X and Y chromosomes, with the smaller Y chromosome to the right
A miniscule bit of DNA from an African American man now living in South Carolina has been traced back 338,000 years, according to a new study.
The man’s Y chromosome — a hereditary factor determining male sex — has a history that’s so old, it even predates the age of the oldest known Homo sapiens fossils, according to the report, published in the American Journal of Human Genetics.
The fellow’s chromosome turned out to carry a rare mutation, which researchers matched to a similar chromosome in the Mbo, a population living in a tiny area of western Cameroon in sub-Saharan Africa.
“Our analysis indicates this lineage diverged from previously known Y chromosomes about 338,000 ago, a time when anatomically modern humans had not yet evolved,” Michael Hammer, who worked on the study, said in a press release.
“This pushes back the time the last common Y chromosome ancestor lived by almost 70 percent.”
Hammer is an associate professor in the University of Arizona’s department of ecology and evolutionary biology and a research scientist at the UA’s Arizona Research Labs.
The DNA detective work began after the South Carolinian submitted a small tissue sample to the National Geographic Genographic Project. The researchers were shocked after they noticed none of the genetic markers used to assign lineages to known Y chromosome groupings were found.
They sent the man’s DNA sample to Family Tree DNA for sequencing. Fernando Mendez, a postdoctoral researcher in Hammer’s lab, led the effort to analyze the DNA sequence. It included more than 240,000 base pairs of the Y chromosome.
Searches through a huge database led to the Mbo connection.
The scientists could then estimate the emergence of the chromosome mutation based on rates of change, creating a sort of “family tree” for the chromosome.
The discovery doesn’t necessarily mean that we all descended from an ancestor living in western Cameroon.
“It is a misconception that the genealogy of a single genetic region reflects population divergence,” Hammer explained. “Instead, our results suggest that there are pockets of genetically isolated communities that together preserve a great deal of human diversity.”
Still, Hammer said, “It is likely that other divergent lineages will be found, whether in Africa or among African-Americans in the U.S. and that some of these may further increase the age of the Y chromosome tree. There has been a lot of hype with people trying to trace their Y chromosome to different tribes, but this individual from South Carolina can say he did it.”
The study has even further implications. It strengthens the belief that there is no “mitochondrial Eve” or “Y chromosome Adam.”
All of humankind, as a result, did not descend from exactly one pair of humans that lived at a certain point in human evolution.
(Image: University of Arizona)