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    Human Y Chromosome Divergence Placed At Least 300,000 Years Ago
    By News Staff | March 6th 2013 09:54 AM | 4 comments | Print | E-mail | Track Comments

    The  divergent lineage of the oldest known genetic branch of the human Y chromosome, the hereditary factor determining male sex, has been pushed back in time.

    The new divergent lineage, found in an individual who submitted his DNA to Family Tree DNA, a company specializing 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.

    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 living 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.

    About 300,000 years ago falls around 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.

     "Our analysis indicates this lineage diverged from previously known Y chromosomes about 300,000 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 percent." 

    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.

    Hammer said "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."



    Prof. Michael Hammer, geneticist at the University of Arizona. His friend is an ancient hominid fossil,. Credit: Michael Hammer 

    Hammer said 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 said. "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 said. "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 said, "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."

    He 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."


    Published in the American Journal of Human Genetics


    Comments

    I hate to burst their bubbles, but just because there is a Y sex Chromosome does NOT insure a male is formed. It is a lot more complicated than that and there is large amounts of evidence stating so. I am saddened by the lack of information in the article. Its 1st grade level reading.

    Gerhard Adam
    It's a press release.  Get over it.  Read the article in the journal if you want the details.
    Mundus vult decipi
    Hank
    If you have evidence of intersex fossils from 300,000 years ago by all means present it. There doesn't seem to be a lack of anything in this press release, it is more that you want exceptions to invalidate the rule. A misplaced gene can cause a male to develop without a Y chromosome but that is outside the scope of this study, which is about divergence of the human Y.
    News articles often do a poor job of explaining scientific concepts. "Y-Chromosome Adam" represents the last man who was the father of everyone on Earth...but that doesn't mean that he was the only human at that point in the past. Nor does it mean he was the first human. Some of the men alive at the same time were fathers of a percentage of today's population, and some were fathers of lines that went extinct. It's also possible, though scientists would hate to admit this, that there never was a "Y-chromosome Adam". Just 50,000 years ago, there were at least five, and possibly six, species (or sub-species: authorities differ) of humans on Earth: sapiens, Neanderthals, Denisovans, Hobbits, Homo erectus, and Red Cave people (from Southern China). Some of these groups interbred, and some didn't. Africans today generally have no Neanderthal DNA, while Europeans and Asians do. If you have to trace back the y-chromosome to the time before sapiens even existed, can we really call this archaic man "human"?

    Also, this one case variant, which subsequently became 12 people (1 from South Carolina and 11 from Cameroon), may represent an incident of interbreeding between "archaic" and "modern" human populations.