Pruning the family tree, chance & inevitability

Gene ExpressionBy Razib KhanMar 11, 2011 6:49 AM


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I picked up Clive Finlayson's The Humans Who Went Extinct: Why Neanderthals Died Out and We Survived mostly for its alternative history value. By this, I mean that it was published in the fall of 2009, less than a year before research which suggests that not all Neanderthals went extinct, in that ~2.5% of the genomes of non-Africans derive from this human lineage. Going by the subtitle I'd assumed that Finlayson's treatment would be useful, despite its likely false premise. Of course the author was no fool, he was channeling the consensus of his time, even citing the mitochondrial DNA studies which indicated no admixture between between the Neanderthal and modern human lineages. But to my surprise the overturning of a central presupposition of the book did little to alter the theses of Finlayson's narrative. In many ways the subtitle was something of a bait & switch. The author clearly was only reluctantly working within the "Out of Africa" framework, which he believed fostered sloppy and incoherent thinking. From what I can tell this seems to be a function of Finlayson's background in paleontology and ecology, rather than genetics. This has some unfortunate consequences, as some of his conceptual assertions wander into a woolly "not even wrong" zone. I simply couldn't understand what was being said on occasion because The Humans Who Went Extinct seems to treat evolutionary genetics as a background prop. I also often had a difficult time following aspects of paleoecology grounded in a deep and thorough understanding of the fossil record and the nature of ecosystem flux and turnover. For someone with my background the impression was of being bombarded by a stream of difficult to assimilate facts, as well as broad overarching theories which were relatively clear and transparent. Unfortunately there was a relative scarcity in comprehension in the "middle-ware" region between thin theory and thick description, because I'm not someone used to thinking in terms of fossils and behavioral ecology. There seem two large theoretical assertions which serve to give overall structure to the argument. First, the importance of contingency and chance in evolution. A climatic or geological event could shift ecologies such that whole taxa die off, while others expand their range. This speaks nothing to inevitability in the proximate sense. But over a longer time scale, there is the second assertion: that generalist, opportunist, lineages in liminal ecological regions between "cores" across the range of a taxon may have a long term fitness advantage because of their flexibility. Note that this second point is at some contradiction to the first. Over the long term it may be inevitable that average time until extinction is higher for specialized lineages than for generalist lineages. But, that extinction event exhibits a probability distribution, and is not inevitable at a specific time. It is when The Humans Who Went Extinct ventures into broad philosophy-of-biology questions that the narrative becomes difficult for me to follow, and in many ways this is all extraneous to the scientific argument within the text. But there is a reason to the rhyme. The author is particularly peeved by the depiction of anatomically modern humans as Übermensch who swept all the inferior "archaic" lineages before them. On the contrary, he suggests that lineages such as Neanderthals were extremely successful and resilient, and that the decline of this subspecies had more to do with longer term changes in climate and ecology than the emergence of a Homo superior. In fact, there were several subspecies of humans which flourished for most of the history of our lineage. Finlayson clearly prefers a "bushy" topology for our family tree. Throughout the text of The Humans Who Went Extinct he reiterates over and over the ways in which the anatomically modern humans of Africa ~150,000 years B.P. were no different from Neanderthals in Europe in technology, cranial capacity, and a host of other traits. He posits that the demise of Neanderthals across northern Eurasia had more to do with exigencies of paleo-climate than competition with anatomically modern humans. Contrary to conventional perception Neanderthals did not inhabit the tundra. Rather, their optimal zone was the northern woodlands, where dwelt their prey. They were ambush predators. Finlayson asserts that as the Pleistocene progressed the cold dry periods became more extreme, and the temperate woodlands which Neanderthals favored became progressively more constrained. Eventually the last Neanderthals were boxed in between the Mediterranean and the tundra at Gibraltar. Unable to cross the straits these relict groups eventually went extinct. Where were anatomically modern humans during this period? Finlayson refers to these as "proto-Ancestors" or "Ancestors," and asserts that by and large they did not overlap with Neanderthals much, and generally engaged in similar behaviors as their northern cousins in any case. In particular, he exposes common prejudices and preconceptions where more advanced archaeological finds are attributed by modern humans simply because their advanced state would not comport with production by Neanderthals. Many of the artistic hallmarks of anatomically modern humans which signaled "behavioral modernity" during the "Great Leap Forward" ~50,000 years ago were not found across much of the world inhabited by modern humans, nor were necessarily produced by the Mesolithic Europeans who succeeded their indubitably behaviorally modern predecessors. Once behavioral modernity is crested the presupposition is that populations possess it in a categorical sense, but Finlayson suggests that there is an inverse burden of proof on "archaic" populations such as Neanderthals. I think this observation about the sociology of science has some merit. Recall that a few years ago scholars routinely wondered whether Neanderthals had spoken language. With the recent findings from the Neanderthal Genome Project this seems more and more likely, and, the possibility of admixture with Neanderthals also probably increases our estimate. But it goes to show how far off the beaten path Neanderthals had been forced. So what about the lucky winners? Here the story turns in particular upon the Gravettian culture of northern Eurasia, which flourished ~30-20,000 years B.P., during the height of the Last Glacial Maximum. The Gravettians were the ones who opened up the tundra to human habitation, and were hunters of mega-fauna. This was a culture which outflanked the last Neanderthals, and was the inverse of a specialized culture suited to particular conditions. Rather, Finlayson depicts the Gravettians as mobile and social generalists, adapting to inclement conditions as no hominid had before. Gravettian-inflected culture in The Humans Who Went Extinct has a long arm, stretching from the Atlantic and reaching the New World. Eventually the descendants of the Gravettians also pushed into South Asia as well, admixing with the older non-Gravettian anatomically modern humans. There is one major problem with Gravettians Über Alles: far less of the world's population derives from Gravettians than Finlayson assumes. This is because he relies on the model in The Journey of Man, common in the early aughts, that anatomically modern humans in Central Asia developed a revolutionary cultural toolkit, and then spread both west and east, leading to Europeans, and East Asians and Amerindians. This story, based in large part on Y chromosomal lineages, is I think false based on the phylogenetics. Total genome analyses tend to indicate that East Asian populations are somewhat closer to Oceanians than they are to West Eurasians & North Africans. If you want cutting edge human population genetics then the The Humans Who Went Extinct is not for you. But, if you want a thorough exposition of fossils and models of paleo-behavioral ecology, then it is. In particular, Finlayson argues that in the Gravettians you saw the instantiation of a low time preference and hyper-social group of humans. The tundra was amenable to hunts of mega-fauna, but only if large numbers of humans cooperated. In contrast, the ambush hunting which the Neanderthals specialized in was not scalable in such a fashion (large groups congregating together does not a successful ambush make). Not only did human groups need to cooperate on a large scale to make tundra hunts economically rationale, but that rationality could manifest in building up long-term stores of protein. The cold dry conditions of the tundra were optimal from preservation of meat. Once the Gravettians stumbled upon the productivity of low time preference strategies, a later expansion into southerly regions stimulated innovations such as preservation through curing and drying. Overall, this is a sloppy, ad hoc story. It is thick in minute detail, and nearly incoherent on the broad theoretical frameworks. But, this is superior and more honest than the clean and elegant models which have been in vogue the past generation (e.g., a genetic mutation in the ancestors of modern humans gave them language, which they used to defeat all other human populations). The rise and spread of modern humanity, as we understand it, was a sloppy affair and lacked elegance. Deep population structure in modern humans, or admixture with long isolated lineages, attests to this reality. The Humans Who Went Extinct characterizes the Ancestors as full-spectrum generalist predators of the north, rather like wolves. They battled nature, and won. In contrast, the Neanderthals and other archaic populations were the specialist pandas, well adapted to local conditions in time and place, but ill-equipped to shift strategy in new circumstances. This is a neat story, but Finlayson explicitly dismisses inter-population competition, and implicitly intra-population competition, as driving forces in human evolution. I think this is a major problem, and derives from the author's thin knowledge of evolutionary genetics. Population fitness is not measured just against the elements, but against other populations, and across individuals within a population. We now know that we copulated with Neanderthals. I see no reason why we could not make war if we could make love.

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