Basal humans in our evolutionary closet

The ancient genome from South Africa was of not because it confirmed what many had long suspected: the deep structure of modern humans in Africa goes back quite a bit further than we had been assuming. A few years ago I co-wrote an op-ed for USA Today where I initially wrote that the Khoisan divergence from other humans was around ~200,000 years ago. For various reasons I let a “fact-checker” change that to ~150,000 years ago. But as I told my co-author Alex Berezow people had access to whole genomes just leaning toward an older date than in the current literature.

Now we know that the date of divergence may be closer to ~300,000 years. Both because of the ancient DNA, and the Moroccan fossils which make it obvious that morphology associated with our own lineage was already beyond incipient before 300,000 years ago.

The past was more complicated than we think. Ancient DNA has made us toss many of our preconceptions out. For example, several years ago researchers concluded that the Out of Africa populations exhibited a different structure than we had thought. Populations of the Near East and Europe had ancestry from a group that was basal to all other non-Africans. That is, if you have a family tree then Oceanians, East Asians, Amerindians, Siberians, and European hunter-gatherers would be on one branch. On the other branch would be “Basal Eurasians” (BEu), named due to their basal position on the tree in comparison to all other non-Africans. Ancient DNA has not uncovered any population which is mostly BEu. Early Holocene Near Eastern populations, the various first farmers, invariably seem to show mixture between BEu and a West Eurasian element similar to what gave rise to the Mesolithic hunter-gatherers of Europe.

It seems possible we may never find “pure” BEu samples. We know that 10,000-15,000 years ago in Europe there was a massive expansion of a population of hunter-gatherers with stronger affinities to modern Middle Easterners than in the past. Perhaps this was part of a broader expansion of this group of hunter-gatherers across vast swaths of Western Eurasia, and in the process they absorbed the BEu populations until there were no pure populations left?

These questions were triggered by novel results which couldn’t be accommodated by current accepted models. This reminds me of a paper from over a year ago which presented some puzzling results which make much more sense now, Ancient gene flow from early modern humans into Eastern Neanderthals. The authors compared the Altai Neanderthal genome, the Denisovan genome, and those of modern humans (as well as a European Neanderthal). The stylized phylogenetic tree is one which the Neanderthal-Denisovan clade spits off from African proto-modern humans ~600,000 years ago. Then around ~400,000 years ago you see the divergence between Neanderthals and Denisovans. This gives us rough expectations as to the nature of the genetic variation in these populations, as they shared quite distinct periods of evolutionary history together and apart.

What the authors found is that a small minority of the Altai Neanderthal genome exhibited much stronger affinities to modern African populations than to the Denisovan. On the order of ~5%. Looking at the length of the haplotypes they estimated that the admixture occurred ~100,000 year ago. Curiously, at least at the time, this modern human population was basal to all modern humans. The authors estimate that the divergence from modern lineages occurred about ~200,000 year ago based on what was understood about modern human differentiation at the time.

At the time that was pushing it, though not unreasonably so. Now that number is probably comfortably defensible. One important point to note is that the modern human admixture was from a group equally related to all Africans. This implied that this group separated before the division of the Bantu and the Yoruba. So if you accept the most recent genomics then this group may date to closer to 260,000 years before the present. And in fact if the fossil record is correct they might have separated as early 350,000 years before the present. Additionally, the group which published the South African genome study posited another group, Basal Africans (BA), who diverged far earlier than the Khoisan from most African (and also non-African) groups.

The Neanderthals are a well studied group. We know what their range was. One can spit all sorts of speculative scenarios, from wide range proto-modern humans pushing deep in Eurasia during the Eemian interglacial 130,000 years ago. Or, it may have been through contact in the Near East and expansion of a somewhat admixed Neanderthal population north and east over time. Who knows? At least until there’s more ancient DNA….