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Newly Discovered Y Chromosome Predates Oldest Known Human Fossils

March 6, 2013
Image Caption: Human sex-determining chromosomes: X chromosome (left) and the much smaller Y chromosome. Credit: University of Arizona

redOrbit Staff & Wire Reports – Your Universe Online

Researchers have discovered a Y chromosome — the hereditary factor determining male sex — that dates back to 100,000 years before the oldest known anatomically modern human fossils.

The chromosome was originally obtained from an African-American man living in South Carolina. It was originally submitted to the National Geographic Genographic Project — a research project that utilizes genetic and computational technology to analyze historical patterns in the DNA of individuals around the world.

However, none of the genetic markers used to assign lineages to known Y chromosome groupings were found to be a match for the sample, it was sent to a private firm, Family Tree DNA, for sequencing. Their findings have been published in the American Journal of Human Genetics.

Those sequencing efforts were led by Fernando Mendez, a postdoctoral researcher in the lab of University of Arizona (UA) associate professor Michael Hammer, and included more than 240,000 base pairs of the Y chromosome, the Tucson-based institution explained in a recent statement.

“Our analysis indicates this lineage diverged from previously known Y chromosomes about 338,000 ago, a time when anatomically modern humans had not yet evolved,” said Hammer, who works in the UA´s department of ecology and evolutionary biology and is also a research scientist at the school´s Arizona Research Labs.

“This pushes back the time the last common Y chromosome ancestor lived by almost 70 percent,” he added. “The most striking feature of this research is that a consumer genetic testing company identified a lineage that didn’t fit anywhere on the existing Y chromosome tree, even though the tree had been constructed based on perhaps a half-million individuals or more. Nobody expected to find anything like this.”

The newly discovered Y chromosome variant is exceptionally rare, Hammer said. He and his colleagues conducted massive database searches and eventually were able to find similar genetic material in the Mbo, an ethnic group living in the western part of Cameroon.

“This was surprising because previously the most diverged branches of the Y chromosome were found in traditional hunter-gatherer populations such as Pygmies and the click-speaking KhoeSan, who are considered to be the most diverged human populations living today,” he explained. “Instead, the sample matched the Y chromosome DNA of 11 men, who all came from a very small region of western Cameroon. And the sequences of those individuals are variable, so it’s not like they all descended from the same grandfather.”

“It is likely that other divergent lineages will be found, whether in Africa or among African-Americans in the U.S. and that some of these may further increase the age of the Y chromosome tree,” Hammer added. “There has been a lot of hype with people trying to trace their Y chromosome to different tribes, but this individual from South Carolina can say he did it.”


Source: redOrbit Staff & Wire Reports - Your Universe Online



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