Thousands of years ago, somewhere in Africa, lived a man who – probably – had no idea that he, among all the other men in his group, would go on to become humankind’s most recent common male ancestor. Scientists would call him “Adam.”
Now, a new paper published in the journal Science significantly narrows the time during which Adam could have lived – about 120,000 to 156,000 years ago – putting him in about the same time period as humankind’s most recent common female ancestor, often dubbed “Eve." The research revises previous findings that dated Adam within a much longer period.
And the findings also ease recent doubts that the Y chromosome can reliably trace ancient lineage, renewing confidence that tracing and dating lineage using mutations in the Y chromosome could be critical in answering some of the vexing questions about how and where the first humans originated.
“We’ve shown that we can do this kind of dating, and that the Y chromosome is a really powerful tool,” says Brenna Henn, a genetics researcher at SUNY Stony Brook. “Now that we can use the Y chromosome in this manner, we can go back and look into other big questions, like exactly where in Africa did humans originate?”
“The ultimate goal is to understand when and where there was a modern human population,” she says.
Dr. Henn and colleagues analyzed the Y chromosome from 69 men from nine globally divergent regions, including Namibia, the Democratic Republic of Congo, Gabon, Algeria, Pakistan, Cambodia, Siberia, and Mexico. The Y chromosome, which in human males is one chromosome of the 23 pairs that form the genome, is a useful means through which to follow paternal lineage, as it is passed as an exact copy from father to son, whereas other chromosomes are shuffled and reshuffled in the making of a new person.
The mitochondrial genome, passed from a mother to both male and female children, is likewise used to plot maternal lineage. It has been used to date Eve.
Still, over hundreds of thousands of years, the Y chromosome is not always faithfully copied. Like mitochondrial DNA, it mutates, and those mutations can be used to trace lineage. Researchers identified about 11,000 mutations in the genetic sequences of the 69 men, after comparing their sequences to what is known as the reference genome. Those mutations were then plotted as a family tree, with the tip of each branch representing an individual’s unique mutation not shared with any of the other men.
Scientists have long suspected that those mutations occur at a regular rate, which would make it possible to then attach a date to those mutations. But since there is not yet a standard mutation rate in genomics, the team had to create their own.
To do so, they referred back to a known event: the migration of humans in North America 15,000 years ago. Mutations common to all modern Native Americans must have existed prior to the peopling of the New World, whereas variants among that population must have developed during the past 15,000 years.
That 15,000-years-ago marker was used to give the scientists a rate at which mutations occur, which was then applied to the Y chromosome tree. All the calculations were then redone with the 69 men’s mitochondrial DNA, to also trace their common female ancestor.
The new research dates the common Y chromosome to about 120,000 to 156,000 years ago and the mitochondrial DNA to some 99,000 to 148,000 years ago, meaning that Adam and Eve now have a 28,000 year period during which they could hypothetically have overlapped. Previous estimates had put the man sometime between the wide range of 50,000 to 150,000 years ago, and Eve at some 120,000 to 156,000 years ago.
The closing of the gap in Adam's possible time period is in part due to improved technology that makes more of the Y chromosome’s base pairs – the units that make up each chromosome and the spots where mutations occur – available for research. Previous studies had looked at hundreds of thousands of base pairs, while this latest paper looked at about 10 million base pairs, Henn said, noting that more base pairs mean more identified mutations and more accurate results.
The new research also makes a major step in genomics in using the same mutation rate to trace back both the Y chromosome and the mitochondria, Henn said. Previously, dates for Adam and Eve had often been calculated using different mutation rates.
And differences in mutation rates could still also explain continued disparities between researchers on pinpointing Adam and Eve’s time period. A separate paper also published Friday in Science also identified about 11,000 mutations in their sample sequences but used a different mutation rate – calculated using an estimated migration date of peoples to Sardinia – to date Adam. That paper’s results put Adam somewhere between 180,000 to 200,000 years ago.
“Mutation rate has really been an unappreciated issue in this kind of analysis,” said Henn, adding that she and colleagues are working to improve the accuracy of their Native American-based mutation rate. Once that is obtained, the team will return to the same data to re-date Adam and Eve and possibly narrow the periods during which the two could have lived.
Despite their biblically inspired monickers, Adam and Eve are not humankind’s first ancestors. Although the pair could have overlapped in time, it’s not necessary – and in fact “statistically unlikely,” says Henn – that the two ever even met.
Instead, Adam’s “Y” chromosome was passed to his male children, and to their male children, and so on. In a separate event, Eve’s mitochondrial DNA was passed to all her children, and to all her children's children, and so on. Other ancient people contemporaneous with Adam and Eve also passed to their children their Y chromosomes and mitochondria, but over the past hundreds of thousands of years, that material was filtered out of humankind’s genetic makeup.
Those Adam and Eve contemporaries also have modern descendants – and in fact parts of their genomes might be common in modern humans – but that lineage is not yet traceable. In short, Adam and Eve are our most recent common identifiable ancestors.
“There are other segments in our genome that we could trace common ancestry from, but those are tricky things,” said Henn.