Friday, 8 December 2017

Human Evolution - Dispersal Out of Africa

Map of sites with ages and postulated early and later pathways associated with modern humans dispersing across Asia during the Late Pleistocene.

Regions of assumed genetic admixture are also shown. ka, thousand years ago.

© Bae et al. 2017. On the origin of modern humans: Asian perspectives. Science.
Image by: Katerina Douka and Michelle O’Reilly
On the origin of modern humans: Asian perspectives | Science

One of the exciting things about science is how knowledge is never fixed and unchanging. It grows and develops as new findings are incorporated into existing understanding, sometimes even needing quite fundamental revisions of what we thought we knew.

One such branch of science is that of human evolution and dispersal out of Africa which is in an exciting phase at the moment as new archaeological discoveries are made and new techniques of DNA recovery and analysis are revealing genetic evidence of relationships and interbreeding.

As this major review of recent research by scientists from the Max Planck Institute for the Science of Human History, Germany and the University of Hawai’i at Manoa, USA shows that what we thought we knew was not quite right, especially in respect of timing and the number of dispersal events out of Africa. Until quite recently it looked fairly well established that anatomically modern humans had dispersed out of Africa in a single event about 60,000 years ago.

Not long before that we thought that all modern humans had evolved all the regional varieties from that single founder species. Although the genetic variation of all non-African people together is far less that that of most African peoples, we now know that there was at least occasional interbreeding with at least three other hominin species - Neanderthals, Denisovans and an as-yet unidentified species. We also know that there were at least two earlier dispersals of pre-modern hominins into Eurasia - Homo erectus and H. floresiensis, and that the Neanderthals and Denisovans at least were descendants of these.

Recently, remains of anatomically modern humans have been found at multiple sites in China dated to between 70,000 and 120,000 years ago.

Now Christopher J. Bae, Katerina Douka, Michael D. Petraglia propose that there were a series of smaller dispersions of groups of hunter-gatherers starting about 120,000 years ago followed by a major dispersal about 60,000 years ago.

Structured Abstract


The earliest fossils of Homo sapiens are located in Africa and dated to the late Middle Pleistocene. At some point later, modern humans dispersed into Asia and reached the far-away locales of Europe, Australia, and eventually the Americas. Given that Neandertals, Denisovans, mid-Pleistocene Homo, and H. floresiensis were present in Asia before the appearance of modern humans, the timing and nature of the spread of modern humans across Eurasia continue to be subjects of intense debate. For instance, did modern humans replace the indigenous populations when moving into new regions? Alternatively, did population contact and interbreeding occur regularly? In terms of behavior, did technological innovations and symbolism facilitate dispersals of modern humans? For example, it is often assumed that only modern humans were capable of using watercraft and navigating to distant locations such as Australia and the Japanese archipelago—destinations that would not have been visible to the naked eye from the departure points, even during glacial stages when sea levels would have been much lower. Moreover, what role did major climatic fluctuations and environmental events (e.g., the Toba volcanic super-eruption) play in the dispersal of modern humans across Asia? Did extirpations of groups occur regularly, and did extinctions of populations take place? Questions such as these are paramount in understanding hominin evolution and Late Pleistocene Asian paleoanthropology.

An increasing number of multidisciplinary field and laboratory projects focused on archaeological sites and fossil localities from different areas of Asia are producing important findings, allowing researchers to address key evolutionary questions that have long perplexed the field. For instance, technological advances have increased our ability to successfully collect ancient DNA from hominin fossils, providing proof that interbreeding occurred on a somewhat regular basis. New finds of H. sapiens fossils, with increasingly secure dating associations, are emerging in different areas of Asia, some seemingly from the first half of the Late Pleistocene. Cultural variability discerned from archaeological studies indicates that modern human behaviors did not simply spread across Asia in a time-transgressive pattern. This regional variation, which is particularly distinct in Southeast Asia, could be related at least in part to environmental and ecological variation (e.g., Palearctic versus Oriental biogeographic zones).

Recent findings from archaeology, hominin paleontology, geochronology, and genetics indicate that the strict “out of Africa” model, which posits that there was only a single dispersal into Eurasia at ~60,000 years ago, is in need of revision. In particular, a multiple-dispersal model, perhaps beginning at the advent of the Late Pleistocene, needs to be examined more closely. An increasingly robust record from Late Pleistocene Asian paleoanthropology is helping to build and establish new views about the origin and dispersal of modern humans.

The traditional “out of Africa” model, which posits a dispersal of modern Homo sapiens across Eurasia as a single wave at ~60,000 years ago and the subsequent replacement of all indigenous populations, is in need of revision. Recent discoveries from archaeology, hominin paleontology, geochronology, genetics, and paleoenvironmental studies have contributed to a better understanding of the Late Pleistocene record in Asia. Important findings highlighted here include growing evidence for multiple dispersals predating 60,000 years ago in regions such as southern and eastern Asia. Modern humans moving into Asia met Neandertals, Denisovans, mid-Pleistocene Homo, and possibly H. floresiensis, with some degree of interbreeding occurring. These early human dispersals, which left at least some genetic traces in modern populations, indicate that later replacements were not wholesale.

Map of sites and postulated migratory pathways associated with modern humans dispersing across Asia during the Late Pleistocene.

Archaic human distributions are also shown. (A) Initial dispersal(s) of Homo sapiens, based on sites predating 60 ka. (B) Subsequent migration pathways of H. sapiens, based on evidence from the most important sites dated to between 60 and 30 ka. Yellow stripes and translucent orange represent tentative ranges for Neandertals and early Homo sapiens, respectively.

Christopher J. Bae et al. Science 2017;358:eaai9067

The presence of archaic H. sapiens in Morocco, North Africa before 60,000 years ago suggests an earlier dispersal within Africa itself which, given the evidence of a wider distribution of Australopithecines across South and East Africa is not really surprising. The operation of the so-called Sahara Pump could explain the presence of H. sapiens on the southern shores of the Mediterranean. This 'pump' was the result of periods of wetter climate that allowed African species to migrate into the Sahara, followed by periods of extreme dehydration which isolated populations in the north from those retreating south again.

There is as yet no evidence that these humans crossed the straights of Gibraltar even though Southern Spain is clearly visible, so any contribution this population made to the dispersal out of Africa would have been across North Africa and into the Levant.

What is emerging is a fascinating picture of dispersing and diversifying human species at different points in evolutionary history, acting much like a ring species with social and sexual contact between related species at the points of overlap of their range, and anatomically modern humans following the paths already trodden by archaic species and their descendants.

So much richer and more impressive a human story than the creationist magic man magicked it all a few thousand years ago superstition, and based on evidence too, not just a book of ancient mythology completely devoid of any evidential support and demonstrably wrong in most of the rest of its 'science'.

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