Geography is one of the things that sets apart modern humans.
Our closest living relatives — chimpanzees and bonobos — are confined to a belt of Central African forests. But humans have spread across every continent, even remote islands. Our species can thrive not only in forests, but in grasslands, swamps, deserts and just about every other ecosystem dry land has to offer.
In a study published on Wednesday, scientists pinpoint the origin of our extraordinary adaptability: Africa, about 70,000 years ago.
That’s when modern humans learned to thrive in more extreme habitats. We’ve been expanding our range ever since. The finding could help resolve a paradox that has puzzled researchers for years.
Our species arose in Africa about a million years ago and then departed the continent a number of times over the past few hundred thousand years. But those migrants eventually disappeared, with no descendants.
Finally, about 50,000 years ago, one last wave spread out of Africa. All non-Africans can trace their ancestry to this last migration. The new study might explain why the final expansion was so successful.
In the new study, Eleanor Scerri, an archaeologist at the Max Planck Institute of Geoanthropology in Jena, Germany, and her colleagues sought to understand what sort of habitats early humans lived in across Africa.
Traditionally, experts have envisioned our species evolving on the savanna, adapted to life in the open woodlands and grasslands of East Africa. But Dr. Scerri and other researchers have found that early humans were more versatile than that.
In February, for example, Dr. Scerri and her colleagues reported that humans lived in a West African rainforest 150,000 years ago. Findings like that prompted the team to figure out just how wide-ranging early humans were.
“We knew there wasn’t a single Garden of Eden, but were there many gardens?” Dr. Scerri asked. To tackle that question, the scientists measured the range of environmental conditions in which humans lived in Africa — what researchers refer to as our niche.
“It effectively tells us what a species can cope with, and therefore it predicts where the species will be found,” said Andrea Manica, a geneticist at the University of Cambridge and an author of the new study.
Scientists can measure the niche of a species by looking at the environmental conditions across its range. Some plants are adapted to the tundra, for example, while others can’t tolerate a single frost.
Dr. Manica, Dr. Scerri and their colleagues analyzed hundreds archaeological sites across Africa to reconstruct the human niche over the past 120,000 years.
The oldest sites revealed that early humans were fairly versatile, living in a range of habitats. But they did not push too far into extreme environments. As a result, populations remained largely isolated from one another, cut off by deserts, towering mountains or impassible marshlands.
The human niche didn’t change for tens of thousands of years. But then a sudden shift occurred 70,000 years ago. Humans pushed into more challenging deserts and forests, filling the gaps on the ecological map of Africa.
“We were not expecting such a sharp change,” said Michela Leonardi, an archaeologist at the University of Cambridge and an author of the study. “We would expect more of a transition.”
William Banks, an archaeologist at the University of Bordeaux who was not involved in the new study, said the study’s conclusions were compelling because the researchers took such a broad view of human history.
In the past, researchers have often limited their studies to just a few archaeological sites. “We need to start looking at environmental conditions across whole regions,” Dr. Banks said. “And we can’t just do that with points on the landscape.”
Dr. Leonardi and her colleagues believe that a change in the climate prompted humans to expand their habitats. “We can see a pressure for this,” Dr. Scerri said.
Before 70,000 years ago, much of Africa was lush and wet. Even the Sahara was green. But then the planet entered an ice age. Ice sheets built up around the North and South Poles and the global climate cooled. In Africa, many regions lost much of their rainfall.
The comfortable habitats that humans had long enjoyed shrank and became fragmented. Dr. Scerri and her colleagues speculate that this change forced humans to move into environments that they had not bothered with before.
“It probably was not just one magical adaptation,” Dr. Manica said.
Instead, humans 70,000 years ago learned different skills to survive in different places. “They’re becoming the ultimate generalist,” he said.
Dr. Manica speculated that the human niche kept expanding in a cultural feedback loop. As humans continued to push into extreme environments, the barriers that had divided them fell away. Now African populations came into more contact with one another.
“Effectively, what you’re talking about is a more connected species,” Dr. Manica said. “It’s really just a ramping up of social networks, of movement of ideas. And that suddenly allows you to be successful in quite a wide range of conditions.”
This dramatic expansion of the human niche might explain why our species was so successful in migrating out of Africa about 50,000 years ago. Humans were now ready to learn about any place that they reached.
“The kind of environments you encountered out of Africa are not the ones that were you seeing in Africa before,” Dr. Manica said. “It’s not that in Africa you suddenly learned how to deal with tundra or the steppes. And yet what we see is that humans at 50,000 years ago finally managed to conquer them.”
Carl Zimmer covers news about science for The Times and writes the Origins column.
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