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.]
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.