New Clues About Human Ancestry
August 2, 2013

DNA Analysis Reveals That Most Recent Human Ancestors Lived During Same Era

redOrbit Staff & Wire Reports - Your Universe Online

For the first time, scientists have been able to trace human ancestry through the male line by sequencing the DNA of multiple entire Y chromosomes - and this so-called "Adam" lived during roughly the same time period as the female most recent common ancestor (MRCA), mitochondrial "Eve," they have discovered.

The MRCAs, mitochondrial Eve and Y-chromosomal Adam, are two individuals who passed down a portion of their genomes to the bulk of humanity, the study authors explained. Much of our lineage remains a mystery, but now scientists have determined the two of them roughly overlapped during evolutionary time.

Stanford University genetics professor Dr. Carlos Bustamante, senior author of the study, and his colleagues have found the male MRCA lived sometime between 120,000 and 156,000 years ago. In comparison, the female MRCA lived between 99,000 and 148,000 years ago.

"Previous research has indicated that the male MRCA lived much more recently than the female MRCA, but now our research shows that there's no discrepancy," Bustamante said, noting previous estimates had placed "Adam" as living between 50,000 to 115,000 years ago.

He and his colleagues, whose findings are detailed in Friday's edition of the journal Science, compared Y-chromosome sequences from 69 men hailing from nine geographically distinct regions, including Namibia, the Democratic Republic of Congo, Algeria, Pakistan, Cambodia and Siberia.

Through the use of high-throughput sequencing technology, they were able to identify approximately 11,000 differences among the sequences, which allowed them to establish phylogenetic relationships and timelines among the sequences with unprecedented accuracy, the study authors explained in a statement.

"Essentially, we've constructed a family tree for the Y chromosome," said lead author and Stanford graduate student David Poznik. "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," he added. "We now have a more complete structure, including meaningful branch lengths, which are proxies for the periods of time between specific branching events."

Poznik, Bustamante and his colleagues were able to obtain highly-accurate sequencing results over a length of approximately 10 megabases of Y chromosome DNA (of 10 million nucleotides) for each of the study participants. They then estimated the annual Y chromosome mutation rate by calibrating it with the settlement of the Americas, which is an event known to have occurred about 15,000 years ago.

The mutations shared by modern-day Native Americans must have existed prior to colonization of the continents, the researchers explained. They repeated their analysis with each individual's mitochondrial DNA in order to create two MRCA timing estimates, and discovered for the first time there is overlap between the two.

Furthermore, it clarified previously undiscovered relationships that occurred among populations as mankind expanded out of Africa and into Eurasia, Bustamante said. For instance, they discovered a single variant demonstrating how three ancient lineages came together approximately 48,000 years ago, he noted.

"We're interested in understanding the historical relationships between many different human populations, and the migration patterns that have led to the peopling of the world," co-author Jeffery Kidd, assistant professor of human genetics at the University of Michigan, said in a statement. "We hope that others will make use of this approach and sequence additional chromosomes of interest that are related to the peopling of specific places."