Biology Magazine

Humans, Neanderthals, Denisovans, and Hominin x

Posted on the 25 August 2016 by Reprieve @EvoAnth

Nowadays us humans are the only surviving members of the hominin family. However, roll the clock back a mere 50,000 years and the world looks a lot different. In Europe, the Neanderthals flourished. The Denisovans were occupying a cave in Siberia. And we humans were beginning our journey around the world. But that's not all. New evidence has revealed there may have been another species living alongside us. The mysterious "hominin x".

What's more, those pioneering humans leaving Africa did know about hominin x. Intimately. Anomalies in the genome reveal that we interbred with this unknown species. Which shouldn't be too surprising, given we did the same to both Neanderthals and Denisovans. However, this would make hominin x the first species to be discovered without every finding physical remains.

Asian arrival

The evidence for this enigmatic new species comes from Asia. Specifically, an isolated group of people living on the Andaman islands, off the coast of India. These appear to be one of the most genetically isolated populations in southeast Asia, featuring a distinct culture and a unique language.

This makes them a key resource when studying the out of Africa migration. Our species began (unsuccessfully) moving out of our African home ~120,000 years ago; but didn't succeed until much later. Why did it take so long for humans to get a foothold elsewhere? What route did they take when they did so? Who did they meet along the way? These are all questions the Andamanese people can help answer.

Or at least, they could if we had their complete genome. Examining the entire genome of a large group of people has only become viable in the last few years, so nobody had gotten around to this isolated Indian island just yet. So researchers decided to go out and fix this issue.

Sure enough, the results were fascinating. Their genes revealed a shared common ancestry with all other non-Africans ~50,000 years ago, confirming that we were all part of the same out of African migration ( except for Aboriginal Australians). This is confirmed by the fact that we all share a similar bit of DNA with Neanderthals, which we probably acquired before the various populations split up. Speaking of which, all European populations appear as an "outgroup". This indicates that Asians and Europeans split early in their history. So that interbreeding with Neanderthals must have happened even earlier.

Hominin x & friends

As if that wasn't interesting enough, evidence of additional interbreeding was found amongst the Andamanese. The Denisovans were living in Siberia during this period, and it looks like they bumped into some humans. In fact, they bumped in a very sexy way. As such some groups, like the Andamanese, also have a bit of Denisovan DNA to go with their Neanderthal genes.

All of this adds up to ~2% Neanderthal, ~2% Denisovan . . . and yet still had ~2% less "human" DNA than expected. All modern humans share a good chunk of DNA with Africans, given that's where we originally evolved. Some of that has since been replaced by unique mutations, some by genes we inherited from our interbreeding with other hominin species. However, in the Andamanese 2% more of these genes had been replaced by something. And this can't be ascribed to unique mutations in that group. Australian Aboriginies and some other populations were "missing" the same part of the genome.

In short, it looked like they inherited an extra few genes from yet another hominin species. This is the enigmatic "hominin x". Existence of it had been hinted at by previous genetic discoveries. However, this is the first solid evidence of it's discovery. With this extra data, it's also possible to estimate that the original "owner" of these mystery genes split from us ~300,000 years ago. This would place their evolution shortly after the emergence of Neanderthals.

With only a tiny fraction of their DNA studied via their interbreeding with modern humans it's difficult to say much about this species. It's still within the realm of possibility that this is just some sort of genetic quirk. Nevertheless, this is the most solid evidence our family was diverse.

Scratch that, even more diverse. With 24 different members of the human family so far, hominin x is about to get a bunch of new friends.

Summary

Genetic evidence from the Andamanese island has a bit of unusual DNA. It's consistent with interbreeding with a previously unknown species of human, which I call hominin x because it sounds mysterious and cool.

References

Hu, Y., Wang, Y., Ding, Q., He, Y., Wang, M., Wang, J., Xu, S. and Jin, L., 2014. Genome-wide scan of archaic hominin introgressions in Eurasians reveals complex admixture history. arXiv preprint arXiv:1404.7766.

Mondal, M., Casals, F., Xu, T., Dall'Olio, G.M., Pybus, M., Netea, M.G., Comas, D., Laayouni, H., Li, Q., Majumder, P.P. and Bertranpetit, J., 2016. Genomic analysis of Andamanese provides insights into ancient human migration into Asia and adaptation. Nature Genetics.


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