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Not the Fittest

The University of California, Berkeley's Kelley Harris and Rasmus Nielsen report that the demise of Neanderthals might have been due to their decreased evolutionary fitness.

In this month's issue of Genetics, the duo uses published estimates of Neanderthal inbreeding and the distribution of mutational fitness effects to infer that Neanderthals had, on average, 40 percent lower fitness than modern humans. This decreased fitness is likely because of their smaller population size, which was due to a strong population bottleneck, and lower genetic diversity.

At Discover's The Crux blog, Bridget Alex writes that Harris and Nielsen's model is "compelling" because it fits with what's currently known about the Neanderthal population size when they met modern humans, the increased number of amino-acid changing variants in Neanderthals, and that Neanderthal DNA has largely been harmful to modern humans.

Indeed, Harris and Nielsen further report that the residual Neanderthal mutational load found in modern non-Africans humans — non-Africans have between 1.5 percent and 2.1 percent Neanderthal-derived DNA — leads to a 0.5 percent reduction in fitness.

"The blame-it-on-population size idea is theoretically and empirically supported, but rather anticlimactic for the narrative of modern humans," Alex says. "Our ancestors may have outcompeted Neanderthals, not through intellect or innovation, but through demographics — the mundane fact that we came from a larger, fitter stock."

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