Why did anatomically modern humans replace Neanderthals?

Extinction of the Neanderthals has long been attributed to pressure on resources following the first influx into Europe by AMH bands and perhaps different uses of the available resources by the two groups. One often quoted piece of evidence comes from the outermost layer in the teeth of deer. Most ruminants continually replace tooth enamel to make up for wear, winter additions being darker than those during summer. Incidentally, the resulting layering gives away their age, as in, ‘Never look a gift horse in the mouth’! Deer teeth associated with Neanderthal sites show that they were killed throughout the year. Those around AMH camps are either summer or winter kills. The implication is that AMH were highly mobile, whereas Neanderthals had fixed hunting ranges whose resources would have been depleted by passing AMH bands. That is as may be, but another possibility has received more convincing support.

Neanderthal populations across their range from Gibraltar to western Siberia were extremely low and band sizes seem to have been small, even before AMH made their appearance. This may have been critical in their demise, based on considerations that arise from attempts to conserve threatened species today (Vaesen, K. et al. 2019. Inbreeding, Allee effects and stochasticity might be sufficient to account for Neanderthal extinction. PLoS One, v. 14, article e0225117; DOI: 10.1371/journal.pone.0225117). The smaller and more isolated groups are, the more likely they are to resort to inbreeding in the absence of close-by potential mates. There is evidence from Neanderthal DNA that such endogamy was practised. Long-term interbreeding between genetic relatives among living human groups is known to result in decreased fitness as deleterious traits accumulate. On top of that, very low population density makes finding mates, closely related or not, difficult (the Allee effect). A result of that is akin to the modern tendency of young people born in remote areas to leave, so that local population falls and becomes more elderly. The remaining elders face difficulties in assembling hunting and foraging parties; i.e. keeping the community going. Many Neanderthal skeletons show signs of extremely hard, repetitive physical effort and senescence; e.g. loss of teeth and evidence of having to be cared for by others. Both factors in small communities are exacerbated by fluctuating birth and death rates and changed gender ratios more than are those with larger numbers; i.e. random events have a far greater overall effect (stochasticity). Krist Vaesen and colleagues from the Netherlands use two modern demographic techniques that encapsulate these tendencies to model Neanderthal populations over  10,000 years.

By themselves, none of the likely factors should have driven Neanderthals into extinction. But in combination they may well have done so, even if modern humans hadn’t arrived around 40 ka. Completely external events, such as epidemics or sudden climate change, would have made little difference. Indeed the very isolation of Neanderthal bands over their vast geographic range would have shielded them from infection, and they had been able to survive almost half a million years of repeated climate crises. If their numbers were always small that begs the question of how they survived for so long. The authors suggest that they ran out of luck, in the sense that, finally, their precariousness came up against a rare blend of environmental fluctuations that ‘stacked the odds’ against them. It is possible that interactions, involving neither competition nor hostility, with small numbers of AMH migrants may have tipped the balance. A possibility not mentioned in the paper, perhaps because it is speculation rather than modelling, is social fusion of the two groups and interbreeding. Perhaps the Neanderthals disappeared because of hybridisation through choice of new kinds of mate. Some closely-related modern species are under threat for that very reason. Although individual living non-African humans carry little more than 3% of Neanderthal genetic material it has been estimated that a very large proportion of the Neanderthal genome is distributed mainly in the population of Eurasia. For that to have happened suggests that interbreeding was habitual and perhaps a popular option

See also: Sample, I. 2019. Bad luck may have caused Neanderthals’ extinction – study. (Guardian 27 November 2019)

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