Allopatric Speciation in the Genus Homo

The uniformitarian principle applies above all to catastrophism

Sooner or later you may find out that subspecies are recognized by systematic biologists in many thousands of species. In fact, the standard theory of speciation is allopatric speciation. This theory of speciation was what economists call a “folk theorem” before Mayr used it systematically to clear up a lot of muddle in many genera in his work (1942, 1963). The idea is that populations of an existing species become genetically isolated from each other because of geographic barriers to gene flow (“allopatry”), usually as a result of the range expansion of species, although barriers may emerge due to external innovations in the species’ environment. Genetic isolation leads to neutral drift and diversifying selection. So, genetically isolated populations become more and more differentiated in both genotype and phenotype over time. At some point they become so genetically differentiated that purifying selection begins to work strongly against hybrids; eventually, with complete lethality, at which point they must be conceded to be good species — capable of complete sympatry without interbreeding in the wild (“friends without benefits”). Long before achieving species status, however, geographically-situated populations may become differentiated enough to warrant subspecific taxonomic recognition — a third formal name, the trinomial.

A well-recognized case of allopatric subspecies is Eastern chimps (Pan troglodytes schweinfurthii) and Western chimps (Pan troglodytes troglodytes), the latter of which was the first to be recognized and hence bears the name of the species as the subspecies name by longstanding convention in systematic biology.

A strict uniformitarian would insist that subspecific taxonomic distinctions be admissible across species; including our own. Obviously, this is jarring to antiracist ears. Paleoanthropologists have stopped using the subspecies concept not just within H. sapiens, and not just within the genus Homo, but in the Hominina subfamily that includes six other extinct genera of hominids. Interesting boundary work is required between the human and the non-human: subspecific taxonomic distinctions are de facto inadmissible in all extinct genera associated in some way with human origins. On the inside, Neanderthals are species; so are Denisovans. You can have species aplenty in the phylogeny, but subspecies are not recognized as if by silent agreement. In fact, back in the mid-1950s, there was a whole war on the pages of Systematic Biology over the usefulness of the trinomial system — on the very concept of subspecies.

Beyond the discipline imposed by ideology and language policing, however, there is good reason to suppose that all members of our species are in some fundamental sense indistinguishable. The reason is the Boas-Chomsky universal, the correct name of our capacity for language and innovation, for reasons I have had occasion to mention.

We find language, dreamworlds, stories, traditions, song and dance across anthropological populations because, Chomsky argued, of the uniformity of the language capacity across our species. Because Boas was the first to observe anthropological universality of culture in the thick sense and Chomsky the first to explain Boasian universality by the uniformity of the language capacity in our species, I have called it the Boas-Chomsky universal.

Policy Tensor, “The Puzzle of Human Origins.”

But, it may be countered, presumably other animals too have their universals, characters which may be equally diagnostic, yet we do recognize subspecies across the animal kingdom. Why, then, should we shy away from using the same reference frame on us? What makes us so special? Did Copernicus not banish this nonsense for good?

OK, fine. Let’s admit subspecies as a scientific reference frame with regard to humans without much more by way of apology. What does modern science have to say about this? albeit, in code, given the politics?

A standard barometer of genetic distance in population genetics is F-st. The Eastern and Western chimps is a fair model for subspecies in humans — chimps are our closest relatives and their subspecies were identified long before the rise of Boasian antiracism. Their genetic distance is 0.32 (100%). By comparison, the genetic distance between Italians and Chinese populations is 0.09 (28%). The Hausa, an Afro-Asiatic speaking population in Nigeria, are roughly equidistant from the two non-African populations — Italians: 0.14 (44%), Chinese: 0.15 (47%). So, by this criteria, the case for a subspecies level distinction between Europeans and Asians is a quarter as strong, and that between Eurasians and Africans is half as strong, as the case for recognizing Eastern and Western chimps as distinct subspecies.

Another criterion, is splitting time (“coalescence time,” ie time-depth of “the last common ancestor,” a tree concept) between the populations. The human/chimp splitting time is about 6.5 million years ago (Ma). The Eastern and Western chimps are estimated to have split about 1 Ma. By comparison, the Out-of-Africa adaptive radiation of H. sapiens, and therefore the splitting time of non-Africans from Africans, is dated to only 0.05 Ma or fifty thousand years ago (ka).

So, we can constrain the splitting time of non-Africans to ~50ka. A case can be made for an additional, earlier dispersal whose relics are found in eastern geographic isolates today. Specifically, the so-called “Negrito” populations in the Indo-Pacific region (eg, Andaman Islanders) and the those of Sahul (the prehistoric supercontinent revealed by the low sea levels during the glacials, comprising today’s Papua New Guinea, Australia and Tasmania), came in an earlier dispersal when Eurasia was still populated by Neanderthals, Denisovans and others. This may have occurred possibly tens of thousands of years before the main Out-of-Africa expansion. Populations native to Sahul also have very high Denisovan and Neanderthal admixture (~10%, compared to 2% in other non-Africans). More important than the lower bound, is the upper bound on the time-depth of the first dispersal — our deepest lineage out of Africa. Most estimates seem pretty modest: ~50-70ka. But numbers as high as 90-130ka have been thrown around. In either case, the time-depth of the oldest colonists out of Africa is unlikely to be much more than a tenth that of the Eastern and Western chimps.

Notice that, if subspecies are to be recognized in humans, the very top of the phylogenetic tree has a special position. For it is the most deeply divergent population, the one with the deepest splitting time with the rest, that has the strongest claim on subspecies status. The deepest splitting time is with the San, 0.3 Ma. For reference, the splitting time of Eastern and Western chimps is 1 Ma, about the same as that between Neanderthals and H. sapiens.

The closest the doyens have come to offering a subspecific taxonomy for our species is when Chris Stringer and coauthors suggested a broad breakdown into five:

The major distinguishable strands of present-day human ancestry could be summarized as including those associated with populations in West Africa, East Africa, the Central African rainforests, southern Africa and the world outside of Africa.

Bergström et al. Nature, 2021.

This is the implication of the discovery of deep population structure in Africa. Populations ancestral to all non-Africans carried away only a small fraction of African genetic diversity, which further dwindled due to the serial founder effect. But the splitting time of non-Africans is so recent as to make their inner distinctions too minor to count. The deep population structure of Africa is the deep population structure of our species — for we are all, in the final analysis, Africans.

One can bicker with Bergström et al.’s taxonomy. There’s a claim to be made to distinguish between Southern and Northern San, as suggested by Hollfelder et al.. In a hypothetical uniformitarian world, they may be an even stronger claim for subspecific taxonomic recognition of the ‘first wave’ populations who stand apart from other non-Africans.

The East-West split dated by paleoanthropologists to 40ka had provided something like a birth certificate to the “racial” distinction between Whites and Asians in the Western imaginary. That goes completely out of the window in the conversation between experts. It’s just too insignificantly recent to be considered for taxonomic attention.

But is the implicit taxonomy of Stringer and others quite correct? Is the case not, in fact, stronger for recognizing Sapiens as the African subspecies H. sapiens sapiens, Neanderthals as the European subspecies, H. sapiens neanderthalensis, Denisovans as the Asian subspecies, H. sapiens denisova —  of a single species of highly-encephalized, Middle Paleolithic populations with a range extending from the temperate tip of southern Africa to Siberia? After all, the splitting time is within the same ballpark as that for Eastern and Western chimps. Moreover, the considerable evidence of successful interbreeding suggests that they may not be good species, while the evidence for selection against Neanderthal DNA suggests strong purifying selection against hybrids. So the case is quite strong for subspecific taxonomic recognition on standard criteria for many Late Pleistocene hominin “species.”

I am in favor of doing this officially not just to rid of the repression of a perfectly useful scientific concept, but also because the story then adds up at all levels. There’s no need to overthrow Copernicus. Humans are not an exception to the reign of science. There is no scientific anomaly. Not even in the adaptive radiation of one subspecies and the attendant extinction of others.

The human race is the African race; we are H. sapiens sapiens. The other geographic races went extinct, likely because of us; a story that has since been repeated many times beginning with the adaptive radiation of Neolithic populations that wiped out most of the world’s language families; all of it culminating in the fury of the European range expansions during the ethnographic present. The uniformitarian principle applies above all to catastrophism.