Scientists researching the genomes of West Africans have discovered signs that a mysterious extinct human species was intertwined tens of thousands of years ago with our own species, the latest evidence of the complex genetic ancestors of humanity.
The study indicated that current West Africans traced a significant proportion, about 2 percent to 19 percent, of their genetic origin to an extinct human species – what the researchers called a “ghost population.”
“We estimate that crossbreeding took place about 43,000 years ago, at great intervals of uncertainty,” said University of California, Los Angeles (UCLA) professor of human genetics and computer science, Sriram Sankararaman, who led the study this week in Science magazine Advances has been published.
Homo sapiens first appeared in Africa just over 300,000 years ago and later spread throughout the world, encountering other human species in Eurasia that have since become extinct, including the Neanderthals and the lesser-known Denisovans.
Previous genetic research showed that our species was intertwined with both the Neanderthals and the Denisovans, with modern human populations outside Africa still carrying DNA from both. But although there is a large fossil record of Neanderthals and a few fossils of Denisovans, the newly identified “ghost population” is more puzzling.
Asked about the details that are known about this population, Sankararaman said, “Not much at this stage.”
“We do not know where this population may have lived, whether it matches known fossils and what the final fate was,” Sankararaman added.
Sankararaman said that this extinct species seemed to deviate about 650,000 years ago from the evolutionary line that led to Homo sapiens, before the evolutionary split between the lines that led to our species and to the Neanderthals.
The researchers examined genomic data from hundreds of West Africans, including the Yoruba people of Nigeria and Benin and the Mende people of Sierra Leone, and then compared them with the genomes of Neanderthal and Denisovan. They found DNA segments in West Africans that could best be explained by ancestral crossing with an unknown member of the human family tree that led to what is called genetic “introgression.”
It is unclear whether West Africans have obtained genetic benefits from this long-standing gene flow.
“We are starting to learn more about the impact of DNA from archaic hominids on human biology,” Sankararaman said, referring to extinct human species. “We now know that both the Neanderthal and Denisovan DNA were generally harmful, but there were some genes where this DNA had an adaptive impact. For example, height adjustment in Tibetans may have been facilitated by a Denisovan introgressive gene.”
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