Friday, 9 October 2015

Africans Are Partly Eurasians

Ethiopian cave where the skull was found
Ancient Ethiopian genome reveals extensive Eurasian admixture throughout the African continent

Did we ever imagine that once anatomically modern humans had left Africa to populate Eurasia, they never went back again until modern times?

This seems to be the main thrust of the news media to today's new that a group of geneticists have analysed the genome from a 4,500 hunter-gatherer from the Ethiopian Highlands and found that he was genetically distinct from modern Africans who have since received a massive dose of Eurasian DNA which came into Africa about 3,000 years ago.

The interesting thing is that the same team found evidence that the genes from these 'Eurasian' immigrants have spread far and wide in Africa, reaching the West African Yoruba and Pygmy people and the Khoisan people of South Africa. This simply illustrates how genes flow through a population over time and how genes evolved in one part of a species range by local selection pressures can appear in another part of the range where they may have little or no particular advantage.

Scientifically, what's interesting here is that DNA could be recovered in sufficient quantity and quality from a tropical specimen at all, when it had been assumed that it would have degraded too far to be useful even if there was any to be recovered.

Modern humans are an example of how a widespread species can begin to diversify according to local selection pressures, especially if relatively isolated for a period of time, and then when conditions change and these local populations come back into contact, genes can flow between the different populations. In this way, over much of our range, human beings behaved like a ring species with some groups progressing to the status of new species, like Homo neanderthalensis, H. Sapiens, H. erectus and the so far scientifically unnamed Denisovans, but still being capable of interbreeding, and sometimes behaving like regional varieties, fully capable of interbreeding.

Humans that had migrated out of Africa some 55,000 years earlier and who had subsequently diversified, were perfectly capable of interbreeding with the descendants of their 'parent' population back in Africa. And of course, human taxonomy, by which these people are defined as different species, different varieties or simply regional variants, is essentially arbitrary anyway.

What is also interesting is that sub-Saharan Africans also have 0.3-0.7% Neanderthal DNA, presumably brought in by these immigrants. Neanderthals are believed to have evolved in relative isolation in Eurasia.

Of course, it goes without saying that the fact that there were these highly divergent genomes 4,500 years ago is utterly inconsistent with the childish notion that humans have all descended from a single family which lived just about 4000 years ago and that the book in which this tale was written is a book of literal and inerrant truth. Once again the evidence of the real world refutes this daft notion because the daft notion is not based on real-world evidence but on the ignorant imagination of people who knew no better.

Further reading:
Prehistoric Eurasians streamed into Africa, genome shows.


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