Why did Neanderthals go extinct while humans prospered? There are volumes full of speculations into the decline and fall of our burly cousin who last walked the Earth 30,000 years ago. Climate change may have reduced the large herbivores on which they depended for food. Humans may have inadvertently spread lethal diseases to them when we first came into contact. Perhaps the most sinister hypothesis is that we extirpated them in an ancient act of genocide (/speciescide?).

Researchers at Oxford now argue that Neanderthal orbit size gives us an insight into the reason for their downfall. They reason that, as Neanderthals had relatively larger eyes than humans, more of their brain was dedicated to visual systems. This was an adaptation to their habitats in the higher latitudes where light conditions were poorer. This came at a cost though because the evolved brain can’t be a master of all trades, there must be some tradeoff. In this case the authors propose that the Neanderthals suffered a reduction in their cognitive abilities. This was significant because it meant that your average Neanderthal could deal with fewer social partners than a comparable human.

The impacts of this in the authors’ words, “First, assuming similar densities, the area covered by the Neanderthals’ extended communities would have been smaller than those of [humans]. Consequently, the Neanderthals’ ability to trade for exotic resources and artefacts would have been reduced, as would their capacity to gain access to foraging areas sufficiently distant to be unaffected by local scarcity. Furthermore, their ability to acquire and conserve innovations may have been limited as a result, and they may have been more vulnerable to demographic fluctuations, causing local population extinctions.”

But this proposal hasn’t gone unchallenged. Anthropologist Trenton Holliday says that by ignoring the relatively larger faces of Neanderthals the inferred larger visual brain region is mistaken. Another criticism comes from Virginia Hughes over at the Only Human blog. She points out that brains aren’t perfectly modular. So by comparing these idealised modules across species isn’t 100% informative. Perhaps Neanderthal brains were set up in a different way to process social information.

I think the visual system-cognition trade-off is something that could be easily explored in extant fauna. Think of related species that differ in latitude et voila a confirmatory or dissenting paper awaits.

Author

Adam Kane: kanead[at]tcd.ie

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