The oasis

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.

Artist's conception of the hot spring refugium [Image credit: artist Jiří Svoboda]

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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A refuge from the cold

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:

Le Moustier Neanderthals by Charles Knight (1920) [Image is in the Public Domain]

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."

Me, I'm curious what happened to these people afterward.  As a linguist, not to mention a white guy of western European descent, I've wondered if we're talking about my forebears, here -- and what languages they spoke.  My suspicion is that we're looking at the ancestors of today's Basques, who still live in northern Spain; they speak a non-Indo-European language that is usually considered a relic of the earliest languages spoken in Europe.  The Indo-European-speaking peoples (therefore the ancestors of the majority of today's Europeans) didn't reach Europe until about four thousand years ago, so long after the heyday of the people who were the subjects of the Posth et al. paper.

So you have to wonder who the descendants of these very early Europeans are.  "Not me" is my general assessment, considering my general cold-hardiness.  Drop me into an ice age where I had to live in a cave, hike on glaciers, hunt mammoth, and fend off cave bears, and I'd last maybe three days, tops.  I'm highly impressed by the ability of these ancient humans to survive, but given a choice I'll stick with my warm house, indoor plumbing, electric stove, and coffee maker.

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