Stanford researchers claim that our most recent common ancestors, known as mitochondrial Eve and Y-chromosomal Adam, roughly overlapped during evolutionary time – between 120,000 to 156,000 years ago for the man, and between 99,000 and 148,000 years ago for the woman.
Previous research has indicated that Y-chromosomal Adam lived much more recently than Mitochondrial Eve, according to Prof Carlos Bustamante, who is a senior author of the study published in the journal Science. “But new research shows that there’s no discrepancy,” he said.
Previous estimates for Y-chromosomal Adam 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 unlikely that Mitochondrial Eve and Y-chromosomal Adam were exact contemporaries. These two individuals had the good fortune of successfully passing on specific portions of their DNA, called the Y chromosome and 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.
The DNA sequences traced by the researchers were chosen because of the unique way they are inherited: the Y chromosome is passed only from father to son, and the mitochondrial genome is passed from a mother to her children. Each can serve as a useful tool for determining ancestral relationships because they don’t undergo the shuffling and swapping of genetic material that occurs routinely in most human chromosomes.
The researchers made their discovery by comparing Y-chromosome sequences among 69 men from nine globally distinct regions – 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.
Lead author David Poznik from the Stanford University School of Medicine explained: “essentially, we’ve constructed a family tree for the Y chromosome. 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. We now have a more complete structure, including meaningful branch lengths, which are proxies for the periods of time between specific branching events.”
The scientists obtained highly accurate sequencing results over a length of about 10 megabases of Y chromosome DNA for each of the 69 individuals. They then estimated the yearly mutation rate on the Y chromosome by calibrating it with a known event: the human settlement of the Americas that occurred about 15,000 years ago. Mutations shared by all Native Americans today must have existed prior to the peopling of the continents, whereas many of those that vary among indigenous American populations arose during the past 15,000 years.
They repeated their analysis with the individuals’ mitochondrial DNA to generate the two estimates of the most recent common ancestors timing, showing for the first time that they overlap.
Bibliographic information: Poznik GD et al. 2013. Sequencing Y Chromosomes Resolves Discrepancy in Time to Common Ancestor of Males Versus Females. Science, vol. 341, no. 6145, pp. 562-565; doi: 10.1126/science.1237619