"Previous research has indicated that the male MRCA lived much more recently than the female MRCA," said Carlos
Bustamante, PhD, a professor of genetics at Stanford.
"But now our research shows that there's no discrepancy." Previous estimates for the male MRCA ranged from between
50,000 to 115,000 years ago.
Despite the Adam and Eve monikers, which evoke a single couple whose children peopled the world, it is extremely
unlikely that the male and female MRCAs were exact contemporaries, the researchers said.
They weren't the only man and woman alive at the time, or the only people to have present-day descendants.
The researchers made their discovery by comparing Y-chromosome sequences among 69 men from nine globally distinct
regions, including some that have only recently been available for study. Regions represented included Namibia,
the Democratic Republic of Congo, Gabon, Algeria, Pakistan, Cambodia, Siberia and Mexico.
New, high-throughput sequencing technologies allowed the researchers to identify about 11,000 differences
among the sequences.
These variants enabled them to establish phylogenetic relationships and timelines among the sequences with unprecedented accuracy.
These two individuals simply had the good fortune of successfully passing on specific portions of their DNA - from the man,
the Y chromosome; from the woman, the mitochondrial genome - through the millennia to most of us, while the corresponding
sequences of others have largely died out due to natural selection or a random process called genetic drift.
"Essentially, we've constructed a family tree for the Y chromosome," said David Poznik, graduate student and the lead author of the study.
"Prior to high-throughput sequencing, the tree was based on just a few hundred variants. Although these variants had revealed the main topology, we couldn't say much about the length of any branch - the number of variants shared by all of its descendants," Poznik said.
"We now have a more complete structure, including meaningful branch lengths, which are proxies for the periods of time between specific branching events."
"We can now date certain events very precisely," said Bustamante. "We found a single variant that shows how three ancient lineages came together about 48,000 years ago, plus or minus only a couple of hundred years. The accuracy is exquisite."
The tree also exemplifies the extraordinary depth of genetic diversity present among modern Africans.