Friday, 29 September 2017

DNA Shows Complex Human Evolution in Africa

Demographic model of African history and estimated divergences. Vertical colored lines represent migration, with down-pointing triangles representing admixture into another group. Southern African hunter-gatherers are shown by red symbols, and Iron Age farmers as green symbols. (For explanation see the original paper)
Modern humans may have been around for nearly twice as long - Uppsala University, Sweden

More evidence today of modern humans emerging gradually from archaic hominins in Africa.

This genetic and archaeological analysis by a team from Uppsala University, Sweden, and the University of Johannesburg, South Africa, suggests, contrary to accepted wisdom, that modern humans may have diversified from archaic forms significantly earlier than generally believed and maybe not in a single location from a single founder species.

The suggestion is that anatomically modern humans could have gradually diversified in different African regions, then gene flow between the emerging groups produced the single, genetically diverse, species we now recognise as Homo sapiens.

This new study has been made possible by improved DNA extraction techniques which are revolutionising archaeology and evolutionary biology.

Southern Africa is consistently placed as a potential region for the evolution of Homo sapiens. We present genome sequences, up to 13x coverage, from seven ancient individuals from KwaZulu-Natal, South Africa. Three Stone Age hunter-gatherers (about 2000 years old) were genetically similar to current-day southern San groups, while four Iron Age farmers (300 to 500 years old) were genetically similar to present-day Bantu-speakers. We estimate that all modern-day Khoe-San groups have been influenced by 9 to 30% genetic admixture from East Africans/Eurasians. Using traditional and new approaches, we estimate the first modern human population divergence time to between 350,000 and 260,000 years ago. This estimate increases the deepest divergence among modern humans, coinciding with anatomical developments of archaic humans into modern humans as represented in the local fossil record.

According to the Uppsala University press release:

The team sequenced the genomes of seven individuals who lived in southern Africa 2,300–300 years ago. The three oldest individuals dating to 2,300–1,800 years ago were genetically related to the descendants of the southern Khoe-San groups, and the four younger individuals who lived 500–300 years ago were genetically related to current-day South African Bantu-speaking groups...

The authors estimate the divergence among modern humans to have occurred between 350,000 and 260,000 years ago, based on the ancient Stone Age hunter-gatherer genomes. The deepest split time of 350,000 years ago represents a comparison between an ancient Stone Age hunter-gatherer boy from Ballito Bay on the east coast of South Africa and the West African Mandinka....

The fossil record of east Africa, and in particular the Omo and Herto fossils have often been used to set the emergence of anatomically modern humans to about 180,000 years ago. The deeper estimate for modern human divergence at 350,000–260,000 years ago coincides with the Florisbad and Hoedjiespunt fossils, contemporaries of the small-brained Homo naledi in southern Africa...

The authors also found that all current-day Khoe-San populations admixed with migrant East African pastoralists a little over a thousand years ago. Of the Iron Age individuals, three carry at least one Duffy null allele, protecting against malaria, and two have at least one sleeping-sickness-resistance variant in the APOL1 gene. The Stone Age individuals do not carry these protective alleles.

Thus, both palaeo-anthropological and genetic evidence increasingly points to multiregional origins of anatomically modern humans in Africa, i.e. Homo sapiens did not originate in one place in Africa, but might have evolved from older forms in several places on the continent with gene flow between groups from different places.

Carina Schlebusch, co-first author
Population geneticist, Uppsala University
Disease resistance evolved in West Africa was carried into South Africa by Iron Age farmers, showing how something evolved in response to local environmental pressures can migrate throughout a population by gene flow. Of course, these disease resistance genes don't need to have evolved in modern humans but may well have arisen in their archaic ancestors.

Incidentally, the diagram the authors use to illustrate this regional divergence is a neat illustration of the difficulty with delineating one 'species' from another as species diverge and evolve over time. Take any of those branches and try to identify where there is a sharp colour change. Small changes in allele frequency over time do not normally lead to large-scale sudden changes. Each generation will always be capable of breeding with its parents' generation as well as with its descendant generation. Evolution ocurrs in populations, not individuals.

The Uppsala University press release continues:

The transition from archaic to modern humans might not have occurred in one place in Africa but in several, including southern Africa and northern Africa as recently reported.

Cumulatively these findings shed new light on our species’ deep African history and show that there is still much more to learn about our process of becoming modern humans and that the interplay between genetics and archaeology has an increasingly important role to play.

It now seems that at least two or three Homo species occupied the southern African landscape during this time period, which also represents the early phases of the Middle Stone Age. It will be interesting to see in future if we find any evidence of interaction between these groups.

Marlize Lombard, co-first author
Stone Age archaeologist, University of Johannesburg.
So the evidence is growing that, just as anatomically modern humans bred with archaic and related species in Eurasia as they spread out of Africa, so our African ancestors may have interbred with other archaic hominins and emerging populations as they spread out from several centres within Africa. In other words, modern humans not only do not have a single founder couple as the followers of the Abrahamic creation myth like to pretend, but we don't even have a single founder species. Different local environmental pressures in different parts of Africa led to local divergence, then later admixing resulted in what we now see as H. sapiens.

I wonder how many creationists are noticing how these new discoveries made possible by newly-developed techniques for extracting the DNA that Darwin never even imagined is not only failing to undermine or discredit any of his and Wallace's Theory of Evolution by descent with modification, but is simply adding new layers of evidence for it.

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1 comment :

  1. Fine article.
    Science keeps progressing ever forward.


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