The timid return of mixing in human evolution

Note: This was prompted by John Hawks’ recent post on introgression

In the pre-war anthropological imagination, human diversity was the result of the gradual mixing of primeval regional stocks. Where these stocks had come from, whether they had sprung out of the earth fully formed, no one quite knew. Human evolution was an inverted candelabrum, with many converging roots and one contemporary branch, “uniform regarding its general character but differing in special appearance” (Weidenreich 1940:381).

The Anthropological project was to disentangle this mish-mash of biologically and culturally mixed populations, and reconstruct the original contributing stocks in order to understand human history and the evolution of the Living Races of Man.

The rejection of mixing

This conception of human diversity as the result of gradual mixing of stocks was only possible because of the very strong human exceptionalism which pervaded the study of human evolution. Evolutionary scientists, even before Darwin, understood that biological diversity is the result of diversification from a common form, rather than a process of mixing from different, originally separate forms. In other words, if time flows from bottom to top of the page, the history of life looks more like the drawing of an upright candelabrum than an inverted one. Not so, until recently, for humans.

In 1942, Howells finally, timidly pointed out the absurdity with which the discipline had been living, if increasingly uncomfortably, for so long: “In other words, this principle [of diversification] might be supposed to have been the ruling one in human development, rather than to have been contravened by man as an exceptional case” (Howells 1942:191).

The implication was significant: “If this is so, it may be more judicious not to try to articulate the other human fossils too closely to Homo sapiens at present. The more dominant the factor of evolutionary radiation is taken to have been, the less reason there is for assuming any connection at all between modern man and fossil forms, excepting only through a remote common ancestor” (Howells 1942:191). And since his understated meaning was plain, that diversification was the only important process, the ancestors of modern human populations were not to be looked for in their local fossil records, and there had to be but one main line of human descent, with dead-end offshoots, such as Neandertal.

This was an important blow against human exceptionalism. It was an important step toward integrating actual evolutionary theory and the insights of the emerging New Synthesis into the study of human evolution. On the other hand, it facilitated the growth of what Brace bitterly condemned as hominid catastrophism (Brace 1964). Since human evolutionary developments had not happened locally, they must have happened elsewhere and for unknown and unexamined reasons, and any local fossil population must have been replaced by invaders with an unexplained advantage.

Anthropology might be ready for the idea that humans had evolved, in a general Darwinian sense even, but an explicit admission that actual, specific evolutionary forces were at work in human evolution, the very same that drove the evolution of other species, would still have to wait.

Not only was the inverted candelabrum of human evolution very inconvenient from a theoretical perspective, it had become politically indefensible by the late 1930s, leading to its final rejection. The quest for original stocks from which the mixed modern populations had allegedly arisen was closely allied with the idea of racial purity, which had become a weapon of war, wielded by a mortal enemy.

The single stream

Interestingly, the rejection of the idea that human populations were the result of mixing, was accompanied by the rejection of the idea that there were pure stocks or races to be found in the past. Without either mixing or purity available as explanatory devices, these two seemingly contradictory reactions were reconciled by the expedient that there was in fact one living human stock, descended from a single, insulated branch of ancestral population, surrounded by conveniently contained, very dead, and utterly extinct offshoots.

A purity was silently extended to cover the entire species, and the entire species was silently declared one stock. There was one and only one fertile channel recognized in what would eventually become the braided delta of our phylogeny. There were dissenters on both sides, of course. Coon tried to sustain the old model for a time. Wolpoff attempted to reconcile it with post-war morality and sensibilities. Montagu and a few others enthusiastically struggled to apply actual evolutionary theory to humans. Most embraced safety, accepted the canons of the post-war Council, and professed the new credo.

The ideological and theoretical grounds were well prepared for the explosive growth of the exclusive out of Africa model, and its most extreme manifestation, the African Eve model. When I was a student in the 80s and 90s, discussion of gene-flow between fossil forms, especially Neandertal and Anatomically Modern Homo was taboo and openly branded as heresy in seminars. Once breathed, “Multi-regionalist” was an indictment from which few were acquitted, and fewer still recovered. The memory of both purity and miscegenation in their pre-war senses were still raw and within living experience.

The braided stream and the return of mixing

The legacy of these ideological struggles is still with us, but their conscious memory has receded somewhat. It has receded enough that mixing, or hybridization and introgression, in their modern guise, can again be invoked in polite company. The genetic revolution, which originally enabled and reinforced our monophyletic mania, is now revealing the full beauty and complexity the braided delta of our connections with our ancestors, close and far, and of our kinship to each other in the present.

In the late 1930s, anthropology reluctantly accepted that, like any other species, modern human populations had diverged from a common ancestor. In a further welcome erosion of human exceptionalism, it is now reluctantly accepting that, just like any other species, the human lineage is reticulated, and that genetic and cultural exchange between phylogenetic cousins is one of the forces driving its evolution.

References

Brace CL 1964. The fate of the “classic” Neanderthals: a consideration of hominid catastrophism, Current Anthropology 5:3-43.

Howells WW 1942. Fossil Man and the origin of races, American Anthropologist 44:182-193.

Weidenreich F 1940. Some problems dealing with Ancient Man, American Anthropologist 42:375-383.

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