Hundreds of thousands of years ago, our species emerged in Africa. Research into the DNA of living humans has shown that early Homo sapiens remained on the continent for a long time, with a small group leaving only 50,000 years ago to populate the rest of the world.
However, these findings raise a puzzling question: Why did it take so long for our species to leave Africa?
Several new studies, including one published Thursday, claim that timeline was wrong. According to new data, several waves of modern humans began leaving the continent around 250,000 years ago.
“It wasn’t just one migration out of Africa,” says Sarah Tishkoff, a geneticist at the University of Pennsylvania. “There were many migrations out of Africa at different times.”
According to Dr. Tishkoff, those past migrations have been largely overlooked until now because the people who moved left no clear fossil evidence of their existence and living people have not inherited their DNA.
But scientists are now discovering clues to these early waves in the DNA of Neanderthals.
The Neanderthal lineage likely began in Africa around 600,000 years ago, before moving into Europe and Asia. In 2010, Svante Paabo, a Swedish geneticist, and his colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, published the first version of a Neanderthal genome, reconstructed from 40,000-year-old fossils found in Croatia.
Dr. Paabo’s team also discovered that living, non-African people carry fragments of Neanderthal DNA, a marker of long-ago interbreeding. In May, a team of researchers estimated that Neanderthals and modern humans interbred in a short period between 47,000 and 40,000 years ago.
But some Neanderthal DNA doesn’t fit into this neat picture. The Neanderthal Y chromosome, for example, looks more like the Y chromosome found in living humans than it does the rest of the Neanderthal genome.
In 2020, researchers offered an explanation: Neanderthal men inherited a new Y chromosome from humans between 370,000 and 100,000 years ago. But that would only make sense if a wave of Africans had spread from the continent much earlier than scientists thought.
Researchers recently found evidence for such an early wave in the genomes of living Africans.
Dr. Tishkoff and her colleagues compared the genome of a 122,000-year-old Neanderthal fossil with the genomes of 180 people from 12 populations in Africa. Previous studies had found no sign of Neanderthal DNA in African genomes. But Dr. Tishkoff’s group discovered small bits of Neanderthal-like DNA spread across all 12 populations they studied.
When they examined the size and order of those genetic fragments, they concluded that Neanderthals inherited them from early Africans. That meant that an early wave of Africans spread to Europe or Asia around 250,000 years ago and interbred with Neanderthals.
Another group of researchers — led by Joshua Akey, a professor of genomics at Princeton University — tackled the same question using a statistical method of their own. After comparing the genomes of 2,000 people from around the world with three Neanderthal genomes, they reached the same conclusion.
As Dr. Akey and his colleagues reported Thursday, modern humans spread out of Africa and interbred with Neanderthals between 200,000 and 250,000 years ago.
But Dr. Akey’s team also found evidence for another early wave. By comparing the genomes of young and old Neanderthal fossils, they concluded that another group of people migrated out of Africa between 120,000 and 100,000 years ago.
Katerina Harvati, a paleoanthropologist at the University of Tübingen who was not involved in the new studies, said some mysterious human fossils from Europe and the Middle East may belong to these early waves. “We are starting to see this more complicated reality in the fossil record,” she said.
In 2019, Dr. Harvati and her colleagues described a skull fragment from Greece dating to over 210,000 years ago that shows some features of modern human anatomy.
The second wave of Africans may have reached Israel, Dr. Akey and his colleagues say. Paleoanthropologists have found modern-looking fossils and stone tools in Israeli caves that are estimated to be 100,000 to 130,000 years old.
Dr Akey said the findings suggested there were other waves of human migration yet to be discovered. “It suggests that there have been repeated African dispersals throughout much of human history,” he said.
But why do the early migrations out of Africa seem to have disappeared? Was there something different about the people in the last wave?
“The short answer is yes, there has to be something else,” said John Hawks, a paleoanthropologist at the University of Wisconsin.
It is possible, he said, that African populations built up cultural knowledge that allowed them to make new inventions, such as arrows, and better adapt to new places.
Dr. Harvati also raised the possibility that early waves of humans struggled to compete with Neanderthals for land and food. But studies by Dr. Akey’s team and others suggest that Neanderthal populations were shrinking as the last wave came along. Perhaps that decline gave humans an edge.