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article imageScientists: Modern, archaic humans interbred

By Elizabeth Cunningham Perkins     Sep 5, 2011 in Science
A study led by University of Arizona (UA) researchers found evidence archaic forms of the genus Homo, such as Homo erectus ("upright man") and Homo habilis ("tool using man"), and Homo sapiens interbred before any modern human migrations out of Africa.
The theory that anatomically modern humans of the species Homo sapiens originated in Africa and migrated over the entire planet in several waves has been widely accepted, and now the question of whether Homo sapiens interbred with more archaic forms of the genus Homo has been answered by a statistical and computational study that suggests ancient humans and closely related primitive forms around them "mixed it up" often, according to UA research scientist Michael Hammer.
Recent DNA research on Neanderthal bones has provided evidence interbreeding occurred after anatomically modern humans migrated from Africa into Eurasia, as Digital Journal reported previously, but the lack of preserved DNA within bones 40,000 to 60,000 years old, unearthed in warmer African climates, has kept what happened earlier between Homo sapiens and their relatives in Africa a mystery.
According to the UA team's paper about their findings, scheduled for publication in the Proceedings of the National Academy of Sciences (PNAS), this new research approached the question indirectly by comparing unusual genome regions identified in DNA from six modern African populations with computer simulations that predicted what ancient DNA sequences would look like within the genes in modern human cells.
Sequencing expanses of human genomes in samples from six populations living in Africa now, and focusing on non-coding segments (that do not contain protein-blueprints), the team attempted to match unusual sequences with results they would expect to find in archaic forms.
They tagged DNA sequences as archaic that differed radically and stretched over shorter areas of chromosomes.
Hammer explained, "We are talking about something that happened between 20,000 and 60,000 years ago -- not that long ago in the scheme of things. If interbreeding occurs, it's going to bring in a whole chromosome, and over time, recombination events will chop the chromosome down to smaller pieces. And those pieces will now be found as short, unusual fragments. By looking at how long they are we can get an estimate of how far back the interbreeding event happened."
Although the team found unusual archaic DNA sequences make up only two to three percent of modern human genomes, they concluded interbreeding was probably extensive and common, with thousands of events likely happening, because many sequences from interbreeding that conferred no particular evolutionary advantage would have been lost over time.
A next step is looking for ancient DNA sequences that provided distinct evolutionary advantages to the populations of Homo sapiens that incorporated them.
"Anatomically modern humans were not so unique that they remained separate. They have always exchanged genes with their more morphologically diverged neighbors. This is quite common in nature, and it turns out we're not so unusual after all," Hammer claimed.
More about ancient humans, Prehistoric man, prehistoric humans, Neanderthals, Homo erectus
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