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Models shift blame for Neanderthal extinction away from modern humans

The notion that Homo sapiens (background skull) drove Neanderthals (foreground skull) extinct has fallen out of favor. Image credit: Shutterstock/Petr Student

The notion that Homo sapiens (background skull) drove Neanderthals (foreground skull) to extinction has fallen out of favor.             Image credit: Shutterstock/Petr Student

Neanderthals vanished about 40,000 years ago, but the reasons for their demise remain shrouded in mystery and a source of debate among archaeologists. The timing coincides with the arrival of modern humans in Europe, which has led some researchers to surmise that interactions between the two groups both led to interbreeding and hastened Neanderthal extinction.

But maybe Homo sapiens weren’t to blame. Other factors were likely more significant, say researchers at Eindhoven University of Technology, in the Netherlands. The multidisciplinary group used approaches from the field of conservation biology to search for more plausible explanations. Their recent findings, published in PLOS ONE, explain the extinction as the result of natural processes and not the consequence of human invasion.

The analyses focused on three influential factors: inbreeding due to small populations; Allee effects, which describe how a small population density can reduce survival; and random variations, such as fluctuations in the death and birth rates.

The Eindhoven researchers studied Neanderthal survival using a model that ran on VORTEX, a software package used by conservation biologists to simulate extinction scenarios for endangered species. VORTEX begins with some population size–set by the use–and estimates how various threats may affect individuals. It then reports, in discrete increments of time, the size of the surviving population.

In the Neanderthal study, the parameter representing inbreeding was represented by the probability that a recessive gene with lethal effects would be carried by both parents. Allele effects were represented using an equation estimating the number of breeding females in a population of a given size. When they ran the simulation with the risks set at varying values–with each run spanning 10,000 years–it became clear that the Neanderthals were doomed with or without the invasion of modern humans, explains Krist Vaesen, the Eindhoven philosopher who led the new study.

The argument that inferior Neanderthals lost out to smarter modern humans is “unsatisfying,” offers Vaesen. He says it’s possible that modern humans played some part, but it wasn’t causal. At most, he says, competition may have accelerated the inevitable.

“Some people have modeled interactions between Homo sapiens and Neanderthals, but they assume that the Neanderthals were inferior,” says Vaesen, whose collaborators included two field archaeologists. “If you assume that Neanderthals were inferior [by assuming a higher mortality rate], then your model will predict that Neanderthals were inferior.” If modern humans did have some effect, says Vaesen, it was to exacerbate the conditions that were already driving the Neanderthals out of existence.

The present study adds to an emerging body of research suggesting that modern humans had little to do with the Neanderthals going extinct, says Alan Rogers, an anthropologist at the University of Utah, in Salt Lake City, who was not involved in the study. “That’s the growing consensus,” he says. “It seems clear that they were in bad shape even before modern humans arrived.”

In 2017, Rogers’ group reported in PNAS that late in the Neanderthals’ existence, they split into several small subpopulations spread out across Europe and the Middle East. The small, isolated populations hastened inbreeding, which leads to less individual variability; less variability lowers a species’ chance of survival, says Vaesen. Small populations are also more vulnerable to the random swings in birth and death rates.

The new study also aligns with recent work reporting a high rate of congenital defects in Neanderthal vertebra fossils led by paleobiologist Antonio Rosas at the National Museum of Science in Madrid, Spain. Those defects, says Rosas, suggest that inbreeding was common.

“We proposed that the reduced size of Neanderthal groups [has] been a fundamental factor in the demography and final extinction,” he says. “I think it’s important that we set aside the old idea that Neanderthal extinction was just a matter of Homo sapiens’ superiority.” The major limitation of the new study, says Rosas, is that it’s based on a computer model rather than real-world evidence from archaeological sites.

“The Allee effect makes sense and might be true,” Rogers adds, “but the authors don’t really provide evidence of it. If such an effect existed, it would exacerbate the situation for Neanderthals, but that’s not terribly surprising.”

Ultimately, Vaesen believes his group’s models help offer support for a simpler explanation of Neanderthal extinction than one that requires a thorough understanding of interactions between competing species. “I don’t think the debate is settled at all,” he says. “I don’t think it ever will be settled.”

Categories: Anthropology | Ecology | Evolution | Genetics | Journal Club | Population Biology and tagged | | | | | | |
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