That the genetic changes have anything to do with brain size or intelligence "is totally unproven and potentially dangerous territory to get into with such sketchy data," stressed Dr.Francis Collins, director of the National Human Genome Research Institute.

"No matter how we [changed] the analysis or assumptions, we couldn't get a date of around 6,000 years," says Gray.

"This kind of study is exactly what linguistics needs," says April Mc Mahon, who studies the history of languages at the University of Sheffield, UK.

If those genes don't work, babies are born with severely small brains, called microcephaly.

Using DNA samples from ethnically diverse populations, they identified a collection of variations in each gene that occurred with unusually high frequency.

"A different way to look at is it's almost impossible for evolution not to happen." Still, the findings also are controversial, because it's far from clear what effect the genetic changes had or if they arose when Lahn's "molecular clock" suggests — at roughly the same time period as some cultural achievements, including written language and the development of cities.

Lahn and colleagues examined two genes, named microcephalin and ASPM, that are connected to brain size.

Years later, a ruler declares one of those copies the definitive manuscript, and a rush is on to make many copies of that version — so whatever changes from the original are in this presumed important copy become widely disseminated.

Scientists attempt to date genetic changes by tracing back to such spread, using a statistical model that assumes genes have a certain mutation rate over time.

Gray and Atkinson analysed 87 languages from Irish to Afghan.

Rather than compare entire dictionaries, they used a list of 200 words that are found in all cultures, such as 'I', 'hunt' and 'sky'.

"The genetic evolution of humans in the very recent past might in some ways be linked to the cultural evolution," he said.