Dating is a science too
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.
"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.
"There's a sense we as humans have kind of peaked," agreed Greg Wray, director of Duke University's Center for Evolutionary Genomics.
If those genes don't work, babies are born with severely small brains, called microcephaly.
The farmers themselves may have moved, or natives may have adopted words along with agricultural technology.
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.
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.