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Why do you not have wings or a tail? Why do fish not
have a beak? Why do chickens not have teeth? If you're
interested in questions like this you should read work
by Stephen Jay Gould, for instance his book 'Hen's
teeth and horse's toes'.
He explains that all vertebrates are 'designed'
according to a ground pattern with extensive variation
in the details. Snakes developed into a life style in
which they didn't need legs, but a very flexible
spine. Eventually the legs disappeared, as with the
tail of primates.
By the way, my biology tutor who is a famous molecular
biologist taught me never to ask 'why' in biological
matters. The question imposes there must be an
intention, a plan, for something to develop. There may
not be such a plan; biology develops into a more or
less random directions, and the functionality of each
mutation is tested case by case. Look at the
present-day sea mammals. They have fins that developed
from legs. But going back in the evolution of mammals,
their ancestors were fish-like. Would it not have been
better if those fishy creatures kept their fins that
the seals needed later? This exemplifies that there is
no plan in evolutionary biology, just adaptation.
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