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Russell Gray and Quentin Atkinson, of the University of Auckland in New Zealand use the rate at which words change to gauge the age of the tree's roots - just as biologists estimate a species' age from the rate of gene mutations.
The differences between words, or DNA sequences, are a measure of how closely languages, or species, are related.
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 work was funded by the Howard Hughes Medical Institute. A family tree of Indo-European languages suggests they began to spread and split about 9,000 years ago.
The finding hints that farmers in what is now Turkey drove the language boom - and not later Siberian horsemen, as some linguists reckon.
The conclusion will be controversial, as there is no consensus on where Indo-European languages came from.
Some linguists believe that Kurgan horsemen carried them out of central Asia 6,000 years ago.
The farmers themselves may have moved, or natives may have adopted words along with agricultural technology.
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.
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.
Aside from not knowing what the gene variants actually do, no one knows how precise the model Lahn used to date them is, Collins added.