All non-Africans descended from a single migration out of Africa: Study

  • HT Correspondent
  • Updated: Sep 22, 2016 22:52 IST

New Delhi: All non-Africans alive today descended from a single migration out of Africa nearly 72,000 years ago, a new research has shown.

Studying hundreds of new genomes from across the globe, researchers, one of them of Indian-origin, found compelling evidence to prove the theory.

The findings are supported by three new genome sequencing studies published in the journal Nature.

One of the studies led by Harvard Medical School (HMS) geneticists sequenced samples from 142 smaller populations.

“Here we report the Simons Genome Diversity Project data set: high quality genomes from 300 individuals from 142 diverse populations. These genomes include at least 5.8 million base pairs that are not present in the human reference genome,” say excerpts from the study.

“Our analysis reveals key features of the landscape of human genome variation, including that the rate of accumulation of mutations has accelerated by about 5% in non-Africans compared to Africans since divergence. We show that the ancestors of some pairs of present-day human populations were substantially separated by 100,000 years ago, well before the archaeologically attested onset of behavioural modernity.”

“We also demonstrate that indigenous Australians, New Guineans and Andamanese do not derive substantial ancestry from an early dispersal of modern humans; instead, their modern human ancestry is consistent with coming from the same source as that of other non-Africans.”

Swapan Mallick from HMS, first author of the study, said, “We wanted to go out into the world and pull together as many of the ethnically, linguistically and anthropologically diverse samples as we possibly could.”

The Harvard geneticists and their international team of colleagues began by selecting two genomes each from 51 populations represented in a collection called the Human Genome Diversity Project.

A key conclusion -- that the vast majority of modern human ancestry in non-Africans derives from a single population that migrated out of Africa -- is also supported by two other whole-genome sequencing studies appearing simultaneously in Nature.

One, led by an Estonian group, focused on 379 whole genome sequences; the other, led by a Danish group, analysed 108 Australians and New Guineans.

Together, the three studies put to rest a lingering question about whether indigenous peoples of Australia, New Guinea and the Andaman Islands descend in large part from a second group that left Africa earlier and skirted the coast of the Indian Ocean.

They do not, the Harvard researchers said.

“Our best estimate for the proportion of ancestry from an early-exit population is zero,” David Reich, Professor of Genetics at Harvard Medical School.

The study that analysed 108 Australians and New Guineans estimated that around 72,000 years ago, an ancestral population common to Aborginal Australians, Europeans and East Asians left the African continent.

“Discussions have been intense as to what extent Aboriginal Australians represent a separate Out-of-Africa exit to those of Asians and Europeans,” said Professor Laurent Excoffier of the SIB Swiss Institute of Bioinformatics and the University of Bern said.

“We find that, once we take into account admixture with archaic humans, the vast majority of the Aboriginal Australian genetic makeup comes from the same African exit as other non-Africans,” Excoffier explained.

The HMS-led study further revealed that the common ancestors of modern humans began to differentiate at least 200,000 years ago, long before the out-of-Africa dispersal occurred.

“It had been unclear whether the group that expanded out of Africa represented a large subset of the populations within Africa,” said Mallick.

“This really shows that there was a lot of substructure prior to the expansion.”

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