Wednesday, April 17, 2013

Father of All Men is 340,000 Years Old !

Albert Perry carried a secret in his DNA: a Y chromosome so distinctive that it reveals new information about the origin of or species. In fact, the last ancestor that Perry shares down the paternal line with most other men walked the Earth before our species appeared! One possible explanation for Perry's bewildering ancestry is the hundreds of thousands of years ago, modern humans interbred with a species that is now extinct.

A man gets his Y chromosome from his father, who in turn got his from his father, and so on. Trace every man's father-son lineage far enough back in time and they all converge on one individual who -- calculations show -- lived between 60,000 and 140,000 years ago. This man was far from the only one alive at the time, but he is known as the ''Y chromosomal Adam'' because by chance all men acquired their sex chromosome from him. Or so we thought. Perry, recently deceased, was an African-American who lived in South Carolina. A few years ago, one of his female relatives submitted a sample of his DNA to Family Tree DNA, a company in Houston, Texas, for genealogical analysis. When the company's technicians tried to place Perry on the Y chromosome family tree, they just couldn't. His was like no other so far analysed.

Chief scientist at Family Tree DNA, Dr. Michael Hammer, heard about Perry and did further tests.  These showed that Perry's Y chromosome lineage probably separated from all others about 338,000 years ago. So the accidental discovery of Perry's DNA means we now know of at least one man who did not descend from the Y chromosomal Adam. The title of the father of all men must be passed to another man....who lived more than twice as far back in prehistory. Perry is unlikely to have been the sole member of his unusual lineage. Hammer estimates that another 1500 American men carry the new Y chromosome. What's more, further analysis uncovered similarities between 11 men living in a village in Cameroon, in sub-Saharan Africa, called Mbo, indicating roughly where his ancestors originally hailed from.  Jon Wilkins of the Ronin Institute, in Montclair, New Jersey says, ''After looking at chromosomes for years, so having to change where the root of the Y-chromosome tree is at this point is extremely surprising.''

The first fossils of our species are 195,000 years old, so the new Y-chromosomal Adam lived long before our species appeared. What are the implications? 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. At some point after our species emerged all those years ago, a female Homo Sapiens may have mated with a male from a cousin species that had split from our own 338,000 years ago. The male's species later vanished, but the random mating meant his Y chromosome was transferred to our species and eventually made its way into Perry's cells. If this happened, it adds to other examples of our interbreeding: with Neanderthals in the Middle East and with the Denisovans somewhere in South-East Asia.

''The theory fits with the fossil record'', says Chris Stringer at the Natural History Museum, London. In 2011, he was involved in an analysis of fossils from a Nigerian site called Iwo Eleru. The fossils showed a strange mix of ancient and modern features, and were found just a few hundred kilometres from the village of Mbo. ''This was surprising because previously the most diverged branches of the Y chromosome were found in traditional hunter-gather populations such as Pygmies and the click-speaking KhoeSan, who are considered to be the most diverged humans living today.  It certainly looks like we have a more complex scenario for the evolution of modern humans in Africa,'' says Stringer.

The Way I See It....we must be cautious against popular concepts of a 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. It is a misconception that the genealogy of a single genetic region reflects the population divergence. Instead, genetic test results suggest that there are pockets of genetically isolated communities that together preserve a great deal of human diversity. It's exciting to think that it is likely other divergent lineages will be found and that some of these may further increase the age of the Y chromosome tree. Dr Hammer sums it up with, ''There has been a lot of hype with people trying to trace their Y chromosome to different tribes, but in this case, Albert Perry from South Carolina can say he did it!'' 

NOTE:  For more information about the Y-Chromosome read my JULY 7, 2010 posting entitled; "X-Celling Over Men !"

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