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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.In fact, the variations were so common they couldn't be accidental mutations but instead were probably due to natural selection, where genetic changes that are favorable to a species quickly gain a foothold and begin to spread, the researchers report.Lahn offers an analogy: Medieval monks would copy manuscripts and each copy would inevitably contain errors — accidental mutations.

The resulting tree matches many existing ideas about language development.

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.

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.