Did Interbreeding with humans give dwindling Neanderthals new lease on life?

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What happened to the Neanderthals?

It’s a question many in the sciences have tried to answer for over a century. The last five years have seen a number of new insights due to major leaps forward in our ability to process ancient DNA. We now know that modern humans interbred with our bulkier cousins in Europe and Asia—enough so that a small fraction of the genomes of some modern humans contain Neanderthal remnants. Some have even suggested that certain behaviors and conditions like drug abuse and depression are from gene variants we got from this ancient ‘co-mingling‘.

Yet, the discovery that humans and Neanderthals interbred only complicates the initial question of what happened to them. Much of the debate surrounding their extinction has focused on the arrival of the “invasive” modern human species to the Neanderthal’s turf. But new evidence suggests that early human immigrants to the European continent may have actually helped Neanderthals stave off extinction—at least for several thousands years.

Neanderthals, like humans, trace their evolutionary origin to Africa. But starting about 400,000 years ago most left and begun to populate what today is Europe and Western Asia. Some scientists suggest that at their peak in numbers—before the arrival of humans—Neanderthals may have totaled around 70,000. A long held belief has been that Neanderthals were unable to adapt to the changing colder climate of this period, but that’s now thought to be mistaken. Neanderthals appear to be particularly well adapted to colder climes because of their short torsos and limbs that helped conserve heat and their large noses that humidified and warmed dry, cold air.

It is hard to pin down exactly when humans first interacted with Neanderthals. The first Homo sapiens are believed to have appeared around 200,000 years ago, also in Africa. It had long been accepted that modern humans began settling in Europe about 45,000-60,000 years ago—probably in more than one migration event. However, newly uncovered fossils, as well as DNA evidence, suggests early humans may have encountered Neanderthals 40,000-50,000 years earlier than that. No matter when the first meeting happened, Neanderthals were completely extinct by about 30,000-40,000 years ago.

There are a number of theories as to why. Although there is no evidence of direct human-Neanderthal combat, many people still believe in the popular notion that humans “wiped them out.” We may have done this indirectly by out-competing them for space and resources, perhaps because of our superior intellect and technology. There’s another mistaken belief that because their DNA can be found in modern human genomes Neanderthals never actually went extinct. A recent popular theory backed by some evidence was that humans (particularly when engaging in sexual intercourse with Neanderthals) may have spread infectious diseases—herpes, tapeworms, and tuberculosis—that the Neanderthals immune system was ill equipped to deal with.

Whatever the reasons for their demise, the arrival of modern humans in Eurasia appears to be the prelude to the permanent departure of the Neanderthals. But Neanderthals didn’t drop from 70,000 strong to nothing quickly. They were already a dwindling species by the time Homo sapiens began making places other than Africa their home. While it is difficult—if not impossible—to say with complete accuracy the global Neanderthal population, estimates suggest that their global numbers could have been as low as 1,500 reproductive-aged females around 70,000 years ago—which coincides with about the time of the largest Homo sapien migration out of Africa. Also during this time, evidence suggests Neanderthals were living in small geographically isolated communities.

This means when humans arrived in Europe and Asia, Neanderthals were likely very few in numbers and living in isolated populations with little contact amongst each other. Anyone who works in the field of species extinction will tell you that this is a very bad combination for long term survival. The fact they existed in small, isolated populations also suggests that Neanderthals likely were inbreeding, which can lead to (often recessive) deleterious traits becoming more and more abundant in the overall population, making them even more vulnerable to disease. (It’s a process that modern humans would see play out often among royal families; e.g. hemophilia in 19 and 20th century Europe).

How inbreeding increases frequency of deleterious traits
How inbreeding increases frequency of deleterious traits

Their geographic isolation would have likely compounded this and made it increasingly more difficult for natural selection to “weed out as bad mutations” as paleoanthropologist John Hawks of the University of Wisconsin at Madison has put it. A recent analysis has also found that the Neanderthal genome was full of genes that reduced their overall fitness—some 40 percent compared to Homo sapiens.

Neanderthals were likely a species on the brink when humans suddenly appeared. Mating and exchanging DNA with these ‘invaders’ quite possibly served as a ‘short in the arm,’ helping to restore their genetic diversity, quite possibly extending their species tens of thousands of years—until some stronger forces, perhaps disease, wiped them out.

One species saving another through interbreeding to rebuild genetic diversity is actually not unheard of. The U.S. Fish and Wildlife Service credits an interbreeding program between the endangered Florida panther and Texas pumas as a driving factor in staving off the former’s extinction. Similar programs have been implemented in conservation efforts for the greater prairie chicken.

Humans may or may not have dealt Neanderthals their final blow; further evidence may reveal an answer or we may never know. But our initial arrival probably did their survival (and gene pool) a lot of good.

Nicholas Staropoli is the associate director of GLP and director of the Epigenetics Literacy Project. He has an M.A. in biology from DePaul University and a B.S. in biomedical sciences from Marist College. Follow him on twitter @NickfrmBoston.

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