Monday, 30th October 2017

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Scent of unknown human ancestor

Scientists have discovered what they believe are genetic fragments of an extinct, previously unknown ancestor of modern humans hidden in the genomes of the Jarawa and Onge people on the Andaman islands.

By G S Mudur
  • Published 15.08.16
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New Delhi, July 29: Scientists have discovered what they believe are genetic fragments of an extinct, previously unknown ancestor of modern humans hidden in the genomes of the Jarawa and Onge people on the Andaman islands.

A study led by scientists in India and Spain has revealed in the genetic makeup of these islanders subtle signatures that they say are best explained through the existence of an extinct archaic species - similar to the Neanderthals and the Denisovans.

Under current views of human evolution, a species called Homo erectus emerged about two million years ago and gave rise to Homo heidelbergensis by about 600,000 years ago. Homo heidelbergensis split into two lineages - the Neanderthals and the Denisovans - before evolving into Homo sapiens about 200,000 years ago.

"Our findings suggest Homo heidelbergensis had a third lineage - the Neanderthals and the Denisovans had at least one other cousin whose genetic residues we see in the Jarawa and Onge," Partha Pratim Majumder, the director of the National Institute of Biomedical Genomics, Kalyani (Bengal), told The Telegraph.

Over the past two decades, scientists across the world have used genomic analysis to reconstruct the migratory history of ancient human populations, using segments of genetic material that can determine the proximity and distance of populations relative to each other. Several such earlier studies have suggested that ancestral human species, starting with Homo erectus, emerged in but trudged out of Africa to populate other continents. Modern humans, or Homo sapiens, also emerged out of Africa several tens of thousands of years ago.

Anthropologists have for long interpreted the dark complexion and frizzly hair of the Jarawa and Onge to assume they are related to African populations. In line with these ideas, genetic studies over the past decade have shown that these islanders are descendants of the earliest modern humans who migrated out of Africa about 60,000 years ago.

In their new study, Majumder collaborating with Jaume Bertranpetit at the Institute of Biological Evolution in Barcelona, Spain, and their colleagues found that the Andamanese genomes appear to be missing two to three per cent of genetic signatures associated with African ancestry.

The scientists have described their findings this week in the journal Nature Genetics.

One possible explanation of this missing African material is to assume it was replaced by Neanderthal and Denisovan genetic material - as multiple earlier studies have established that the Neanderthals and Denisovans intermingled and mated with Homo sapiens before going extinct.

But equal amounts of Neanderthal and Denisovan ancestry in the Andamanese and east Asian populations make this explanation unlikely. The scientists say the best explanation is that genetic fragments from an unknown and archaic, extinct human species replaced the two to three per cent missing African genetic segments.

While an abundance of skeletal remains of Neanderthals have been unearthed across northern Asia and Europe since the first discovery in Germany in 1829, the evidence for Denisovans comes from only a fingerbone found in 2010 and a wisdom tooth - both recovered from Denisova cave in Siberia.

Majumder said the absence of fossil evidence thus far for the proposed extinct species is not surprising. "Skeletal remains are not easily preserved - they're hard to find," he said. But genetic studies such as this one, scientists are hoping, will spur fossil hunters to continue looking for skeletal remains.

The analysis suggests that the extinct archaic species could have emerged as far back as 300,000 years ago, but its geographic spread and the time of its extinction remain unknown, the researchers say.

"We expect that people engaged in ancient DNA research will retrieve more information about this species in the future," Betranpetit told this newspaper. "The whole picture of our ancestry is more complex than we have thought."

Their study also questions suggestions made earlier that modern humans may have had multiple out-of-Africa migrations. About five years ago, Danish population geneticist Mortem Rasmussen and his colleagues studied Australian aboriginal material and proposed that Australian aborigines owe their ancestry to an out-of-Africa migration of modern humans that occurred between 62,000 and 75,000 years ago, thus making them "the oldest continuous (human) population outside Africa."

But the new findings by the Majumder-Bertrainpetit collaboration suggest that the existence of an unknown cousin to the Neanderthals and the Denisovans could under certain conditions make a single out-of-Africa migration appear as two distinct migrations. The results support a single origin for the Andaman tribes, Asian populations, and the Australian aborigines - in other words, a single out-of-Africa modern human migration led to the populations of Asia and Australia.

"This is a fascinating paper - the most exciting new result is that the present-day inhabitants of the Andaman islands, the Pacific and the aboriginal Australians harbor a genetic signature that one or more archaic population contributed to the human gene pool," Carlos Bustamante, a population geneticist at Stanford University who has been involved in human origin studies.

"The evidence is strong - they (the Majumder-Bertranpetit collaboration) are careful to sugges there may be more than one archaic population different fromNeanderthals and Denisovans that contributed to diversity in Asian genomes," Bustamante said.

"They also make a strong case to suggest that the previous evidence for multiple migrations of modern humans may need to be reevaluated," he said. "My hope is that we continue to diversity the set of human genomes we sequence and continue to gain insights into how we became such a beautifully diverse species."