The New York Times, July 11, 2024
Hundreds of thousands of years ago, our species arose in Africa. Research on the DNA of living people has indicated that early Homo sapiens stayed on the continent for a long while, with a small group leaving just 50,000 years ago to populate the rest of the world.
But those findings have raised a puzzling question: Why did our species take so long to move beyond Africa?
Several new studies, including one published on Thursday, argue that the timeline was wrong. According to new data, several waves of modern humans began leaving the continent about 250,000 years ago.
“It wasn’t a single out-of-Africa migration,” said Sarah Tishkoff, a geneticist at the University of Pennsylvania. “There have been lots of migrations out of Africa at different time periods.”
Those earlier migrations went largely overlooked until now, Dr. Tishkoff said, because the people who moved did not leave a clear fossil record of their existence, nor did living people inherit their DNA.
But scientists are now discovering hints of those early waves in the DNA of Neanderthals.
The Neanderthal lineage most likely began in Africa about 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 draft 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 signature of interbreeding from long ago. In May, a team of researchers estimated that Neanderthals and modern humans interbred during a short period of time, between 47,000 and 40,000 years ago.
But some Neanderthal DNA does not fit into this neat picture. The Neanderthal Y chromosome, for example, is more similar to the Y chromosome found in living humans than it is to the rest of the Neanderthal genome.
In 2020, researchers offered an explanation: Neanderthal males inherited a new Y chromosome from humans between 370,000 and 100,000 years ago. But that would have made sense only if a wave of Africans had expanded out of the continent much earlier than scientists had thought.
Researchers have 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 across Africa. Previous studies had found no sign of Neanderthal DNA in African genomes. But Dr. Tishkoff’s group detected tiny pieces of Neanderthal-like DNA scattered across all 12 of the populations they studied.
When they examined the size and sequence of those genetic fragments, they concluded that Neanderthals inherited them from early Africans. That meant an early wave of Africans expanded into Europe or Asia about 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 with its own statistical method. After comparing the genomes of 2,000 people from across the world with three Neanderthal genomes, they reached the same conclusion.
As Dr. Akey and his colleagues reported on Thursday, modern humans expanded out of Africa and interbred with Neanderthals between 200,000 and 250,000 years ago.
But Dr. Akey’s team also found evidence for yet another early wave. By comparing the genomes of young and old Neanderthal fossils, they concluded that another group of people migrated from 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 that some mysterious human fossils from Europe and the Middle East might belong to these early waves. “We’re 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 back over 210,000 years that bears some hallmarks of modern human anatomy.
The second wave of Africans might 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 hinted that there are other waves of human migration left to be discovered. “It suggests that there were repeated African dispersals for much of human history,” he said.
But why do the early migrations out of Africa seem to have fizzled away? Was there something different about the people in the last wave?
“The short answer is, Yes, there has to be something different,” said John Hawks, a paleoanthropologist at the University of Wisconsin.
It’s possible, he said, that African populations built up cultural knowledge that led them to make new inventions, like arrows, and adapt to new places more successfully.
Dr. Harvati also raised the possibility that early waves of humans might have struggled to compete with Neanderthals for land and food. But studies by Dr. Akey’s team and others suggest that Neanderthal populations were shrinking when the last wave emerged. Perhaps that decline gave humans an edge.
Copyright 2024 The New York Times Company. Reprinted with permission.