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Y chromosome analysis moves Adam closer to Eve

Genetic studies push back age of men's most recent common ancestor

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Men might need some more pages in their family album.

The largest analyses to date of the human Y chromosome suggest that modern men can trace their family tree further back in time than previously thought. One of the studies, an analysis of 69 men from nine populations worldwide published in the Aug. 2 Science, finds that their most recent common ancestor lived 120,000 to 156,000 years ago. That’s roughly the same time that the last common ancestor of women is estimated to have lived, researchers report.

The Y chromosome, passed down from father to son, and mitochondrial DNA, passed down from mother to child, are useful in retracing ancestry because they don’t undergo genetic reshuffling as the rest of the genetic instruction book does. Researchers analyze mutations in these parts of the genome to assess when groups split apart. The hypothetical common ancestors of these genetic lineages are sometimes called Y Chromosome Adam and Mitochondrial Eve.

“We’re not saying they’re exact contemporaries or they actually met or all men and women descended from the same couple,” says study coauthor Carlos Bustamante of Stanford University. Y Chromosome Adam and Mitochondrial Eve aren’t the first human male and female but instead represent the common ancestors of the modern Y chromosome and modern mitochondrial DNA.

The findings may overturn previous results that suggested Y Chromosome Adam was only a half or a third as old as Mitochondrial Eve. Previous analyses date the Y chromosome common ancestor to between 50,000 and 115,000 years ago and the mitochondrial DNA common ancestor to between 150,000 and 240,000 years ago.

Bustamante and colleagues also looked at mitochondrial DNA in their study population and found a common female ancestor 99,000 to 148,000 years ago.

Another study in the same issue of Science pushes the Y ancestor back even further in time. Paolo Francalacci of the University of Sassari in Italy and colleagues analyzed DNA from 1,204 Sardinian men and determined that Y Chromosome Adam lived 180,000 to 200,000 years ago.

Since the Y chromosome and mitochondrial DNA are inherited separately and have independent evolutionary histories, their trees do not necessarily spread from contemporaneous roots. Still, the apparent lag between the mitochondrial DNA and Y chromosome lineages has been a head-scratcher for researchers who expected the ancestors to be contemporaneous. “People tied themselves in knots to come up with an explanation,” says Rebecca Cann, an evolutionary biologist at the University of Hawaii at Manoa.

One idea implicated mating differences between the sexes. Women bear approximately similar numbers of children, but men can vary widely in their fertility, Bustamante says. One man might leave behind hundreds of descendants, another only one or a few. That variation in the number of offspring of men and women could account for different patterns in the Y chromosome and mitochondrial DNA trees.

The studies look at longer stretches of the Y chromosome than earlier work, which could help explain why they find an older male ancestor, says Bustamante, whose team analyzed complete Y chromosome sequences.  

But even these studies are missing pertinent data, says Michael Hammer of the University of Arizona in Tucson. In March, Hammer and colleagues reported in the American Journal of Human Genetics the discovery of a rare Y chromosome in an African-American and other Y chromosomes from the same lineage in 11 men in western Cameroon. Hammer’s team traced the most recent common ancestor of the Y chromosome back 338,000 years.

In this scenario, the Y chromosome ancestor is much older than the mitochondrial DNA ancestor — and even predates the earliest known fossils of Homo sapiens by more than 100,000 years. The great antiquity may imply that H. sapiens is older than the fossil evidence currently suggests or that early humans mated with a closely related hominid species that contributed to the Y chromosome gene pool.

The new studies didn’t consider the Cameroonian population, so they are missing crucial genetic diversity in their analyses, Hammer says. In general, scientists are overlooking lots of Y chromosome diversity because populations in sub-Saharan Africa have been poorly sampled, he says.

Melissa Wilson Sayres, a geneticist at the University of California, Berkeley, agrees there are still a lot of data to collect. Part of the problem has been the complicated nature of the Y chromosome itself. It’s highly repetitive and therefore has taken a long time to properly read. In fact, she says, it took almost as long to sequence the Y chromosome as it did to sequence all the rest of the human genome.

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