Man’s rare Y chromosome traced back 338,000 years
U. ARIZONA (US) — DNA from an African American in South Carolina pushes back the time of the most recent common ancestor for the Y chromosome lineage tree to 338,000 years ago.
This time predates the age of the oldest known anatomically modern human fossils.
“Our analysis indicates this lineage diverged from previously known Y chromosomes about 338,000 years ago, a time when anatomically modern humans had not yet evolved,” says Michael Hammer, an associate professor in the University of Arizona’s department of ecology and evolutionary biology. “This pushes back the time the last common Y chromosome ancestor lived by almost 70 percent.”
Unlike the other human chromosomes, the majority of the Y chromosome does not exchange genetic material with other chromosomes, which makes it simpler to trace ancestral relationships among contemporary lineages.
If two Y chromosomes carry the same mutation, it is because they share a common paternal ancestor at some point in the past. The more mutations that differ between two Y chromosomes the farther back in time the common ancestor lived.
Originally, a DNA sample obtained from an African American in South Carolina was submitted to the National Geographic Genographic Project. When none of the genetic markers used to assign lineages to known Y chromosome groupings were found, the DNA sample was sent to Family Tree DNA for sequencing.
Fernando Mendez, a postdoctoral researcher in Hammer’s lab, led the effort to analyze the DNA sequence, which included more than 240,000 base pairs of the Y chromosome.
“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,” Hammer says.
Traced to Cameroon
About 300,000 years ago, the time the Neanderthals are believed to have split from the ancestral human lineage. It was not until more than 100,000 years later that anatomically modern humans appear in the fossil record. They differ from the more archaic forms by a more lightly built skeleton, a smaller face tucked under a high forehead, the absence of a cranial ridge and smaller chins.
Hammer says the newly discovered Y chromosome variation is extremely rare. Through large database searches, his team eventually was able to find a similar chromosome in the Mbo, a population living in a tiny area of western Cameroon in sub-Saharan Africa.
“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.”
“Instead, the sample matched the Y chromosome DNA of 11 men, who all came from a very small region of western Cameroon,” Hammer says. “And the sequences of those individuals are variable, so it’s not like they all descended from the same grandfather.”
Hammer cautions against popular concepts of “mitochondrial Eve” or “Y chromosome Adam” that suggest all of humankind descended from exactly one pair of humans that lived at a certain point in human evolution.
“There has been too much emphasis on this in the past,” he adds. “It is a misconception that the genealogy of a single genetic region reflects population divergence. Instead, our results suggest that there are pockets of genetically isolated communities that together preserve a great deal of human diversity.”
Still, Hammer says, “It is likely that other divergent lineages will be found, whether in Africa or among African Americans in the US and that some of these may further increase the age of the Y chromosome tree.”
The study, published in the American Journal of Human Genetics, came about by combined efforts of a private business, Family Tree DNA, the efforts of a citizen scientist, Bonnie Schrack, and research capabilities at the University of Arizona.
Source: University of Arizona
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