by Greg Mayer
At least since Socrates explored the meaning of the Greek maxim “Know thyself”, and Alexander Pope added that “the proper study of Mankind is Man”, we have been interested in knowledge about ourselves. But who are we? A paper in press in the Proceedings of the National Academy of Sciences by Ron Pinhasi and colleagues raises this issue with regard to Neanderthals, an issue which Jerry considered a while back: are they us?
In several senses, they obviously are us: fellow mammals, fellow primates, fellow hominids, and fellow members of the genus Homo, and thus men in the generic sense (in both the vernacular and technical senses of generic). But are they members of the same species as us, Homo sapiens? Or members of a distinct species, Homo neanderthalensis?
The question of whether they are a different species from us is the question of whether or not we could interbreed with each other. And not just mate– but successfully produce fertile offspring. For most of the time since the first reported Neanderthal in 1856, reproductive compatibility could only be inferred based on morphological data, and opinions varied as to whether Neanderthals were a subspecies of H. sapiens or a separate species. The great Finnish paleontologist Bjorn Kurten proposed in his novel, Dance of the Tiger, that Neanderthals and modern H. sapiens could mate and produce offspring, but that the offspring, while showing somatic luxuriance (they were really smart and strong), were completely sterile (a form of post-mating reproductive isolation). Published in 1980 before there was any genetic evidence, a novel, rather than a scientific paper, was probably the right venue for Kurten’s reasoned but entirely speculative proposal.
Early genetic data from mitochondrial DNA indicated that Neanderthal mitochondria were well outside the variability of modern populations, supporting the ideas of those (such as Kurten) who supposed that Neanderthals were a separate species. More recently obtained nuclear DNA sequences, however, showed that 1-4% of the genome of modern Eurasians was derived from Neanderthals (modern Africans lack this admixture of Neanderthal DNA). Thus there was enough successful interbreeding to leave a noticeable signature in modern genomes. Even more recently, a previously unknown fossil Asian population called Denisovans, related to but distinct from Neanderthals, has been shown to have contributed about 5% of the genome of modern Melanesians. Thus, measurable interbreeding occurred between anatomically modern humans and more archaic Eurasian populations as the former spread out of Africa across the remainder of the Old World.
The paper by Pinhasi et al. revises the dating of Neanderthal fossils from the Caucasus, finding them to be older than previously thought (about 40,000 years BP). The authors also suggest that other younger dates are unreliable, and that it is unlikely that anatomically modern humans and Neanderthals coexisted for any substantial length of time. If they are correct, then the Neanderthal (and Denisovan) contribution to modern genomes speaks even more strongly of conspecficity, as the gene flow had to occur over shorter periods of time. There are, regrettably, quite a few historical instances where two peoples (both of course undoubted anatomically modern H. sapiens) met, and one quickly disappeared, with relatively little measurable gene flow having occurred, so the rapid demise of Neanderthals in the face of anatomically modern humans is no bar to their having been the same species. I would interpret the genetic evidence so far as indicating that the Neanderthals, indeed, are us.
In addition to the references below, John Hawks of the University of Wisconsin, Madison, has a fine blog in which he often comments on Neanderthals and other paleoanthropological topics. His take on the Pinhasi et al. paper, which deals more with the dating issues, is here; his view on the species question is here. [JAC: I also discussed the species problem in Neanderthals, reaching the same conclusion as Hawks.]
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Green, R.E. et al. 2010. A draft sequence of the Neandertal genome. Science 328:710-722.
Green, R.E., et al. 2008. A complete Neandertal mitochondrial genome sequence determined by high-throughput sequencing. Cell 134:416-426.
Kurten, B. 1980. Dance of the Tiger. Reissued by University of California Press, Berkeley, 1993.
Pinhasi, R., T.F.G. Highamb, L.V. Golovanovac, and V.B. Doronichev. 2011 Revised age of late Neanderthal occupation and the end of the Middle Paleolithic in the northern Caucasus. Proceedings of the National Academy of Sciences in press.
Reich, D., et al. 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature 468:1053-1060.
I think I’ve met that guy in the “reconstruction”.
This raise the question of what constitutes a species.
And when does the color green become blue on the color wheel?
Colors are bad examples though, since they _are_ standardized AFAIK.*
* The interesting thing is it is done because of biology. We have different receptors for some spectral ranges, and can define response regions as we do for all responses.
Now you can start to ask what variation (different alleles) makes out of colors in the population. But that is separate from the standardization issue.
The idea that the Neanderthals and Denisovans are ‘us’ has some important implications for the age of the human species – at least in terms of how it is commonly described. Until recently the human species has been commonly described as being around for 200,000 years (or 70,000 years if you want to put the mark at the out of Africa point).
With the Neanderthal and Denisovans included the ‘human’ species becomes three to four times as old.
Great point.
What does it say (if they are us) about the out-of-Africa story? Did the Neanderthals (or their priors) also emerge from south-west Africa? If so, I imagine there must have been a lot of to and fro from Africa, over a longer time.
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@ Donn Well, of course the Neanderthals’ priors (aka ancestors) came out SW Africa. Where else would they have come from?
@Charles – Thanks. I am just an interested amateur. My sense of the story has been that Neanderthals were from the cold north (kind of Russia-ish). They sort of ‘poof’ into the picture as semi-human mysteries with a long past, but I have no feeling for the timeline.
If “1-4% of the genome of modern Eurasians” is from Neanderthals, and we all came from SW Africa, how did modern Africans come to avoid it? I suppose the N’s came first, left Africa, then modern humans developed and left to meet them in the north. Close?
Neanderthals probably left Africa before interbreeding with modern Homo sapiens. They may have more in common with Homo erectus, who left Africa even earlier.
Ok. This makes me wonder how geographically close N and Homo Sapiens where while they both occupied Africa. It’s interesting and well put in your colour wheel analogy.
Thanks for the link! It’s so funny because I have a post scheduled that links your epigenetics post from the other day!
Greg, that clever intro sentence is right out of Science News articles. If ever you’re looking for work…
Regarding the question of whether they could interbreed with humans as making them the same species – Red Deer, Cervus elephas, & Sika Deer, Cervus nippon, are not usually found in the same area as each other & are separate species, however they are easily able to form hybrid populations, so surely this is just a case of Sapiens & Neandethals being in different areas for most of their history. Indeed recent finds suggest they died out 10,000 years before it was previously thought, so perhaps contact was very limited between the two populations.
One question, from the Neanderthal genome, do we know that they had pale skin as in this reconstruction or is that just a assunption based on the latitude they lived at?
The following stories are relevant – I forgot to put the links –
http://www.nature.com/news/2011/110509/full/news.2011.276.html
for the Neanderthal dating, &
http://news.bbc.co.uk/1/hi/scotland/edinburgh_and_east/7842458.stm
for hybrid Red/Sika Deer.
Sorry – I am repeating what you said – read too quickly! apologies…
Nope, recent finds suggest they died out 10,000 years after it was previously thought, that is as thought last Tuesday.
I mean, this was known already last Thursday; you have to try to keep up. 😀
Oh, man, where have I heard “last Tuesdayism” before?…
1. Gnothi sauton is the Delphic maxim, not the Greek maxim.
2. The quotation mis-attributed to Pope is by Pico della Mirandola (from the 900 theses written in the 1490s). You probably missed because he wrote in Latin and the first translation is only now due out shortly from HUP.
Could it be that the small portion of Neanderthal genome with sequence similarity to humans be the result of parallelophyly, and the result of DNA that was shared by the population that gave rise to both H. sapiens and H. neanderthalensis? Presumably H. hiedelbergensis? Isn’t this an equally likely scenario?
No, I think the data is different to what you are suggesting.
Basically the non African human population shares a couple of percent DNA with the neanderthal genome (or rather with the neanderthal specific genome SNP markers – the actual shared complete genome sequence is going to be close to 99% plus).
The African human population doesnt seem to have these neanderthal markers – indicating that the mixing took place after humans had left Africa and before they migrated around the world – so something like 70,000 years ago (perhaps!)
An additional factor is that Melanesians (Australian aborigines and Papua New Guineans etc) seem to derive from an ancestor who also mated with another type of human that was not neanderthal – the Denisovan. The Melanesians seem to have about 4 or 5% of their genome markers derived from this source.
The model that fits the data best is that humans migrated out of Africa in several waves, including the Denisovans and the Neanderthals and then ‘modern’ homo sapiens who encountered and mated with these original emigrants as they colonised the world from 70,000 years ago onwards.
Well put.
GCM
I don’t know whether the genetic data is best interpreted as evidence of a single species or as evidence of introgression as a result of occasional hybridization between two (or more) species. In fishes there are examples of situations were two syntopic species create hybrid swarms here, but have no reproductive interaction there. I understand that mitochondrial DNA introgresses much more than nuclear DNA, but have no idea why.