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Human Evolution: Jaw dropper
A chimpanzee's jaws are so powerful it can bite off a
person's finger in one chomp. That is not a theoretical calculation;
more than one primate researcher has lost a digit that way.

The team dated the mutation to 2.4
million years ago - just before our brain expansion took off. But
another study, which sequenced a longer section of the muscle gene, came
up with an earlier estimate for when the mutation occurred - 5.3
million years ago (Molecular Biology and Evolution, vol 22, p 379).
Whichever date is right, the mutation
still happened after we split from our last common ancestor with chimps.
Why would our ancestors switch to a weaker bite? Stedman speculates
that rather than changes in diet being the catalyst, it could be that
our ancestors no longer used biting as a form of attack. "At some point,
perhaps through social organisation, this form of weaponry became more
optional for our ancestors," he says.
further information : source
further information : source
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