Tuesday, 4 February 2014

Stirring The Human Gene-Pool

Humanity's forgotten return to Africa revealed in DNA - life - 03 February 2014 - New Scientist

A new analysis of the DNA from a distinct African population - the Khoisan of Southern Africa - has helped shed some light on why humans didn't diversify into different species as they spread out of Africa into the rest of the world, even though they developed regional varieties. It seems we were simply too mobile and didn't stay isolated for very long.

One of the 'causes' of evolution is the isolation of a population from the rest of the species for long enough for the two populations to diverge genetically or to adopt other isolating mechanism such as mating rituals, ways of attracting the opposite sex, etc, so that, if and when the two original populations come into contact again, interbreeding doesn't normally occur. Taxonomists then regard them as different species.

As effectively isolated gene-pools any selection pressure acting on one population can't have any effect on the other, so each population is free to evolve in it's own way. In fact, if they do come back into contact they may well compete for resources and each is merely a component of the other's environment and part of the selecting mechanism. For example, look what happened when two populations of squirrel which had diverged at some point in their history to form grey and red squirrels, came back into contact.

These are very special, isolated populations, carrying what are probably the most ancient lineages in human populations today. For a lot of our genetic studies we had treated them as groups that had split from all other present-day humans before they had split from each other.

David Reich, Harvard University
The Khoisan of Southern Africa are as distinct from the other sub-Saharan African peoples as they are from Europeans and are thought to have diverged from the East African Homo sapiens who migrated out of Africa into Euro-Asia some 60,000 years ago. They are thought to have had a much more extensive range in earlier times only being pushed south and west into the Kalahari as Bantu people migrated east and south - a process only really completed as recently as 500 CE. Until then the Khoisan were thought to have existed in near-isolation from the rest of the H. sapiens.

So, it was something of a surprise when David Reich of Harvard University and his colleagues found a fairly hefty dose of European DNA in the genomes of 32 Khoisan. These pieces of DNA most closely resemble the DNA found in people from Southern Europe such as those from Sardinia, Italy and the Basque regions of spain and France. The evidence is that this DNA entered the Khoisan genepool sometime between 900 and 1800 years ago, some considerable time before modern Europeans even became aware of Southern Africa.

These populations were always thought to be pristine hunter-gatherers who had not interacted with anyone for millennia. Well, no. Just like the rest of the world, Africa had population movements too. There was simply no writing, no Romans or Greeks to document it.

Brigitte Pakendorf, Linguist,
University of Lyon, France
However, this ties in with linguistic evidence which suggests the group of Khoisan who speak a distinct language known as Khoe-Kwadi originated in East Africa around the Ethiopia region and migrated south around the right time before merging with the Khoisan. These people in turn are thought to have come into Ethiopia about 3000 years ago, probably from the Middle East, from a population which also spread along the northern shore of the Mediterranean. This population was identified by an earlier study in 2012 by Luca Pagani of the Wellcome Trust Sanger Institute in Hinxton, UK.

So we can see that, far from being isolated, even a population of H. sapiens in remote South West Africa was subject to injections of DNA from other people. The human population may have formed transient isolated populations for long enough for some distinct characteristics to evolve but we were also subject to frequent migrations for a variety of reasons - climate change, technology, cultural innovations such as agriculture or pastoralism, the latter requiring movement to new pastures as populations grow or because other people are migrating into existing pastures. Whole chains of population movement could be set in train by one people somewhere moving out of their 'homeland' into other areas, so giving the human gene-pools another stirring. Because of this, H. sapiens has never evolved into distinct species and has never progressed beyond regional varieties.

This was not so for our cousins the Neanderthals, Denisovans and maybe other members of the Homo genus including us, who seem to have evolved into almost distinct species some 100,000 years ago, able to successfully interbreed only occasionally, but enough to give the Euro-Asian people a sprinkling of Neanderthal DNA. The question now is did this Neanderthal DNA find its way into Africa as the Euro-Asians in the Middle East found their way back into Africa from whence they had come some 57,000 years earlier?

Either way, it seems the European and Arab colonialists who found their way down the west and east coasts of Africa respectively in modern times may not have been the first Europeans to have migrated into sub-Saharan Africa after all, a reflux of the much earlier colonisation of Europe and Asia by our African forebears.

The difference may well have been that the modern colonialists carried memes for a cultural superstition which made them think of themselves as a superior people, entitled to enslave and exploit the 'lesser races' they found in Africa, sure in the knowledge that that was what a god had put them there for, and armed with holy books which told them everything was created just for them, that slavery was perfectly acceptable and cultural imperialism was just what that god had ordered in order to bring to these lesser people the 'good news' in the same holy books that provided the pretext for their enslavement and the eradication of their 'inferior' cultures with the promise of jam tomorrow after they were dead - if they did what the new masters told them today.

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