By Hannah Devlin, The Guardian
Results of largest ever genetics study of a single population could also help refine dates for major events during human evolution
Humans are evolving more rapidly than previously thought, according to the largest ever genetics study of a single population.
Scientists reached the conclusion after showing that almost every man alive can trace his origins to one common male ancestor who lived about 250,000 years ago. The discovery that so-called “genetic Adam”, lived about 100,000 years more recently than previously understood suggests that humans must have been genetically diverging at a more rapid rate than thought.
Kári Stefánsson, of the company deCODE Genetics and senior author of the study, said: “It means we have evolved faster than we thought.”
The study also shows that the most recent common male ancestor was alive at around the same time as “mitochondrial Eve” — the last woman to whom all females alive today can trace their mitochondrial DNA.
Unlike their biblical counterparts, genetic Adam and Eve were by no means the only humans alive, and although they almost certainly never met, the latest estimate which gives a closer match between their dates makes more sense, according to the researchers.
When the overall population size is stable – as it has been for long periods in the past – men have, on average, just one son, and women, just one daughter. This means that for any given man, there is a high chance that his paternal line will eventually come to an end. This means any male descendants, for instance his daughter’s son, would have Y-chromosomes inherited from other men. If you travelled back far enough in time, the theory goes, there would be only one man whose paternal line extends unbroken to the present day: this man is Y-chromosome Adam.
The researchers dated the existence of this man by comparing the Y-chromsomes of 753 Icelandic men, who were also grouped into 274 paternal lines. The researchers used a “molecular clock”, based on the number of DNA mutations that arise with each generation, to estimate Adam’s age.
The study, published in Nature Genetics, put the new age for genetic Adam at between 174,000 and 321,000 years ago. Genetic Eve is thought to have walked the Earth around 200,000 years ago: well within the new error margin for Adam.
“It gives us enormous confidence to have a timeline that is similar,” said Stefánsson.
Previous dates for ancestral Adam ranged from far more recent, just 50,000 years ago, right back to around 500,000 years ago, with some estimates showing major mismatches with the dating of ancestral Eve. Some researchers had suggested that polygamy could explain the gap, in the cases where Adam was more recent, by reducing the number of men who would pass on their Y-chromosome. Stefánsson describes this argument as a “crock of shit”. “The two sexes are inseparable,” he said. “It doesn’t matter how many women a man has children with – half of them will be boys and half girls.”
Agnar Helgason, also of deCODE, said the latest findings could help refine dates for major events during human evolution, such as when humans first migrated out of Africa and arrived in Europe. “We’re curious about where we came from and when,” he said. “This gives us a bit more solid information about when.”
Previous research also suggested that humans are evolving more quickly now than at any time since the split with the ancestors of modern chimpanzees 6m years ago. The study, by the University of Wisconsin, found that at least 7% of human genes have undergone recent evolution. Some of the changes included the emergence of fair skin and blue eyes in northern Europe, greater resistance to malaria in some African populations and the appearance of a gene that allows lactose to be digested.