A magnified Y chromosome. High-resolution DNA sequencing has revealed… (Andrew Syred / Photo Researchers…)
The most recent common ancestors of modern-day men and women — dubbed “Adam” and “Eve” -- lived during roughly the same time period, contrary to previous findings indicating that Eve was tens of thousands of years older, according to two studies published Thursday in the journal Science.
The researchers sequenced DNA from two key sources -- the Y-chromosome, which is passed only from father to son; and mitochondria, which provides energy for cells and is transmitted only from a mother to her children. Both can serve as useful tools for tracing ancestry since they don’t swap genetic material, unlike the rest of the human genome.
Until now, scientists estimated that Eve lived about 190,000 to 200,000 years ago, while Adam lived much more recently -- between 50,000 to 115,000 years ago. But that “didn’t make a lot of genetic sense,” said Theodore Schurr, an anthropological geneticist at the University of Pennsylvania who was not part of the study. How could two individuals living as much 150,000 years apart not give rise to separate species?
The disparity might have resulted from limitations in gene sequencing. Up until about five years ago, scientists could sequence only a few regions of the genome. As a result, they could study only a few snippets of the long, structurally complex Y-chromosome genome, Schurr said.
But they were able to examine a significant portion of the mitochondrial genome, which is thousands of times shorter than the Y chromosome and easier to work with.
Thanks to recent technological advances, researchers can sequence more DNA cheaper and faster, allowing them to flesh out the details of humanity’s origins.
Geneticists from Stanford obtained DNA samples from 69 men living all over the world, including Namibia, Algeria, Pakistan, Siberia and Mexico. When they decoded the DNA from the Y chromosomes, they identified about 11,000 mutations. Earlier studies had found only a few hundred.
Since each mutation represents a branching point on the evolutionary tree, the new data enabled the researchers to establish ancestral relationships with remarkable detail.
When they were finished constructing their tree, they traced it back to a single event — the migration of humans to the Americas 15,000 years ago, based on mutation patterns among the subjects indicating a rapid migration to the region. Then, by examining mutations among Native Americans alive today, they were able to calculate how quickly changes accumulated in DNA. Then they used this to determine the length, in years, of each branch of the family tree.
When they were done, they estimated that Adam had lived around 120,000 to 156,000 years ago. After repeating their analysis with the subjects’ mitochondrial DNA, they concluded that Eve lived around 99,000 to 148,000 years ago -- showing for the first time that both ancestors were alive around the same time period.
In a separate study, Italian researchers used a similar approach, sequencing Y-chromosome DNA from 1,204 men from the Italian island of Sardinia. Using the spread of humans across Sardinia as a calibration point, they also identified about 11,000 mutations and estimated Adam to have lived roughly 180,000 to 200,000 years ago.
David Poznik, a geneticist at Stanford University who led the first study, cautioned that the most recent common ancestors don’t necessarily represent the “founding man or founding woman of humanity,” or the only people to have present-day descendants.
“The male most recent common ancestor is just the individual whose Y chromosome happens to survive to this day, whereas those of his contemporaries have died out,” he said. The same goes for his female counterpart.
Paolo Francalacci, a geneticist from the University of Sassari in Italy who led the second study, said he thinks his research will spur similar sequencing efforts that will add more detail to the human ancestral tree. His group has already begun sequencing the genomes of other populations along the Mediterranean.
“Up until now we’ve done these studies with very few markers,” he said. “It was like looking at a photo with a few pixels,” while current genome sequencing technology now allows geneticists to examine a “photo with lots of pixels.”
The new studies mark “a new era of evolutionary research,” he said.
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