New Light on Genetic Adam and Eve
The results of two recent studies challenge earlier studies that suggested genetic ancestral "Eve" lived long before genetic ancestral "Adam".
A fisherman on a traditional Sardinian boat made of common reeds. [Photo by Gianluca Dedola]
Previous genetic research has indicated the existence of two ancient modern human individuals who passed their genes along to all humans living today, what scientists have referred to as our most recent common ancestors, or MRCAs. The first, designated "Mitochondrial Eve", lived between 190,000 and 200,000 years ago, and the second, "Y-chromosomal Adam", between 50,000 and 115,000 years ago.
Now, a team of researchers led by Stanford University's Carlos Bustamente and David Poznik have redefined the ranges for Y-chromosomal Adam and Mitochondrial Eve, placing them at 120,000 to 156,000 years ago and 99,000 to 148,000 years ago, respectively. The most significant finding relates to the relative timing of their existence.
"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." The study resuts suggest that genetic Adam and Eve lived much closer together in time than previous studies have suggested.
The researchers compared Y-chromosome sequences among 69 men from nine globally distinct regions, recording 11,000 mutations that have influenced the Y chromosome over time. From this they were able to reconstruct a "Y chromosome tree" to identify a time period for the MRCA.
However, the new chromosome tree did much more than identify the overlap between Y-chromosome Adam and Mitochondrial Eve. It also revealed some previously unknown relationships that occurred among populations as humans expanded out of Africa into Eurasia.
"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 new tree also confirmed the wide genetic diversity present among modern Africans, as compared to other world populations.
In another similar recent study by an international team led by Paolo Francalacci and colleagues, a genetic analysis of 1,204 men from the island of Sardinia in the Mediterranean Sea identified thousands of mutations on the Y chromosome, 6,751 of which had never been documented before. Their results suggested that human paternal lineages coalesced between 180,000 and 200,000 years ago.
Although the numbers differ between the two studies, taken together, they both suggest that Y-chromosome Adam did not appear on the planet significantly later than Mitochondrial Eve, as previous studies have implied.
These DNA type sequences, the Y chromosome and mitochondrial genome, are used by researchers because of the way they are inherited in individuals: the Y chromosome is passed only from father to son, and the mitochondrial genome is passed from a mother to her children. This makes it possible to determine ancestral relationships because they don't undergo the same changes that other genetic material does through time in human population dynamics.
But, according to scientists, it was not likely that Y-chromosome Adam and Mitochondrial Eve lived within the same generation, nor were they the only man and woman alive at the time or the only people to have present-day descendants. They simply had DNA portions that, because of their unchanging nature, made them more detectable and traceable, while the same sequences of others have become extinct because of natural selection and genetic "drift" (change in the frequency of a gene variant in a population due to random sampling).
Details of the studies have been published in the August 2, 2013 issue of the scientific journal Science, a publication of the American Association for the Advancement of Science.