Human Expansion During Ice Age Could Explain How We Vanquished the Neanderthals

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When Homo sapiens reached Bacho Kiro cave in Bulgaria, it was much colder than science had realized until now
When Homo sapiens reached Bacho Kiro cave in Bulgaria, it was much colder than science had realized until nowCredit: Sarah Pederzani / MPI-EVA Leipzi
Ariel David
Ariel David

The first modern Homo sapiens to reach Europe more than 45,000 years ago faced much colder temperatures than had been thought, archaeologists have deduced – yet they slogged on and survived during one the peaks of the last Ice Age, a new study shows.

The findings contradict the prevailing paradigm that Homo sapiens migrated out of Africa and the Levant, and dispersed throughout Eurasia, chiefly because the climate at this time had warmed. It isn’t so, we learn. The study, published Wednesday in Science Advances, could change how we think about the dispersal of our early ancestors and what caused the extinction of other hominin groups, chiefly the Neanderthals.

An international team led by Sarah Pederzani, an archaeologist at the Max Planck Institute for Evolutionary Anthropology in Germany, gleaned the data from the chemical analysis of animal bones found in Bacho Kiro Cave, a site in Bulgaria recently identified as the oldest known home of modern Homo sapiens in Europe

A single tooth and a handful of bone fragments have shown that already 46,000-45,000 years ago the cave was occupied by humans linked to the so-called Initial Upper Paleolithic tool culture, the first group of modern Homo sapiens that left Africa to colonize large swathes of Eurasia.

Tools associated with the Initial Upper Paleolithic first appear in what is today Israel’s Negev desert around 50,000 years ago and then spread relatively quickly (in evolutionary terms) as far as Mongolia and Eastern Europe. 

Extreme Ice Age cold was thought to constrain early human migration. It didn'tCredit: Ed Roberts

The new research by Pederzani and colleagues didn’t look at the paltry sapiens remains found at Bacho Kiro. The team analyzed the leftovers of animals hunted and eaten by the cave’s Initial Upper Paleolithic residents, specifically teeth of horses and bovids.

The researchers found that 45,000 years ago the climate in this region of Eastern Europe was considerably colder than previously thought, with temperatures similar to what is experienced today at the higher latitudes of Russia and Scandinavia.

While the mean temperature at Bacho Kiro is around 10 degrees Celsius (50 degrees Fahrenheit) today, back then it was closer to 5 degrees below zero, with lows of 20 degrees below zero in winter.

“It’s hard to say how well they were adapted to these conditions but it’s clear that they survived and managed to exploit this almost polar environment for at least a couple of millennia,” Pederzani tells Haaretz. “The idea that only warm conditions are favorable for early Homo sapiens dispersal is not supported by this data.”

It bears adding that tools apparently used to skin animals and prepare their pelts for clothing were discovered in Morocco and dated to 120,000 to 90,000 years ago. So the sapiens leaving Africa for the cold may have been at least suitably dressed, but the real challenge in the rigid European climes would be finding enough food to avoid starvation – all while competing for scarce resources with the continent’s original inhabitants, the Neanderthals.

Analyzing the horse’s mouth

The findings also belie the somewhat comforting idea that we had nothing to do with the disappearance of Neanderthals, which occurred around 42,000 years ago and which some previous paleoclimate studies have blamed on a prolonged cold spell.

We know from earlier DNA studies that the Bacho Kiro sapiens had interbred with Neanderthals. So low temperatures cannot be entirely blamed for their disappearance if our close evolutionary cousins were still around in what seems to have been one of the coldest periods of the last Ice Age, and so were we. Also, the nice, warm Levant was thronged by Neanderthals starting about 80,000 years ago and until their final extinction.

But, I hear you asking, how does looking into a prehistoric horse’s mouth tell us what the temperature was tens of thousands of years ago?

Howling Ice Age conditions at Bacho Kiro when humans arrived were deduced from these horse teeth Credit: Sarah Pederzani / MPI-EVA Leipzi

The research relies on isotopic analysis of the teeth enamel, a scientific method that is increasingly used by archaeologists. The ratio of isotopes, variants of the same element with a different number of neutrons, changes from place to place and through different climes. In the formative years of an animal’s life, humans included, these isotopes accumulate in our dental enamel as we absorb them from the water, air and food. So researchers can study the isotopes in ancient teeth to reveal how populations migratedwhat they ate and what environmental conditions they were exposed to

In the case of Bacho Kiro, the researchers looked at the isotope ratios of two elements: strontium and oxygen. The ratio for the first element varies from place to place, and the fact that it was the same for all animals studied confirmed that these horses and bovids hadn’t migrated from elsewhere and the information locked in their enamel could be used as an indicator of the local climate. That information comes from the ratio of oxygen isotopes, which varies as a result or more or less evaporation, which in turn is a function, and therefore an indicator, of temperature, Pederzani explains.

To summarize: young wild horses and cattle drink water 45,000 years ago, absorbing a telltale composition of oxygen atoms in their tooth enamel; later in their lives these animals are hunted and eaten by Homo sapiens at Bacho Kiro Cave; millennia later an arguably smarter version of sapiens uses the leftover teeth to figure out that during the Upper Paleolithic it was much chillier than we thought.

At this point you might be wondering whether, given the polar winter temperatures at Bacho Kiro, the occupation of the cave may have been seasonal, with people living there only during the summer. But that’s not the case, the paper in Science Advances notes. Since some animals tend to be born at a specific time of the year, the researchers were able to check the state of development of their bones and determine in what season they had been killed. And indeed part of the remains found in the cave belonged to animals who had been hunted down in winter.

Excavating inside Bacho Kiro, BulgariaCredit: Tsenka Tsanova / MPI-EVA Leipzig

A paleolithic thermometer

If confirmed by future research, the implications of the discovery would be much broader than just delivering a weather report from the Stone Age. They suggest that modern sapiens was much more adaptable than we believed and the factors behind our dispersal from Africa were much more complex than a supposedly favorable climate, Pederzani explains.

“A lot of different factors may have driven the dispersal, such as cultural and demographic ones, as well as other environmental factors,” she says. “For example, a colder steppe environment with vast open grassland may be more favorable to highly mobile hunters than a warmer, forested environment.”

Researchers have been able for a while to study paleoclimate by analyzing ice cores extracted from Arctic ice sheets or isotopes deposited in cave speleothems

The problem with this data is that it tends to be fairly broad in geographic or chronological terms, says Professor Jean-Jacques Hublin, head of the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology. The new method developed by Pederzani and colleagues gives us an indication of the climate at the time and place where early humans actually lived, says Hublin, who is also one of the authors on the study.

“I’ve been dreaming for years of something like this, of finding a thermometer that tells us what was the temperature of a place when humans were there, rather than looking at broad data from ice cores in Greenland,” he tells Haaretz.

Archaeologist recording artifacts found in the Initial Upper Paleolithic Layer of Bacho Kiro caveCredit: eljko Reek / MPI-EVA Leipzig

The find sheds light on a key and still quite mysterious step in human evolution.

Neanderthals and sapiens diverged around 700,000 years ago and for hundreds of thousands of years they pretty much stuck to their corners of the world: early sapiens roamed Africa while Neanderthals shared Eurasia with the elusive Denisovans.

The Middle East was sort of a “border zone” which at times was controlled by either sapiens or Neanderthals, and there is evidence the two groups coexisted and possibly had the first of a series of interbreeding events in the region. 

These forays into each other’s Levantine territories were brief and limited in scope, Hublin says. They may have well been modulated by climate fluctuations: moist periods brought sapiens more to the north and drier spells pushed the Neanderthals south.

But the new study shows that after 50,000 years ago, with the first appearance of the Initial Upper Paleolithic culture outside Africa, something had happened in our evolution that broke this equilibrium.

Evolutionary chess

“The common scenario was that there was this very cold episode in Europe that reduced a lot the Neanderthal populations and then this milder period where modern Homo sapiens moved from the Levant into the higher latitudes and replaced the Neanderthal,” Hublin notes. “This is completely false: the time when we have the first modern sapiens in Europe is a phase that is much colder than we thought.”

The secret to our success was not a random mild spell, but that humans had developed much more complex social and cultural ties that allowed them to come up with more advanced technologies and adapt to all but the most inhospitable environments on Earth, Hublin posits.

This means that even if we didn’t actively exterminate other hominins, we likely had a hand in their extinction, by outcompeting them.

Previous studies have already showed that Neanderthals had survived earlier cold periods of the Ice Age, and though their numbers may have been diminished, they were quickly replenished once conditions improved, Hublin notes.

“If the Neanderthals disappear at the time when humans arrive in Eurasia it’s because of us, because of the competition with us,” he says.

Because sapiens was more adaptable, as the climate fluctuated, modern humans tended to be the first to occupy an inhospitable region as soon as temperatures started to rise and the landscape changed, Hublin says.

“I see Europe at this time as a giant chessboard with squares that open and close depending on the climate situation,” he concludes. “If you have one group that is more flexible and faster in occupying those squares when they open up then this group is eventually going to replace the others.”

It took a few thousand years but soon, for the Neanderthals, the Denisovans and other hominins with whom we once shared the planet, it was checkmate.

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