African ancestors: Evolution’s past is modern human’s present

“The past is present when it comes to human evolution.” 

That seems to be the takeaway from new research that concludes “archaic” humans, somewhere in Africa during the last 20-60 thousand years, interbred with anatomically modern humans and transferred small amounts of genetic material to their offspring who are alive today.

University of Arizona geneticist Michael Hammer and a team of evolutionary biologists, geneticists and mathematicians report the finding in today’s Proceedings of the National Academy of Sciences.

The National Science Foundation (NSF) in Arlington, Va., funded the study.

Caption: National Science Foundation-funded scientists studied DNA sequence data from three sub-Saharan African populations to test whether their ancient, anatomically-modern relatives interbred with "archaic" humans inside Africa. Credit: © 2011 Jupiter Images Corporation

“It appears some level of interbreeding may have occurred in many parts of the world at different times in human evolution,” said Hammer, whose work is the first to definitively suggest interbreeding between separate human forms inside of Africa.

Previous studies primarily examined modern human interbreeding with Neanderthals in Europe or other archaic forms in Asia. Analyses of ancient DNA in those studies suggest a small amount of gene flow occurred from Neanderthals into non-African offspring sometime after anatomically modern humans left Africa.

Additionally, researchers discovered evidence that a distinct group of extinct archaic humans who lived in Southern Siberia, called ‘Denisovans,’ contributed genetic material to the genomes of present-day Melanesians.

As Hammer and colleagues write in their recent report, “Given recent fossil evidence, however, the greatest opportunity for introgression was in Africa” where anatomically modern humans and various archaic forms co-existed for much longer than they did outside of Africa.

“We estimate that the archaic DNA fragments that survive in modern African genomes come from a form or forms that diverged from the common ancestor of anatomically modern humans 700 thousand to 1 million years ago,” said Hammer.

“This archaic genetic material is more prevalent in west-central African populations, possibly reflecting a hybridization event or process that took place in central Africa,” he said. “The populations that interbred were similar enough biologically so that they were able to produce fertile offspring, thus allowing genes to flow from one population to the other.”

Hammer and team studied DNA sequence data from three sub-Saharan African populations: Mandenka, Biaka and San to test models of African archaic admixture.

The Mandenka are an agricultural population from West Africa, while the Biaka and the San are historically isolated hunter-gatherer populations from Central and Southern Africa, respectively.

The team conducted extensive simulations to test the likelihood of gene flow from an archaic population to anatomically modern humans. The simulations rejected the default position, or null hypothesis, that no admixture took place.

The result gave the scientists confidence to infer that contemporary African populations contain a small proportion of genetic material–about 2 percent–that moved from a species of archaic humans into the gene pool of anatomically modern humans about 35 thousand years ago.

They surmise that this archaic population split from the ancestors of anatomically modern humans about 700 thousand years ago.

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Source: Biomechanism

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