323666111). The authors, from Germany, Italy and France, challenge the single-exit hypothesis based on genetic data, suggesting that those data are also commensurate with several Out of Africa dispersals beginning as early as 130 ka. They favour the Bab el Mandab exit point and migration around Eurasia at that time when sea-level was extremely low during a glacial maximum. They hint at the ancestors of living native Australians and Melanesians being among those first to leave Africa, other Asian and European populations having dispersed from a later wave.

The origins of the first Americans

Whatever controversies still linger about when they arrived in the Americas, there can be little doubt that humans crossed what are now the Bering Straits from NE Asia using the landmass of Beringia exposed by sea-level fall during the last ice age. Of course, there have been controversies too about who they were; probably of East Asian origin but the waters muddied by the celebrated case of 9300 year-old Kennewick Man whose skull bears close resemblance to those of modern Europeans but also to those of the Ainu of northern Japan. Genetic studies of Y-chromosome DNA suggested that all early Americans stemmed from 4 separate colonising populations who may have entered via Beringia by different routes (coastal and across the interior of North America) and at different times. Now, perhaps unsurprisingly, a new kind of data seems set to stir things up immeasurably.

Sitting Bull, Red Cloud, Swift Bear, and Spott...

Famous Lacotans of the Dakotas (credit: Wikipedia)

After the triumphs of reconstruction of the Neanderthal and Denisovan genomes and the corollary that both interbred with anatomically modern humans, it was only a matter of time before the palaeogenetics of humans would be pushed back in time. The oldest remains to yield DNA are those of a boy from near Lake Baikal in Siberia excavated by Soviet archaeologists along with a rich trove of cultural remains, including female effigies. Such figurines are rare in Siberia, most being known from western Eurasia. Radiocarbon dating of the bones gave an age of around 24 ka, just before the last glacial maximum. The genetic information, specifically mtDNA and Y-chromosome DNA are potentially revolutionary (Raghavaan and 30 others 2013. Upper Palaeolithic Siberian genome reveals dual ancestry of Native Americans. Nature online doi:10.1038/nature12736).

The mtDNA (passed down the female line) places the individual in haplogroup U, but with little relation to living members with that ‘signature’. Modern haplogroup U is mainly confined to people now living in North Africa, the Middle East, south and central Asia, Europe and western Siberia up to the area where the skeleton was found but rare further to the northeast. The male-specific Y-chromosome DNA is related to haplogroup R widely spread today among men living in western Eurasia, south Asian and in the vicinity of the find. When the data were subject to statistical tests routinely used in distinguishing existing p[populations and lineages within them (principal component analysis) a surprise emerged. The boy plots separately from all living populations but halfway between modern Europeans and the genetic trend of native Americans: i.e. descendants from the population to which he belonged could have evolved towards both extant groups but certainly not to East Asians. Plotted on a map, the degree of shared genetic history of the ice-age south Siberian boy to modern humans shows links westward to Europeans and eastwards to northeastern Siberians and hence to native Americans.  Up to 38% of native American ancestry may have originated by gene flow from the population to which the boy belonged, similarly for Europeans as a whole.

The research helps explain traces of European genetic ‘signatures’ in native Americans rather than the commonly held view that this resulted from post-Columbian admixture with European invaders. It also links with the European-looking skulls of a number of early Americans which do not resemble those of East Asians once thought to be their forebears.