The human family is a diverse bunch, featuring several different species living alongside each other. A few years ago, enigmatic finds from the Red Deer Cave in China revealed this co-existence may have been going on as recently as 14,000 years ago.
Our species was on the brink of settling down and starting civilisation, whilst a completely different species was still happily living alongside us. A weird species with massive cheekbones.
Or maybe not. The skull of the species is rather weird. However, the original paper noted that for the most part this weirdness did not push it outside of norms seen in some early African fossils (with the exception of the massive cheekbones. Those were genuinely weird).
Could this simply be a late-surviving example of an early modern human? New research suggests its actually a bit more complicated than that. In a really sexy way.
The messy origins of modern humans
Part of the issue of understanding where Red Deer Cave fits in human evolution is the fact that human evolution is a rather messy thing.
Even relatively recently, there were all sorts of different populations living side by side. There are dozens of "weird" fossils in Africa and the Middle East that appear to show a mixture of old and new traits, a lot like Red Deer Cave (except not quite so many old, weird features). The status of those other weird fossils has a big impact on what we make of these Chinese fossils.
If we count them as full-blooded Homo sapiens then accepting the Red Deer Cave fossils as Homo sapiens becomes that little bit easier. On the other hand, some suggest that these might be "relic" populations. Old leftovers from before true modern humans emerged. This helps explain why they look so weird and retain all those old traits.
If this is the case, then the Chinese fossils could also belong to that category. Which itself raises a whole host of new problems given how much more recent the Red Deer Cave fossils are. The other African fossils are closer to 100,000 years ago. How could these "relic" populations survive for so long?
Many place the other "relic" fossils in various sub-species of human; which only serves to exacerbate this messiness. How are they connected to modern humans? Are they a sister sub-species? Our ancestors? Failed descendants? Did we absorb them into us, or did they go extinct?
And where does Red Deer Cave fit in?
Possible explanations for Red Deer Cave
When all of this messy evolution is taken into account, a few possible explanations for the Red Deer Cave fossils emerges.
The first is based on the fact that it contains a lot of traits from older hominin species. This is part of the reason it's so darn weird in the first place. These features include its weird wide face, chunky teeth, a relatively small(ish) brain, and much more. All of these old traits could indicate that this fossil is actually just an old species. These earlier populations left their home in Africa (and arriving in China shortly after their departure) in several "waves" of migration over a period of about a million years. Perhaps one of these waves managed to cling on for a few hundred thousand years; only going extinct relatively recently.
The second explanation is almost the exact opposite, being based on the fact that it contains a lot of traits that seem more similar to modern humans. In various statistical tests performed when the species was first discovered, most aspects of the skull were found to fall on the edge of variation in Homo sapiens. So despite being weird, this fossil is actually just a boring old modern human. However, it retained a few traits from earlier populations; giving it that classic weirdness we've come to know and love.
Or maybe it's something else entirely. New research raises a very interesting possibility.
Stupid sexy fossils
None of the existing explanations are particularly good. It seems too modern to be a long lasting old species, yet has too many old traits to be a weird modern species.
A recent re-analysis of the skull confirmed that it didn't fit into either camp. The authors subjected a bunch of these old/modern skulls (along with a few definitely old and definitely modern ones) to a barrage of statistical tests. These tests consistently picked out certain skulls as either definitely modern humans or definitely not. Often with confidence levels of >80%.
Except for the Red Deer Cave skull. The tests identified this as non-human, but with a probability of only ~50%.
Which is where the new idea comes from. Since it seemed impossible to put the Red Deer Cave individual into a nice neat category, the authors concluded that perhaps it doesn't belong in a category. That it's actually a hybrid.
Genuine "older" species lived until relatively recently outside of Africa. Perhaps as one group of humans was migrating across Asia they bumped into these remnants and had some weird, wide-cheeked babies. They reinherited all these older traits, hence the weird combination seen in these fossils.
Ordinarily I would be a skeptical of these claims, but the Red Deer Cave fossils are just so weird (and so definitely not fitting into any one species) that it seems plausible. Plus it's not like this is without precedent. Modern humans are renowned for their inter-species relationships.
Just ask the Neanderthals.
tl;dr
Weird fossils from Red Deer Cave may actually be evidence of modern humans mating with an yet another different species.
References
Curnoe, D., Xueping, J., Herries, A.I., Kanning, B., Taçon, P.S., Zhende, B., Fink, D., Yunsheng, Z., Hellstrom, J., Yun, L. and Cassis, G., 2012. Human remains from the Pleistocene-Holocene transition of southwest China suggest a complex evolutionary history for East Asians. PLoS One, 7(3), pp.1-28.
Curnoe, D., Ji, X., Taçon, P.S. and Yaozheng, G., 2015. Possible signatures of hominin hybridization from the early Holocene of Southwest China. Scientific reports, 5.
Curnoe, D., Ji, X., Liu, W., Bao, Z., Taçon, P.S. and Ren, L., 2015. A Hominin Femur with Archaic Affinities from the Late Pleistocene of Southwest China. PloS one, 10(12), p.e0143332.
