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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.
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.
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.
"The genetic evolution of humans in the very recent past might in some ways be linked to the cultural evolution," he said.
Other scientists urge great caution in interpreting the research.
Rather than compare entire dictionaries, they used a list of 200 words that are found in all cultures, such as 'I', 'hunt' and 'sky'.
Words are better understood than grammar as a guide to language history; the same sentence structure can arise independently in different tongues.
The resulting tree matches many existing ideas about language development.
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.
So suggests new research that tracked changes in two genes thought to help regulate brain growth, changes that appeared well after the rise of modern humans 200,000 years ago.