Washington: Researchers have discovered an extremely rare African American Y chromosome that pushes back the time of the most recent common ancestor for the Y chromosome lineage tree to 338,000 years ago.
University of Arizona geneticists have discovered the oldest known genetic branch of the human Y chromosome - the hereditary factor determining male sex.
The new divergent lineage, which was found in an individual who submitted his DNA to Family Tree DNA, a company specialising in DNA analysis to trace family roots, branched from the Y chromosome tree before the first appearance of anatomically modern humans in the fossil record.
The study was published in the American Journal of Human Genetics.
"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 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 per cent," Hammer said in a statement.
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.
"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 said.
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 said the newly discovered Y chromosome variation is extremely rare.