LOS ANGELES -- Human DNA contains myriad individual differences that influence a host of traits, be they eye color or the ability to digest milk. Now a study shows that most of those tiny genetic variations are rare -- and they arose in the very recent history of our species.
Joshua Akey, a geneticist at the University of Washington in Seattle, led a consortium of scientists who examined the DNA of 4,298 European Americans and 2,217 African-Americans. Limiting their analysis to the parts of the genome that contain instructions for making proteins, the study authors found more than 1 million sites where the building blocks of DNA -- the nucleotides known by the letters A, C, G and T -- varied in at least one of the subjects.
Most of those individual variants were rare, with each one found in fewer than 0.5 percent of the people in the sample. Nearly half of the mutations were detected in only one person, according to their report last month in the journal Nature.
The scientists were even able to estimate the age of variations based on how rare or common they were -- and found that about 73 percent of the mutations had occurred in the last 5,000 to 10,000 years.
The percentage of rare mutations was greater in European Americans than African-Americans, the team said. That fits with the history of Homo sapiens: Ancestors of Europeans went through explosive population growth when they left Africa tens of thousands of years ago.
The results give a sense of just how quickly human expansion has affected our genetic makeup, said Shamil Sunyaev, a computational geneticist at Brigham and Women's Hospital in Boston. Without that population explosion, there would have been fewer chances for mutations to occur and the proportion of recent mutations would not have been so high.