The Brain
The adult human brain weighs, on average 1350 grams (about three pounds), and is about the size of your two fists pressed together. Our closest living cousins — the chimpanzees — have brains only one-third as large, while blue whales have brains five-times larger than ours. More significant, however, is not the absolute weight of a brain, but the brain/body weight ratio; here we find that the human brain is six-times as large as what would be expected from the ratio found in other mammals.
Brains are biologically expensive: with only 3% of the body’s weight, they consume 17% of the total calories — this caloric requirement suggests a close relationship between the growth in human brain size and our diet during the course of our evolution.
Neurons — the cells that comprise about one-half the bulk of the brain — come in over 200 varieties, and there are about 100 billion of these cells in the brain. The interconnections among these neurons are estimated at 1000 trillion.
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