Modern humans evolved about 200000 years ago — and by 20000 years ago we had taken over the world, wiping out all our would-be competitors, like Neanderthals. It's all because we chose the right place to wait out the Ice Age.
That's the theory being put forward by John Stewart of Bournemouth University and Chris Stringer of London's Natural History Museum. They argue that many of the same climate processes that we've documented in the evolution of other animals can also be used to explain two things: first, why various distinct groups of archaic humans emerged. And second, why we humans ultimately emerged triumphant.
The key idea here is that of refugium, which as you can imagine is related to taking refuge. A refugium is a small area of relatively livable conditions that a population can use to survive the lethally harsh conditions of an Ice Age. These refugia are important because, as Stewart and Stringer explain in their paper in Science, they "will generally be the smallest space occupied by the fewest numbers of individuals over time." That gives these areas of refuge tremendous influence on the evolutionary future of the species.
Genetic drift and the natural selection pressures unique to given refugia will make populations more distinct over time. The Ice Age further isolates these groups from one another, and the time spent apart generally serves to fracture a once unified species. Previous research into animals as varied as hedgehogs and polar bears suggest that, even once the Ice Age ends and the different populations start intermingling again, they never really merge back together as a single group. The refugia serve as an evolutionary wedge.
In this view, various populations from the Homo genus spread out of Africa during interglacials, the time periods between Ice Ages. In general, Homo is a broadly adapted genus, meaning it can survive in a huge variety of environments, which explains why multiple species - including but hardly limited to Homo erectus, Neanderthals, and ultimately us - had so much success traveling so far from their evolutionary birthplace.
But these multi-generational migrations into Eurasia created the climate equivalent of musical chairs - once the good weather stopped and the glaciers started rolling in, these groups needed somewhere to survive. The "chair" the different groups selected as their refuge area was immensely important, because it helped determine not just their short-term survival prospects but also their long-term evolutionary fitness.
While early modern humans tended to pick refugia in relatively temperate climates, Neanderthals were essentially trapped up north, carving out an existence in still palatable but much colder areas. Those circumstances were key in shape the morphological differences between Neanderthals and us, and the divergent evolutionary paths created by our choices of refuge might have helped determine which group went extinct. Stewart and Stringer explain in their paper:
Neandertals have been described as a cold-adapted, even hyperarctic-adapted, human species. Several of their traits, such as relatively short limbs, high body mass, and enlarged sinus cavities, have been interpreted as related to temperature regulation...There are grounds to infer that Neandertals were cold-adapted to an extent, and certainly more so than were their African ancestors. Furthermore, they were probably better physically adapted to cold conditions than the succeeding anatomically modern human populations, who instead possessed additional behavioral adaptations that allowed them to cope well with the Late Pleistocene Eurasian environment.
If Neanderthals hadn't been so far north when the Ice Age came in, they might have avoided the specialized adaptations that, while vital for their immediate survival, helped doom them when placed in competition with us. On the flip side, our more temperate refugia helped move us towards evolving behaviors that served us well once the Ice Age ended. Not being restricted to the frontiers of Europe also helped - humans were able to spread out all across Eurasia as soon as the glaciers receded, while the Neanderthals were stuck in a far less prime location for expansion.
A similar phenomenon - though minus the extinction - can be seen in polar bears, which are essentially brown bears who were forced to adapt drastically when they found themselves in northern refugia during glacial periods. I'm not totally sure this justifies giving Neanderthals the nickname of "polar humans", but it's too cool an opportunity to pass up.
Another advantage of looking at human evolution through the prism of refugia is it helps explain why we see so many distinct species and divergent populations. While humans, Neanderthals, Denisovans, and the just discovered Red Deer people were certainly separated by vast differences - Eurasia is a big place, after all - it's not as though we were actually isolated from each other by any insurmountable barriers like, say, an ocean. Retreating to these refuges during the Ice Ages would help explain just why humans and Neanderthals ended up so different when geography alone won't explain it.
It's still early days for this model, and we're still missing tons of vital information that would help us better understand how all these different groups fit together. As we add more to both the fossil and genetic records for these different groups, we will better understand how they spread throughout Eurasia and how they evolved to meet their particular challenges. But this does all point to one overwhelmingly clear point - if you're going to choose a place to wile away an Ice Age, you better be damn sure you've picked the right one.
Via Science. Digital painting by James Ives.