“Adam,” humankind’s most-common male ancestor, is from an older era than once thought, living about the same time as genetic “Eve,” a study found.
Using the complete strand of DNA that determines male sex, researchers have determined that Y Chromosome Adam lived 120,000 to 156,000 years ago, overlapping with Mitochondrial Eve, who probably lived 99,000 to 148,000 years ago, according to a paper published today in the journal Science. Researchers previously thought Adam lived 50,000 to 150,000 years ago.
The new data provides a richer picture of the outlines of human history, helping scientists more precisely understand the evolutionary tree. Previous efforts to date a common ancestor didn’t use the whole Y chromosome, leaving undetected the rare variations that help define commonality with past generations.
“When we put it together, we realized we had the very best map at the time of human genetic variation,” Carlos Bustamante, a genetics professor at Stanford University and a study author, said in a telephone interview. “And when we started looking at this classic question, we were getting an answer that was different than before.”
The study analyzed the Y chromosome for 69 men from nine populations, and traced backward to the time of man’s presumed beginnings. Previous estimates for Adam were based on a smaller amount of data and less-diverse population, Bustamante said.
Y chromosomes that carry the same mutations share a common male ancestor in the past. It’s possible to determine how far back that ancestor was by looking at how many mutations differ between the chromosomes: The more mutations that aren’t shared, the longer ago the common ancestor lived.
The common ancestor in today’s study probably isn’t the first, said Jennifer Hughes, a researcher with the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts. Rather, he’s one of probably thousands of very successful and very lucky fathers, whose offspring were also successful. He just happens to be easy to track, thanks to the qualities of these portions of DNA, Hughes said in a telephone interview.
The study had the largest number of Y chromosomes anyone has sequenced so far, she said.
Chromosomes are composed of DNA tightly wound around proteins that provide structure. Humans have 23 chromosomes, including two sex chromosomes, X and Y. Women carry two X chromosomes, while men carry an X and a Y.
Ordinarily, during reproduction, DNA strands break apart and rejoin in order to produce genetic variation, creating sequences in offspring that weren’t present in parents. However, the Y chromosome contains the longest stretch of genes that don’t recombine in the entire human genome. This is despite the fact that the Y chromosome changed “a huge amount” over recent evolutionary history, Bustamante said.
Previous research has shown that the Y chromosome used to contain 800 genes 200 million years ago. It’s since shrunk to only 30, though that number has remained stable for the last 25 million years, according to Hughes’s research, which was published in the journal Nature in February 2012.
Since much of the Y doesn’t exchange genetic material with other chromosomes, it’s easier to use to trace common ancestries, according to Bustamante’s paper.
In order to estimate the yearly rate of mutations, Bustamante’s group used a known event: the settlement of the Americas, which occurred 15,000 years ago. Mutations shared by Native Americans must have existed prior to ancestral migration; those that vary must have occurred after.
What’s more, the full Y chromosome data showed ancient branches on the family tree of which Adam is the root, revealing how quickly mutations sprung up and spread throughout the population. The differences in the non-recombining DNA can be used to trace genetic diversity. People who have the same genes in the same places are sorted into what are called haplogroups.
Mitochondrial DNA, like portions of the Y chromosome, is also passed from mothers to offspring without shuffles. That makes it possible to track the foremother of most current humans.
Despite the Adam and Eve nicknames, the two most likely didn’t coexist, Bustamante said. There were other people around when Adam and Eve were alive; these two were simply the most successful at producing surviving offspring.
The relatively few amount of changes make the Y chromosome an excellent way of measuring human migration across the globe, Bustamante said. His group works frequently in Latin America, and Y chromosome data may help distinguish migration events there, he said. More research should also be done in Africa, as that continent has the longest history of human inhabitation, and is relatively understudied.
“It’s interesting when you think something is solved and the question is dead, and then you get more information and it upends itself,” Bustamante said.
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