I've always wondered how our distant ancestors survived during the various ice ages.
After all, we're mostly-hairless primates evolved on the warm, comfy African savanna, and it's hard to imagine how we coped with conditions like you often see depicted in books on early humans:
Despite the bear pelts around their nether regions, I've always wondered how they didn't all freeze to death. When the weather's nice, bare skin is fine; I only wear a shirt during the summer under duress, and can't remember the last time I wore swim trunks when I went swimming in my pond. But when the weather's cold -- which, here in upstate New York, is more often than not -- I'm usually wearing layers, and that's even indoors with our nice modern heating system. Okay, admittedly I'm a wuss about the cold, but the fact remains that we're evolved to dwell in temperate regions. Which, for a significant part of the Pleistocene Epoch, most of the world was not.
In particular, during the Last Glacial Maximum, between twenty-six and twenty thousand years ago, much of the Northern Hemisphere was experiencing a climate that the word "unpleasant" doesn't even begin to describe. The average temperature was 6 C (11 F) colder than it is today, which was enough to cause ice sheets to spread across much of North America and northern Europe (where I currently sit, in fact, was underneath about thirty meters of ice). Much of the non-glaciated land experienced not only dreadful cold, but long periods of drought. The combined result is that the sea level was an estimated 130 meters lower than it is today, and broad dry valleys lay across what are now the bottoms of the Bering Sea, the North Sea and English Channel, and the Gulf of Carpentaria.
These conditions opened up passageways for some people, and closed off living space for others. This was the time that the various pulses of immigrants crossed from Siberia through Beringia and into North America, where they became the ancestors of today's Indigenous Peoples of North and South America. (If you want to read a brilliant account of how this happened, and some of the science behind how we know, you must read Jennifer Raff's wonderful book Origin: A Genetic History of the Americas.) The same sort of thing happened from southeast Asia into what is now Australia.
In Europe, though, things got dicey to the point that it's a wonder anyone survived at all. In fact, what brings this up is a study that appeared in Nature last week by a humongous team led by paleogeneticist Cosimo Posth of the Max Planck Institute of Evolutionary Anthropology. The team did a complete genomic analysis of 356 individuals whose remains range from thirty-five thousand to five thousand years of age -- so right across that awful Last Glacial Maximum period -- to try to figure out how groups moved when the ice started coming in, and afterwards, once it retreated.
What they found was that only one part of Europe showed a consistent human genetic signature throughout the time period: the Iberian Peninsula. What this indicates is that modern Spain and Portugal were a "climate refugium" during the worst of the glaciation, where people came to stay when the climate turned very cold, and pretty much stayed put. Other areas that you might think were possible candidates for comparatively warm hideouts, such as what are now Italy and Greece, show a significant genomic shift across the Last Glacial Maximum, indicating that the people there before the cold set in either migrated or else died out, and were replaced by immigrants who moved in after things warmed and the area once again became more hospitable for humans."At that time, the climate warmed up quickly and considerably and forests spread across the European continent," said Johannes Krause, senior author of the study, in an interview with Science Daily. "This may have prompted people from the south to expand their habitat. The previous inhabitants may have migrated to the north as their habitat, the 'mammoth' steppe, dwindled,. It is possible that the migration of early farmers into Europe triggered the retreat of hunter-gatherer populations to the northern edge of Europe. At the same time, these two groups started mixing with each other, and continued to do so for around three thousand years."