A study published in Nature this week, has found that the majority of human genetic variation occurred in the past 5000 to 10,000 years, relatively recently in evolutionary terms.
They looked at more than 1 million single letter variations that changed one nucleotide: A, C, G or T, in the DNA sequence and put a date on when that mutation arose. The results suggest that 86% of harmful mutations arose in Europeans between the last 5,000 and 10,000 years.
"The spectrum of human diversity that exists today is vastly different than what it was only 200 to 400 generations ago," says Joshua Akey, associate professor of genome sciences at the University of Washington in Seattle.
To put the time-scales involved into context, it is important to understand that humans have been around for the past 100,000 years so mutations arising in the past 5,000 years are considered very recent in human history.
"One of the most interesting points is that Europeans have more new deleterious (potentially disease-causing) mutations than Africans," says Dr Suzanne Leal, a professor of molecular and human genetics at Baylor College of Medicine in Houston, and one of the report’s authors.
The researchers believe that the diversity of young mutations in Europeans is linked to a population bottleneck which reduced their populations to small numbers when they left Africa around 50,000 years ago. Known as the ‘Out of Africa’ model, the dramatic reduction in population size meant that rare mutations became more common, as well as new mutations appearing at a greater rate as the populations expanded.
Akey adds, "the Out of Africa bottleneck led to inefficient purging of the less-harmful mutations."
"The recent dramatic increase in human population size, resulting in a deluge of rare functionally important variation, has important implications for understanding and predicting current and future patterns of human disease and evolution," write the authors in the paper in Nature.
The study also found that the older a mutation was, the less likely it was to be deleterious.