Read moreIn 2012, Dr. Paabo and his colleagues took samples from the bone to search for DNA. To their surprise, it held a number of genetic fragments.
“This is an amazing and shocking and unique sample,” said David Reich, a geneticist at Harvard Medical School and co-author of the new study.
The researchers used the DNA fragments to recreate a high-resolution copy of the man’s complete genome. A Y chromosome revealed that the thighbone belonged to a man.
The scientists then compared the genome of the so-called Ust’-Ishim man to those of ancient and living people.
They found that his DNA was more like that of non-Africans than that of Africans. But the Ust’-Ishim man was no more closely related to ancient Europeans than he was to East Asians.
He was part of an earlier lineage, the scientists concluded — a group that eventually gave rise to all non-African humans.
Homo sapiens, our own species, appeared in Africa around 200,000 years ago. Previous studies — both on genes and fossils — have suggested that they then expanded through the Near East to the rest of the Old World.
The Ust’-Ishim man’s genome suggests he belonged to a group of people who lived after the African exodus, but before the split between Europeans and Asians.
Dr. Paabo and his colleagues also found that the Ust’-Ishim man had pieces of Neanderthal DNA in his genome, just as living non-Africans do. But his Neanderthal DNA has some important differences.
Fossils indicate that Neanderthals spread across Europe and Asia before becoming extinct an estimated 40,000 years ago. Today, the Neanderthal DNA in each living non-African human is broken up into short segments sprinkled throughout the genome.
Dr. Paabo and his colleagues have hypothesized that this arrangement is the result of how cells divide.
During the development of eggs and sperm, each pair of chromosomes swaps pieces of their DNA. Over the generations, long stretches of DNA get broken into smaller ones, like a deck of cards repeatedly shuffled.
Over thousands of generations, the Neanderthal DNA became more fragmented. Dr. Paabo and his colleagues predicted, however, that Neanderthal DNA in the Ust’-Ishim man’s genome would form longer stretches.
And that’s exactly what they found. “It was very satisfying to see that,” Dr. Paabo said.
By comparing the Ust’-Ishim man’s long stretches of Neanderthal DNA to shorter stretches in living humans, Dr. Paabo and his colleagues estimated the rate at which they fragmented. They used that information to determine how long ago Neanderthals and humans interbred.
Previous studies — based on only living humans — had yielded an estimate between 37,000 and 86,000 years. Dr. Paabo and his colleagues have now narrowed down that estimate dramatically: Humans and Neanderthals interbred between 50,000 and 60,000 years ago, according to the new data.
out of africa theory
Ancient Arabian Stones Hint at How Humans Migrated Out of Africa
Previous research had suggested that the exodus from Africa started between 70,000 and 40,000 years ago. However, a genetic analysis reported in April hinted that modern humans might have begun their march across the globe as early as 130,000 years ago, and continued their expansion out of Africa in multiple waves.
In addition, stone artifacts recently unearthed in the Arabian Desert date to at least 100,000 years ago. This could be evidence of an early modern-human exodus out of Africa, scientists say. However, it’s possible that these artifacts weren’t created by modern humans; a number of now-extinct human lineages existed outside Africa before or at the same time when modern humans migrated there. For instance, the Neanderthals, the closest known extinct relatives of modern humans, lived in both Europe and Asia around that time.
To help shed light on the role the Arabian Peninsula might have played in the history of modern humans, scientists compared stone artifacts recently excavated from three sites in the Jubbah lake basin in northern Saudi Arabia with items from northeast Africa excavated in the 1960s. Both sets of artifacts were 70,000 to 125,000 years old. Back then, the areas that are now the Arabian and Sahara deserts were far more hospitable places to live than they are now, which could have made it easier for modern humans and related lineages to migrate out of Africa.
“Far from being a desert, the Arabian Peninsula between 130,000 and 75,000 years ago was a patchwork of grasslands and savanna environments, featuring extensive river networks running through the interior,” Scerri said.
The northeast African stone tools the researchers analyzed were similar to ones previously found near modern-human skeletons. The scientists found that stone artifacts at two of the three Arabian sites were “extremely similar” to the northeast African stone tools, Scerri told Live Science. At the very least, Scerri said, this finding suggests that there was some level of interaction between the groups in Africa and those in the Arabian Peninsula, and might hint that these Arabian tools were made by modern humans.
Surprisingly, Scerri said, tools from the third Arabian site the researchers analyzed were “completely different.” “This shows that there was a number of different tool-making traditions in northern Arabia during this time, often in very close proximity to each other,” she said.
One possible explanation for these differences is that the artifacts were made by different human lineages. Future research needs to uncover skeletal remains with ancient tools unearthed from the Arabian Peninsula to help solve this mystery, Scerri noted. Unless skeletal remains are found near such artifacts, it will remain uncertain whether modern humans or a different human lineage might have made them.
“It seems likely that there were multiple dispersals into the Arabian Peninsula from Africa, some possibly very early in the history of Homo sapiens,” Scerri said. “It also seems likely that there may have been multiple dispersals into this region from other parts of Eurasia. These features are what make the Arabian Peninsula so interesting.”
Ancient migrants out of Africa and from Eurasia might have encountered a number of different populations in the Arabian Peninsula, Scerri said. Some of these groups may have adapted to their environment more than others had, which raises the intriguing question:
“Did the exchange of genes and knowledge between such groups contribute to our ultimate success as a species?” Scerri said.
Ancient Arabian Stones Hint at How Humans Migrated Out of Africa
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