Humans may have been evolving for millions of years, but some of our traits only developed recently. I’m always fascinated by these modern traits, they’re often the last thing you expect. Who would’ve thought that our forehead is a recent development? Or our chin? These seemingly inconsequential attributes are what help distinguish modern humans from our close relatives, such as Homo neanderthalensis or Homo heidelbergensis (Boyd and Silk, 2009). Maybe then obesity is something to be embraced. As we develop more chins we’re just becoming more human.
A human ancestor (Homo heidelbergensis). Note the sloping, un-human-like forehead and chin.
That said there are some Neanderthals that appear to have developed more human-esque chins. Many that lived 30,000 – 45,000 years ago (just before their extinction) have a chin quite similar to that you or I possess (Condemi et al. 2013). This raises all kinds of interesting possibilities. Did the benefits of a chin lead to its development in both lineages? Was it just chance? Or was there interbreeding between the two species?
The last option is particularly interesting. We’re all fascinated by the sex lives of our ancestors, aren’t we? However, it was also the least supported by the evidence for many years. The earliest human occupation of Europe seemed to occur after the appearance of these human chins, so they couldn’t have arisen via interbreeding. However, recent discoveries have revealed that humans actually migrated into Europe ~45,000 years ago (Benazzi et al., 2011). This earlier migration means humans arrived early enough to have been the source of chins in Europe (now there’s a sentence I never thought I’d write).
As an aside, the redating itself is fascinating and probably worthy of a blog in its own right. The first definitive human toolkit in Europe is the Aurignacian, which contains many new technologies never seen on the continent before. However, it is preceded by “transitional” industries which contain “primitive” versions of Aurignacian technologies. For years there was debate over whether these industries were the result of Neanderthals developing the technology independently, borrowing it from humans or whether it was humans who had arrived early. Benazzi et al. (2011) concluded it was the latter based on the new discovery of human remains with these transitional industries.
A sample of Aurignacian tools, the first made by humans in Europe
Anyhoo, all of this means that humans are in the right place at the right time to have interbred with Neanderthals and thus introduced the chin into some populations. But is there any way to tell this is what actually happened? Finding a Neanderthal with an almost perfectly modern chin would be pretty strong evidence for it. Chance or convergent evolution rarely result in completely identical traits.
Fortunately for those of us who want to know about the private lives of our ancestors, Condemi et al (2013) report on an Italian Neanderthal with a chin that falls slap bang in the middle of the human range. This provides strong evidence that it is the result of human interbreeding. Unfortunately all we have of this individual is the lower jaw. It’s hard to tell species apart based purely on the jaw, particularly when the jaw is notable for being so similar to another species! It may be they aren’t Neanderthal at all.
The left grouping is human jaws, the middle is Neanderthals and the right is Homo heidelbergensis. The Neanderthal jaw in question is Mezzena
So Condemi et al. also analysed the genetics of their jaw bone. They were only able to recover mitochondrial DNA (since there are more copies of that and so it is more likely to survive intact. Interestingly that’s also why claims that genetics have identified a bigfoot or alien hybrid report the mtDNA of the specimen is normal. It’s the most accurate data and it reveals nothing is afoot, surprise surprise. Don’t pardon the pun, it was awful).
This mtDNA showed that they had a full-fledged Neanderthal on their hands, not a human with a funky chin. Unfortunately this means didn’t provide any evidence of human DNA being mixed in there. But then mtDNA is only inherited from the mother, so we can’t rule out the guy had no human genes at all. They could just be present in the nuclear DNA, part of which is inherited from daddy. Plus mtDNA doesn’t contain the genes for the chin, so we wouldn’t expect any human DNA to be there anyway.
But there are less copies of nuclear DNA in each of us, so it’s more likely to have broken down to the point where it is irretrievable And that is what happened in this case, there was no nuclear DNA left to look for human genes in. Without this nuclear DNA confirming there was a hybirdisation event this evidence remains interesting but inconclusive. Humans were in the right place at the right time to introduce chins into Neanderthal populations, populations which know had very human-like chins. So there is a strong possibility interbreeding between humans and Neanderthals was occurring here, but a possibility is all it remains for now.
Nonetheless, the fact that there is such a possibility is intriguing. All the evidence we currently have shows that the interbreeding between humans and Neanderthals was one way, with them introducing genes into us. If it could be shown that it was more of a two way exchange then this could revolutionise how we view interactions between our species.
The Neanderthal chin in question
References
Benazzi, S., Douka, K., Fornai, C., Bauer, C. C., Kullmer, O., Svoboda, J., … & Weber, G. W. (2011). Early dispersal of modern humans in Europe and implications for Neanderthal behaviour. Nature, 479(7374), 525-528.
Boyd, R., & Silk, J. B. (2009). How Humans Evolved. WW Norton & Company, New York.
Condemi, S., Mounier, A., Giunti, P., Lari, M., Caramelli, D., & Longo, L. (2013). Possible Interbreeding in Late Italian Neanderthals? New Data from the Mezzena Jaw (Monti Lessini, Verona, Italy). PLOS ONE, 8(3), e59781.
EvoAnth 
Interesting!
What if most of us human, are just neanderthals with a chin, due to miscegenation with early H.S & other arcaics hominids, some of them extinct, during the last 60 000 – 30 000 yrs ago?
The genetic differences between modern humans and Neanderthals are large enough that this is unlikely.
Well, humans and chimpanzees are something like 99% genetically identical, and sapiens and neanderthalensis/sapiens neanderthalensis are even more so.
At the same time, there are several possible gradations between “multiregional hypothesis” and “recent African replacement”, and even the RAR accepted hybridization, only stressing that it could have been small and don’t leave genetic signs that it ever occurred.
So perhaps the main objection to the notion wouldn’t be much “genetic difference” from neanderthals, but that the remaining neanderthal inheritance is very small, not necessarily specific to neanderthals, and that populations could even be technically descendants of neanderthals with many individuals not having any genetic trace of it. It starts to become more or less like saying a white person is “African” or “Asian” because he or she may have had a great-great-great-great-parent that was African or Asian, even though no one could tell that by looking at the phenotype or even pick that up with 100% accuracy with a genetic analysis. It’s arguably the most extreme standard of “hyperdescent” ever (or perhaps sapiens hypodescent).
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I think that in a paper by Tattersall or someone else it’s said that neanderthal’s proto-chins/weak chins are likely to be homoplasies|”convergences” (if I recall there wasn’t speculation about function of the chin, so it doesn’t imply “adaptive”/selective convergence, could be just “convergence of form” for randomness or different adaptive forces), because they have a different morphology and they show up earlier in the development in Homo sapiens.
It doesn’t really say anything/much about what “sapiens chin genes” sneaking into the neanderthal population would look like, though. Could well be that it wouldn’t look as homologous, and perhaps it could be “postponed” in the embrionary development. And if we’re actually saying of a small sample/isolated incidents of hybrids then the developmental timing aspect is really clouded, as we don’t have any data for a developmental series in the same individual or very closely related individuals thought to be such hybrid instances.
But I think that perhaps the bulk of “plain neanderthalness” in these fossils could rule out the notion of they’re being likely “instances” of “F[ilial] small number” hybrids , rather than just having some “creeping sapiens genes” after a long chain of hybrid gradations and generations.
There’s also some evidence from Atapuerca that there were Neanderthal/human homoplasies in the teeth; so it’s not an implausible idea. I’m not sure I gave it enough coverage in the post, but I’ll be sure to mention it more the next time a “hybrid” is found. That said, given we do know some interbreeding was occurring, I wonder which explanation becomes the more pragmatic and least likely to be cut by occam’s razor.
Also, fixed your post.