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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.
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.
"We, including scientists, have considered ourselves as sort of the pinnacle of evolution," noted lead researcher Bruce Lahn, a University of Chicago geneticist whose studies appear in Friday's edition of the journal Science.