I've always thought it was astonishing that anything short of extremely cold-adapted species could make it through an ice age.
During the last major glacial period, which peaked about twenty-one thousand years ago, the spot where I'm sitting right now was under a thirty-meter-thick layer of ice. In fact, the hills about fifty kilometers south of us -- the Elmira Moraine -- marks the terminus of the glacier, where rocks, gravel, and soil that had been pushed forward by the advancing ice sheet got left behind as it melted. During this period, the average global temperature was 6 C colder than it is now, and so much water was locked up as ice that the sea level was over a hundred meters lower than it is today.
My picture of how species survived (excluding the aforementioned cold-lovers) was that everything shifted range toward lower latitudes as the temperature cooled and the ice advanced, then reversed the process as the glacial period ended and the ice receded. Species that couldn't shift quickly enough, or for which the climatic changes happened too fast to adapt, became extinct. But according to a paper last week in Science Advances, the picture may not have been quite so simple.
One clue that our understanding was incomplete had to do with genetic diversity. For a lot of species, we have a pretty good understanding of how quickly genetic mutations accrue, so looking at the genetic makeup of various populations within a species gives you an estimate of how long ago they had a common ancestor. (And also tells you how closely each of those populations are related to the others.) And in Europe, the populations of warmth-loving tree species like oaks suggested strongly that modern individuals weren't all descended from southern survivors which gradually expanded their ranges back northward as the glacial period ended twenty-odd-thousand years ago. Their genetic diversity was too high for that to be plausible -- and some of the northern populations of modern oaks seemed to be a genetic cluster only distantly related to their southern cousins.
Fossils from the Czech Republic strongly suggest that what happened was that patches of the original forest were able to survive, clustered around hot springs that even at the height of the glacial period never froze over. Geologist Jan Hošek of the Czech Geological Survey, who was lead author of the paper, found fossils of warmth-loving tree species preserved in geyserite -- a sedimentary rock produces by hot water dissolving and then depositing layers of opaline silica on exposed surfaces. The hot springs created an oasis covering an estimated fifty square kilometers. Not huge, but enough that a population of oaks and other temperate woodland plants (and presumably the animals they hosted) were able to survive the worst of the cold.
Being a warmth-lover myself, I always find it astonishing that species made it through some of these climatic extremes. Not only the cold ones, of course; episodes like the Paleocene-Eocene Thermal Maximum, when the global temperature was 8 C above what it is now, can't have been pleasant, either. But the recent discoveries show that given even a small refuge, living things will hang on despite all odds.
As Ian Malcolm famously put it, "Life, uh, finds a way."
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