Shrinking brains in evolution

In _Evolving Brains_, John Allman mentions that early modern humans had
brains that averaged 1450 grams, whereas the average for contemporary
humans is about 1300 grams. He opines that this is a result of the
"domestication" of humans. The development of agriculture and
domestication of animals as sources of food and clothing have served as
major buffers against environmental variability.

Reduction in brain size and neuron number have been observed in many
domestic animals relative to their wild counterparts. Dogs have brains
that are about three quarters the size of the brains of wolves of
comparable body size. Humans have assumed the responsibility for
providing food and shelter for dogs, and thus the dog's necessity for
maintaining a larger brain is decreased. Large brains are also very
energy intensive, and so there is a selective advantage in making them
smaller.

Another example comes from the comparison of the retinas of European
wild cats and domestic cats. Domestic cats have 40% fewer retinal
ganglion cells, which relay the input of the retinas to the brain. This
major loss of neurons might be produced by programmed cell death in
development and linked to domestication.


--dkomo@xxxxxxxx





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