Europe was considerably colder 44,000 years ago than previously thought, according to new research. The finding is forcing a rethink about early human migration patterns and where our ancestors preferred to settle.
“The expansion of Homo sapiens across Eurasia marked a major milestone in human evolution that would eventually lead to our species being found across every continent,” write the authors of new research published today in Science Advances.
But scientists still aren’t sure how early modern humans managed to pull off this remarkable migrational trick, given considerable environmental variations around the world. The new study, co-authored by Sarah Pederzani from the Max-Planck-Institute for Evolutionary Anthropology, sought to explore the climatic conditions experienced by Homo sapiens when venturing from southwest Asia to Europe.
“This process is very interesting for us to understand, because we believe that it holds key answers about how our species was able to spread across the whole globe and adapt to many different environments and climates, while other human species, such as Neanderthals, eventually disappeared,” Pederzani explained in an email.
Evidence sourced from a Bulgarian cave showed that the climate in southeast Europe during the Upper Paleolithic — about 44,000 years ago — was considerably colder than previously thought. Today, the mean annual temperature in Bulgaria is about 10 degrees Celsius, but back then it was somewhere between 0 degrees C and -5 degrees C, according to the study. Those conditions are akin to what’s currently experienced in the subarctic climates of northern Scandinavia and Siberia. The researchers were studying ice age Europe, so they naturally expected a colder climate, but not to that degree.
“I really was very surprised to see that the temperatures we reconstructed for this site and time period were so low,” said Pederzani. “I first double-checked all my measurements to make sure it wasn’t a mistake, but in the end I was sure it was indeed the case.”
The revised local climate model was derived from an isotopic analysis of butchered animal remains found in Bulgaria’s Bacho Kiro Cave. This cave was home to both Neanderthals and Homo sapiens, and it has yielded a trove of archaeological and genetic evidence over the years. The team analysed strontium and oxygen isotopes taken from the teeth of contemporaneous horse ancestors and bison found in the cave. By sequentially analysing the teeth layer by layer, the team was able to reconstruct seasonal temperatures across the animal’s life. The analysis of 179 samples allowed them to reconstruct local temperatures while the cave was occupied by humans across a 7,000-year span, including summers and winters.
Early modern humans reached Europe by way of southwest Asia, but anthropologists figured their early occupations of Europe coincided with short warm phases, during which time temperatures were similar to what’s experienced in the region today. When the researchers first embarked on their study, “most of the available evidence showed that the ages of early modern human sites in Europe and northern Asia seem to coincide with phases of warm climate that are indicated in long-term climatic records from ice cores in Greenland or plant remains, such as pollen in cores drilled in the Mediterranean sea or in Greece,” said Pederzani.
In addition, humans migrating from Africa didn’t immediately spread out, choosing instead to hang out in southwest Asia for a prolonged period of time before dispersing into Eurasia. This was some indication, said Pederzani, that humans only later developed an ability to live in colder climates. Human groups at the time “maybe didn’t yet use tailored, as in completely sewn, clothing, so there are a few reasons to suggest that colder climates posed a barrier to our species at some point,” she said.
By this time, Neanderthals were living in ice age Europe and parts of Asia, as they had done for hundreds of thousands of years. There’s ample archaeological evidence for this. As for early modern humans doing the same, not so much. But as the new study suggests, the cold never bothered Homo sapiens anyway.
“The new study suggests that the common assumption that humans migrate and occupy places only during warm and wet times needs revision,” as Jessica Tierney, a paleoclimatologist at the University of Arizona who wasn’t involved in the study, wrote in an email.
Indeed, the humans of Bacho Kiro cave appear to have endured subarctic conditions for several thousand years. In a press release, Jean-Jacques Hublin, director of the Department of Human Evolution at the Max Planck Institute and a co-author of the study, said early humans displayed a “higher degree of climatic flexibility” than previously appreciated.
Since 2015, Hublin and his colleagues have been collecting archaeological evidence in the cave, finding animal bones, stone tools, pendants, and human fossils. This rich archaeological record helped to establish a human presence in the cave for the time period studied.
“The finding that the climate was cool is not too surprising to me, because 45,000 years ago, the Earth was in an Ice Age state,” said Tierney. “Although the Greenland ice core shows some rapid climate oscillations during this time, including some brief warmings, it is not clear that these events affected the whole world.”
In 2017, Tierney published a study in which she and her colleagues reconstructed temperatures in East Africa during the major out-of-Africa migrations starting some 65,000 years ago. From around 70,000 to 40,000 years ago, “sea-surface temperatures off the east African coast were really cold, the coldest in 200,000 years,” she said. “Homo sapiens migrated out-of-Africa during this cold — and also dry — time.”
That said, Tierney said it’s important to be able to detect signs of climate change when it’s intermixed with archaeological evidence. It’s this that “makes this particular paper special,” she said.
Looking ahead, Pederzani hopes anthropologists will conduct studies that generate climate data from archaeological materials, such as animal teeth, bones, and shells.
“Research into how modern humans may have adapted to colder environments is also very important if we’re to gain a better understanding going forward,” she said. “How far and how often did human groups move across the landscape? What animals were they hunting and when? What was their community structure? These are all important questions that we need to re-examine with the use of colder environments in mind.”