Longtanshan Cave in China contains something it shouldn't. A series of modern human teeth, dated to ~80,000 years ago. These teeth are out of place because genetics shows that humans didn't arrive in the region until closer to 50,000 years ago. These are teeth out of time.
The implications seem clear. A group of humans left Africa before the migration that gave rise to modern populations. They made it as far east as China before dying out. Those initial pioneers were ultimately failures, leaving behind little more than mysterious teeth. Our wave of migration arrived later and the rest is (pre)history.
However, new genetic data indicates these early pioneers may have been more successful than we thought. Although they still went extinct, they may have lived long enough to meet our migration. And as always happens when two hominins meet, interbreeding ensued.
Australian migration
Modern humans evolved in Africa around 200,000 years ago. Later they left their homeland and spread around the world. However, "Out of Africa" is a simple description for an incredibly complex event. It's actually a story of groups leaving Africa, returning, leaving, occupying new areas, and retreating once more.
One of the bigger unknowns surrounds the colonisation of Sahul. You probably haven't heard of that place because it doesn't exist anymore. It's the continent formed of Tasmania, Australia, and Papua New Guinea; before the ocean came and carved them up. However, when humans arrived Sahul still existed. But beyond that, there's a lot we don't know about this occupation.
For instance, scientists were able to recover DNA from one of the oldest skeletons in Australia. You'd think that Mungo Man's (since he came from near Lake Mungo) DNA could shed light on ancestry. However, initial results seemed more confusing. They suggested Mungo was unrelated to modern Aboriginal Australians. Instead, it seemed that he was part of an earlier, separate and extinct migration into the country.
Later results confirmed this was a false conclusion. Mungo Man is related to modern Australians after all, indicating there was just the one migration into Sahul. But the confusion doesn't stop there. Debate sparked up over whether Mungo's migration was a branch of the same migration that populated the rest of the world. Or was it a separate event.
Initial results appeared to indicate the latter. Mungo and the rest of Sahul came from an early, separate out of Africa movement. The ancestors of Europeans, Americans, and everyone else left later. However, not everyone was convinced
Studying Sahul
A series of studies have just been published in Nature to try and get to the bottom of this Australian migration. Was it a unique wave of migration, or simply part of a wider population movement?
Much like the initial conclusions about Mungo Man, it seems the first findings of it being a unique wave are false. So I've got a disproven article sitting on this website. Darn.
Anyhoo, these studies included more data from Sahul than previous examinations. This revealed several interesting facts about the occupation of this ancient continent. For example, the split between people living on different islands began before those landmasses were fully separate.
But they key discoveries are that aborigines from all over Sahul are equally distantly related to those from elsewhere. So Sahul was occupied by a single migration from the mainland. And that this single migration was part of the same movement that led to the colonisation of the rest of Eurasia. Key evidence for this comes from the fact both branches show evidence of the same population bottleneck, around 72,000 years ago.
Intrabreeding
Whilst trying to study the migration into Sahul, this research came across an anomaly. If the Sahul movement was part of the same migration as, for example, Europeans, then both should be equally distantly related to Africans. However, they weren't. Estimates revealed Sahul-ians were more distantly related than expected.
This could be explained by interbreeding with an older group. The inclusion of more ancient DNA would increase the genetic distance to their African ancestors.
This opens up a whole raft of fascinating possibilities. Were they interbreeding with that early, failed out of Africa migration? Or maybe they had a bit of extra fun with archaic species like Neanderthals or Denisovans. Perhaps there's even some unknown species they got intimate with.
The key to telling all these options apart comes from Africa. These Sahul-ians had some segments of African DNA not present in the successful out of Africa migration. This would indicate that they got them from another group fo ex-Africans. Perhaps the people who left behind the teeth at Longtanshan.
But given how complex the out of Africa idea has turned out to be, it might be some other unknown population movement for all we can tell. Regardless, they're still definitely extinct. Some might live on in modern groups, but it's only 2%.
Our species doesn't have a 100% success rate.
Conclusion
The "first" migrants into Australia interbred with an earlier, now extinct group of humans who had left Africa much earlier. It turns out we weren't the first modern humans to leave Africa. Our efforts to conquer the world were complex and it took many tries.
References
Curnoe, D., Ji, X., Shaojin, H., Taçon, P.S. and Li, Y., 2016. Dental remains from Longtanshan cave 1 (Yunnan, China), and the initial presence of anatomically modern humans in East Asia. Quaternary International, 400, pp.180-186.
Heupink, T.H., Subramanian, S., Wright, J.L., Endicott, P., Westaway, M.C., Huynen, L., Parson, W., Millar, C.D., Willerslev, E. and Lambert, D.M., 2016. Ancient mtDNA sequences from the First Australians revisited. Proceedings of the National Academy of Sciences, p.201521066.
Malaspinas, A.S., Westaway, M.C., Muller, C., Sousa, V.C., Lao, O., Alves, I., Bergström, A., Athanasiadis, G., Cheng, J.Y., Crawford, J.E. and Heupink, T.H., 2016. A genomic history of Aboriginal Australia. Nature.
Pagani, L., Lawson, D.J., Jagoda, E., Mörseburg, A., Eriksson, A., Mitt, M., Clemente, F., Hudjashov, G., DeGiorgio, M., Saag, L. and Wall, J.D., 2016. Genomic analyses inform on migration events during the peopling of Eurasia. Nature, 538(7624), pp.238-242.
Reyes-Centeno, H., Hubbe, M., Hanihara, T., Stringer, C., & Harvati, K. (2015). Testing modern human out-of-Africa dispersal models and implications for modern human origins. Journal of human evolution.
