Where Do We Come From?

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Remember how European explorers defined different races based on a few stops along African and Asian coasts – when in fact there were clines and continuous distributions? But the labels they came up with lasted long after scientists realized that skin color, for example, did not neatly fall into categories of blacks, white, red and yellow. I suspect we have been doing the same thing with human ancestors, so “Neanderthals” were a single sample of a much larger distribution of variation. As the authors point out so well, we have a non-random distribution of samples heavily weighted towards Europe and Africa, and our view of human evolution is going to change dramatically as the blank spots in time and space get filled in. This leads me to wonder at the hubris of anthropologists who name a new species – long before we know enough about the context to tell if it is really just a population. Thanks to the authors for casting light on a set of linked issues and open questions. We don’t know so much more than we know! But none of this is going to stop NatGeo from trumpeting the next fossil find as a “lost human ancestor.”

The authors begin on a good note by noting that species is the only taxonomic level with a definite biological foundation, namely the restricted ability of one species to successfully interbreed with another species. They then muddy their argument by accepting the use of the term species to designate morphological differences between one population and a descendant population (allopatric species) without noting that the non-interbreeding criterion does not apply to allopatric species, except in the vacuous sense that they cannot interbreed since they do not coexist. This difference between sympatric and allopatric species means that selection for one population to not interbreed with another population, the basis for sympatric speciation, does not apply to allopatric species. Consider the following simplified example. Suppose a species has a quantitative trait controlled by a single locus with two alleles, a and A. Assume that of the three possible values for the quantitative trait due to the three genotypes aa, Aa and AA, the aa value is optimal for the species only in ecological zone 1 and the AA value is optimal only in ecological zone 2, but the Aa value is sub-optimal in both ecological zones (hence there would be selection for aa individuals to forage in zone 1 and for AA individuals to forage in zone 2). Suppose, initially, there is a single population with access to both ecological zones. Of the possible mating types, both the aa x aa and the AA x AA mating, only giving rise to aa and AA offspring, respectively, will be optimal in comparison to all other mating types due to the suboptimal Aa genotypes produced by these other matings types, regardless of ecological zone. Hence, there will be selection for any trait that reduces the likelihood of individuals mating with individuals having a different genotype, thus, there will be selection for the population to bifurcate into two species. Obviously, the same argument does not apply to allopatric species. Differences in fossils obtained from hominin populations ancestral to Homo sapiens have been used to characterize ancestral hominin populations. These are allopatric species, hence they did not arise through selection for speciation and instead reflect morphological change in an evolving population of hominins through time. This also means that these allopatrically defined hominin species may simply be an artifact of sparse sampling due to the difficulty in discovering hominin fossils. Consider a population with a morphological trait changing continuously through time. With sparse sampling of this evolving population, the more ancient specimens will appear morphology different in comparison to the less ancient specimens and this difference may then be treated as if it were an evolutionary event localized in space and time in which one species became the other species even though there was just a single population changing continuously through time. Thus, asking for the origin of the species, Homo sapiens, as if it arose through a distinct evolutionary event is not, as the authors argue, a meaningful question. Equally, the presumed sympatric species difference between Homo sapiens and Homo neanderthalensis, needs to be shown to arise through selection for a speciation event, especially since the differences may simply be due to geographic isolation. Not surprisingly, the DNA evidence shows that the populations involved could and did interbreed when hominin ancestral populations of Homo sapiens from Africa came in contact with hominin populations of Neanderthals in Europe and Asia. The authors rightly question assumptions that require presuming that species defined allopatrically or on the basis of geographic isolation also have biological reality as sympatric species.

This is a terrific essay that helped coalesce several lines of scholarship of which I had only a peripheral understanding. Looking forward to catching up with the contributing papers and the author’s own work.

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