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Scientists
aren’t exactly sure how many new mutations crop up each generation
in humans—for years, the standard estimate was round 100-200, but
one
2011 study,
using whole genome information from two families, has put it at about
30-50. And the rate of mutation is more than a curiosity: Seeing how
many mutations separate us from cousin species and multiplying that
by the rate of change is one way scientists measure the time since we
diverged from each other—a “molecular clock.” So that new,
lower mutation rate—30-50 changes per generation—implies that our
common ancestor with chimps was not 5 million years ago, as had been
thought, but 7 million years. (For
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