The End of the Ice Age

The Younger Dryas and the Mesolithic

For the vast majority of Homo sapiens’ time outside Africa - the past 115,000 years or so, not counting the possible earlier dispersals - we lived in a world defined by ice. The Earth regularly oscillates between periods of cold and warmth, and our species came of age in a time defined by the cold.

The ice sheets extended much further south and north from the poles, and outward from the planet’s towering mountain ranges. Sea levels were dramatically lower, as much as 120 meters (roughly 400 feet) lower than today. Landscapes looked different: The mammoth steppe and other grasslands predominated rather than tropical jungle and boreal forest, while deserts were often larger and harsher than they are today.

These conditions weren’t uniform; they came and went, with warmer periods following cooler ones at a variety of intervals. Patterns of moisture shifted, bringing prosperity to one region and disastrous drought to another. Large chunks of the Sahara Desert, for example, changed from inviting, river-strewn grassland to uncrossable desert on several occasions over the past 115,000 years.

But eventually, the ice reached its peak extent. We call this the “Last Glacial Maximum,” and for most of the world, it took place between about 26,000 and 20,000 years ago. Much of Europe was simply uninhabitable, either covered by ice sheets or near enough to them that frigid arctic desert was the main landscape. On the frozen grasslands of the Ukraine, people clung on by hunting and scavenging woolly mammoth, even using their bones and tusks as the framework for large, elaborate houses. This is a replica of a house excavated at the village of Mezhyrich:

After about 20,000 years ago, however, the ice sheets began to recede for good. The arctic desert became mammoth steppe, the mammoth steppe became new-growth woodland, and people left their refuges to explore the new landscapes. They adopted new ways of life to fit their emerging environments: new cultural practices and new subsistence strategies for a new world. The Clovis hunters who made their way south into the vast and un- or barely inhabited tracts of the Americas were one of these groups, following the mammoth, mastodon, and bison across a largely unexplored land. As Europe became increasingly forested, the mammoth and horses who had wandered the mammoth steppe disappeared, and deer, aurochs, and boar became the favored prey. The Natufian hunter-gatherers of today’s Near East exploited huge expanses of wild grasses, the forerunners of domesticated crops, and hunted gazelle.

This was the final Paleolithic, the last gasp of the old Stone Age.

For much of the world, times were good during the moist, temperate period known as the “Bølling–Allerød warming,” which lasted from about 14,690 to 12,900 years before the present. But the good times didn’t last. Before the clock turned to the Holocene, the warmer period in which we live, the ice - quite suddenly - advanced once more. This marked the onset of the Younger Dryas, a severe cold snap with global reach and implications. January temperatures in Britain dropped to 10 to 30 degrees Celsius (18-54 degrees Fahrenheit) lower than they are today. Much of coastal Northern Europe would have been effectively uninhabitable.

The effects of the Younger Dryas varied a great deal from region to region. I mentioned Northern Europe a moment ago; in what are now the eastern United States, summer temperatures were as much as 7.5 degrees Celsius (13.5 degrees Fahrenheit) colder than they had been prior to the onset of the Younger Dryas. In other spots - central Europe or the western United States, for example - the effects were noticeable, but much less severe: summers weren’t notably colder, but they were shorter, to name one widespread phenomenon. The upshot of all this was that over a brief period, decades or even years, many formerly productive and viable landscapes became much less welcoming to people.

Those encroaching forests that had thrived in Europe in warmer, wetter conditions died off, replaced once more by the mammoth steppe. Yet the mammoth were gone, and didn’t return. Neither did the horses. The deer who had made their homes in the forest didn’t care for the open grassland, and retreated along with the trees to Alpine valleys and hillsides. The aurochs, bison, and wild boar could survive in either landscape, and the herds of reindeer eventually returned, but there were far fewer resources available for people to exploit. The people who survived in Northern Europe, for example, turned to intensive reindeer hunting with a new weapon, the bow and arrow.

All of that meant fewer people in a harsher landscape, and for some 1200 years (12,900 years before the present to 11,700), these conditions persisted.

Why did the climate so suddenly take a turn for the worse? The general mechanism for widespread cooling was almost certainly a shift in the currents of the North Atlantic, which led to the formation of more sea ice, which then altered the wind patterns, reflected sunlight, and helped keep it cooler. But why did it happen in the first place? Why did the currents shift?

There are a number of theories, none of which are a perfect fit for the available evidence. First, the rather sudden cold snap that precipitated the Younger Dryas took place in a context of cooling temperatures. It was already getting colder on average, one of the numerous up-and-down temperature oscillations within a long-term period of general warming. Second, some scholars have argued for a sudden pulse of extremely cold glacial meltwater - perhaps from Glacial Lake Agassiz in what’s now Canada - into either the Atlantic or more likely the Arctic Ocean, which would’ve had the effect of stimulating the formation of sea ice. Third, there was a volcanic event in what’s now western Germany at precisely this time (12,900 years before present), the Laacher See eruption. This was as large as the 1883 eruption of Krakatoa, and could have had a marked impact on global temperatures, at least in the short term.

Finally, there is widespread evidence for a series of extraterrestrial impacts right around the time the Younger Dryas began. According to this line of reasoning, fragments of a comet or perhaps a meteor shower bombarded the planet some 12,900 years ago, precisely at the Younger Dryas boundary. While no individual impact in this encounter might have been particularly large, there were enough of them to cause widespread burning of ground cover and eject particulate matter into the atmosphere, which would have reduced the absorption of sunlight and lowered the temperature. The more extreme advocates of this theory give it far too much explanatory power, but there’s no reason to reject it out of hand. Extraterrestrial objects hit the Earth all the time!

These are all plausible, and the most likely explanation for the Younger Dryas involves some combination of a general cooling trend, a meltwater pulse, a volcanic eruption, and extraterrestrial impacts, perhaps in rapid succession. An event that might cause notable but short-term cooling in one place could have global consequences if it were effectively simultaneous with several others.

After 1200 years, around 11,700 years before the present, the Younger Dryas came to an end. The warming resumed, and the Holocene - our present era - began. The ice melted, sea levels rose, and the environment changed once more. A whole clutch of new cultures and practices emerged to capitalize on the new possibilities. In Europe, these new cultures belonged to the Mesolithic period. They were quite different from what had come before. Genetic evidence suggests that Mesolithic people only had a small bit of ancestry from the Paleolithic Europeans who had lived there for tens of thousands of years. As the population changed, cave painting, a practice that had lasted for perhaps 20,000 years, came to an end. The tools changed, too: Rather than making large spearpoints and blades, they created small, modular blades called “microliths,” which could then be placed into a variety of antler, wood, or bone handles and used for a number of different purposes. Rather than making an arrowhead and a separate scraper to clean hides, Mesolithic people made microliths, which could be used for either purpose in the right setting.

As the environments changed, Mesolithic people adopted a substantially different way of life than their Paleolithic predecessors. Microlithic tools, with their greater adaptability, are one signal of this flexibility: Mesolithic folks were generalists, rather than specialist mammoth- or reindeer-hunters. The rising sea levels, warmer temperatures, and greater moisture created a diverse array of wetland environments on sea- and lakeshores across Europe, and this was where Mesolithic people often resided. At the site of Star Carr in Yorkshire, which was then a reed-covered lakeshore, Mesolithic Britons plied the waters in small boats, lived in wigwam-like houses, and hunted deer. They wore deer skulls, with antlers attached, as masks: Whether they did this as a disguise while hunting or during some kind of shamanic ritual is impossible to say, but the masks look pretty cool:

They made effective barbed points:

And finally, here’s an illustration of a young Mesolithic who lived in what’s now Denmark, by the paleoartist Tom Bjorklund, showing an eel spear and woven eel trap suitable for exploiting a wetland environment:

Tom Björklund @TomBjorklundArt

Once more, ”Lola” the Danish WHG girl chewing birch tar. Made it for a very interesting article that caught a lot of attention when it was published in Nature last December. theguardian.com/science/2019/d…

The Mesolithic way of life was one way of adapting to the new climatic conditions offered by the end of the Ice Age. Elsewhere - in the Fertile Crescent of the Near East - other hunter-gatherers were experimenting with other options. This led to the birth of agriculture and a whole new way of life. That will be next week’s topic.

If you’d like to learn more about the Mesolithic, check out today’s episode of Tides of History, my podcast, where I discuss all of this in much greater depth. Next week, I’ll be discussing those first farmers I mentioned a moment ago, and the much different kind of world they built.