When modern Homo sapiens left their evolutionary home in Africa, they (we?) encountered and eventually displaced the older species Homo neanderthalensis.
But it wasn’t all fightin’ and killin’ – recent genetic sequencing has shown that to some extent our ancestors interbred with Neanderthals, and that this probably helped their survival by boosting their immune systems.
When the Neanderthal genome sequence was published in 2010, parts of it were shown to be common to most modern humans. In fact, people from all over the world – yes, even Australia – shared 1-4% of their genes with Neanderthals; everywhere that is, except Africa (Ref 1).
This suggests that when modern humans left Africa – commonly believed to be about 70,000 years ago – they met and mated with Neanderthals in the Middle East. Whereas those who stayed in Africa didn’t meet Neanderthals and so didn’t pick up their genes (at least at first).
Now, new research has shown that some of the genes we share are important for our immune system (Ref 2).
These genes are part of the human leukocyte antigen system, or HLA. The HLA genes, or rather the proteins created from their code, are used by our immune systems to distinguish our cells from invaders. Picking up these new genes from locals who had already adapted to their environments would have helped our ancestors fight the many new diseases they encountered as they moved around.
But the HLA genes didn’t come from just one source: some are also common to the Denisovans, a newly discovered species whose mitochondrial DNA – sequenced from toe and finger bones found in Siberia – is distinct from both Neanderthals and modern humans (Ref 3).
What’s even more startling is that, even though the Neanderthal and Denisovan genes are most common in Eurasians, they are actually found to a small degree in Africans too, implying that they were brought back in at a later point.
This prehistoric promiscuity and the propensity to spread genes around definitely seems to have made a contribution to human success, and the outlasting of our hominin predecessors.
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- Krause J, Fu O, Good JM, Viola B, Shunkov MV, Derevianko AP & Pääbo S 2010, “The complete mitochondrial DNA genome of an unknown hominin from southern Siberia”, Nature 464, pp. 894-897, doi:10.1038/nature08976