More excerpts from Chris Stringer's "The Origin of Our Species"

I read this a few months ago, but never got around to posting anything on it.

On group selection:

However, more recently, biologists and anthropologists such as Paul Bingham and Samuel Bowles have returned to the issue by recruiting weaponry and genes to the cause of group selection. The argument goes that by joining together to use effective projectile weaponry, individual risks were reduced, and thus coalitions of warriors would have been advantageous for group defence and offence. Bingham proposed that this development would also have been important within societies by deterring free-riders who tried to reap the rewards of group membership without contributing their fair share of commitment to the associated costs or risks. However strong individually, they could soon be brought into line when faced with a coalition of spear-armed peers, who could act as general enforcers of within-group rules and solidarity. Bowles posited the idea that if Palaeolithic groups were relatively inbred and genetically distinct from each other, and warfare between groups was prevalent, then group selection via collaborative defence and attack could evolve and be maintained. Without warfare, a gene with a self-sacrificial cost of only 3 per cent would disappear in a few millennia, but with warfare, Bowles's model showed that even levels of self-sacrifice of up to 13 per cent could be sustained. He used archaeological data (although mainly post-Palaeolithic) to argue that lethal warfare was indeed widespread in prehistory, and that altruistic group-beneficial behaviours that damaged the survival chances of individuals but improved the group's chances of winning a conflict could emerge and even thrive by group selection. Moreover, the model could work whether the behaviour in question was genetically based or was a cultural trait such as a shared belief system. As mentioned above, Bowles's archaeological data do not come from the Palaeolithic, but there is one observation that does resonate with his views: the French archaeologist Nicolas Teyssandier has noted that the period of overlap of the last Neanderthals and first moderns in Europe was characterized by a profusion of different styles of stone points. This might reflect a sort of arms race to perfect the tips of spears, perhaps to hunt more efficiently, but equally, this could suggest heightened intergroup conflict.
On modern behavior:
In terms of innovation, we saw in chapter 1 that the apparently sudden florescence of the rich Upper Palaeolithic societies of Europe seduced many in the last century to consider that this period marked the real arrival of fully modern humans, even if areas like the Middle East or Africa had been rehearsal grounds for the revolution that was to be finally expressed in the caves of France. But as we have also seen, this Eurocentric viewpoint that the Cro-Magnons were the first "modern" people has been largely abandoned, although that is not to deny that something special did happen in the Upper Palaeolithic of Europe. If Africa was actually at the forefront of Palaeolithic innovations more than 40,000 years ago, why was that? As anthropologist Rob Foley has pointed out, the sheer size of Africa (one could easily fit China, India and Europe into its surface area), and its position straddling the tropics, certainly gave it advantages over any other area inhabited by early humans. The rapidity and repetition of climatic oscillations outside of Africa probably continually disrupted long-term adaptations by human populations in those regions. Thus Neanderthals in Europe and the descendants of Homo erectus in northern China were constantly faced with sudden range contractions and the extinction of large parts of their populations every time temperatures sank rapidly, as they often did. [. . .]

The complex climates of Africa may also explain why there seems to be no single centre of origin for the earliest signals of behavioural modernity. Perhaps North Africa (and the Middle East?) led the way 120,000 years ago, but as conditions deteriorated, populations there shrank back or even became extinct, as favoured environments rapidly vanished. Perhaps the torch of modernity was then kept alive further south at sites like Blombos and Klasies River Mouth, as conditions favoured that region for a while (give or take the interruption of events like the Toba eruption).Waves of population expansion and contraction could explain the brief but extensive florescence of the Still Bay culture with its rich symbolism, and the subsequent rise and fall of the Howieson's Poort with its innovative tiny hafted blades and engraved ostrich eggshells (recently described from Diepkloof rock shelter) more than 5,000 years later. And it is my guess (though we lack much data to support it) that East Africa became one of the next centres for behavioural evolution, about 60,000 years ago, as it was from there that modern humans (and their developing suite of modern behaviours) made their way out of Africa. [. . .]

The big picture is that we are predominantly of recent African origin, so is there a special reason for this? Overall, I think that the pre-eminence of Africa in the story of modern human origins was a question of its larger geographical and human population size, which gave greater opportunities for morphological and behavioural variations, and for innovations to develop and be conserved, rather than the result of a special evolutionary pathway. "Modernity" was not a package that had a unique African origin in one time, place and population, but was a composite whose elements appeared at different times and places, and were then gradually assembled to assume the form we recognize today.

On genetic evidence for archaic admixture:
Up to now, the big picture, from our autosomal, mitochondrial and Y-chromosome DNA, has generally lacked signs of introgression from other human species, although scientists such as John Relethford, Vinayak Eswaran, Henry Harpending and Alan Templeton have argued that indications were indeed there. Short branches in our gene trees, particularly in Y and mtDNA, have pointed to a simple, recent African origin, and simulations from mtDNA data of the level of possible Neanderthal and Cro-Magnon admixture had suggested that it was either zero or very close to zero. However, despite the fact that mtDNA and Y-DNA provide such clear genealogical signals, they constitute only about 1 per cent of our total DNA, and signs of hybridization were clearly lurking in the rest of our genome. [. . .]

A recent example of such work is the study by Jeffrey Wall and colleagues of 222 SNPs (see chapters 7 and 8) in the genes of people from West Africa (Yoruba), China and Europe. Many of the SNPs were tightly clustered, and so deviations from the expectation of them all sharing the same pattern of inheritance from a single recent African ancestral population should have shown up clearly. The majority met Out of Africa expectations, but analysis suggested that the populations did display unusual mutations in some genes, and these had different histories from each other, and when compared between the geographical samples. Wall argued that the most likely explanation was that there was not a single ancestral population for all the SNPs -- most fitted the bill, but some were apparently descended from ancestral groups that had been isolated from each other long enough to develop separate SNP mutational patterns, which had then been bequeathed in slightly different ways to the modern regional populations. Interestingly, although each showed a signal of some "archaic" (rather than recent African) genetic contribution, the strongest pattern was not in Europe (where the Neanderthals might have been the source), nor in China (where it might have come from Denisovans), but in West Africa -- a puzzling result. The work has been criticized because some of the anomalous genes might have developed via recent drift or strong selection, if the mutations were regionally advantageous, but enough have been found to convince sceptics like me that there probably was ancient admixture in Africa as well.

On Iwo Eleru:
West Africa, where the oldest known fossil, from the Iwo Eleru rock shelter in Nigeria, is thought to be less than 15,000 years old. This poorly preserved skeleton was excavated from basal sediments at Iwo Eleru in 1965 by archaeologist Thurstan Shaw and his team, and was associated with Later Stone Age tools. That latter fact alone would suggest a relatively young age, and a radiocarbon date on a piece of charcoal suggested an age of about 13,000 years. The skeleton, and particularly the skull and jaw, was studied in 1971 by Don Brothwell, my predecessor at the Natural History Museum, and he argued that while the specimen could be related to recent populations in West Africa, it actually looked rather different from them. I studied the skull for my Ph.D., with surprising results. I also found that it did not closely resemble recent African populations, but in its long and low shape it was actually closer to early moderns such as those from Skhul, and even to more primitive specimens such as Omo 2. This was decidedly odd for such a young skeleton, and so I recently collaborated in a new study of the specimen with archaeologist Philip Allsworth-Jones, dating expert Rainer Gruen and anthropologist Katerina Harvati. We first checked with Thurstan Shaw whether there were any hints that the skull could have been much older than previously suggested, and there were none. With the help of Nigerian archaeologist Philip Oyelaran, I obtained a fragment of bone from the skeleton and passed it to Gruen in order to check its age directly. His determination from a direct uranium-series age estimate is that the bone is unlikely to be older than 20,000 years, consistent with the stratigraphy, and associated archaeology and radiocarbon date. Finally, could Brothwell and I have been wrong about the unusual shape of the skull? Harvati used state-of-the-art geometric morphometric scanning techniques on an exact replica of the skull (which is now in Nigeria), and found, as we did, that it was quite distinct from recent African crania, and indeed from any modern specimen in her comparative sample. Her results placed the skull closest to late archaic African fossils such as Ngaloba, Jebel Irhoud and Omo 2 -- all thought to be at least 140,000 years old. So what does this mean? Because of the poor preservation of Pleistocene bones in West Africa, we have no other data on the physical form of the inhabitants of the region during the whole of the Pleistocene, so we have to be careful in interpreting an isolated specimen such as Iwo Eleru. But it does not seem to be diseased or distorted, and does indeed seem to indicate that Africa contained archaic-looking people in some areas when, and even long after, the first modern-looking humans had appeared. Support for this view comes from the work of anthropologist Isabelle Crevecoeur. Her restudy of the numerous Ishango fossils from the Congo has shown that these Later Stone Age humans were not only similar to Iwo Eleru in age, but also in the surprisingly archaic features found in their skulls, jaws and skeletons. [. . .]

Africa today has the greatest internal genetic variation of any inhabited continent, and its skull shapes show the highest variation. This is usually attributed to its greater size, larger ancient populations and deepest timelines for humanity. But could those timelines go back even further than we thought? Did the early modern morphology evolve gradually, and then spread outwards from a region like East Africa, completely replacing archaic forms within Africa, and then outside (as mtDNA data would suggest)? Or, could there have been a version of assimilation or multiregional evolution within Africa, with modern genes, morphology and behaviour coalescing from partly isolated populations across the continent? Given its huge size, complex climates and patchworks of environments, Africa could have secreted distinct human populations just as easily as the rest of the inhabited world. So was the origin of modern humans there characterized by long periods of fission and fusion between populations, rather than representing a sudden single event? And was the replacement of the preceding late archaic peoples not absolute, so that they were partly absorbed by the evolving moderns rather than completely dying out? In which case, did early Homo sapiens forms, and even the preceding species, Homo heidelbergensis, survive alongside descendant modern humans?

Possible reason we don't have pigmentation genes from Neanderthals:
If the interbreeding actually happened earlier, in a warmer region or a warmer period, maybe the Neanderthals involved were not light-skinned and cold-adapted European examples? In fact, the interbreeding might even have happened when people like those from Skhul-Qafzeh and Tabun were in the Middle East 120,000 years ago. If a thousand of those early moderns mixed with just fifty Neanderthals and then survived somewhere in Arabia or North Africa, could they have subsequently interbred with the Out of Africa emigrants 60,000 years later, and passed on their hidden component of Neanderthal genes?

1 comment:

Steve Sailer said...

Stringer writes: "Given its huge size, complex climates and patchworks of environments, Africa could have secreted distinct human populations just as easily as the rest of the inhabited world."

I have a romantic attachment to the idea of isolated refuges, but I've never been able to think of any in sub-Saharan Africa, other than the Ethiopian highlands.