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The expensive tissue hypothesis relates brain and gut size in evolution. It suggests that in order for an organism to evolve a large brain without a significant increase in basal metabolic rate , the organism must use less energy on other expensive tissues; the paper introducing the ETH suggests that in humans, this was achieved by eating an easy-to-digest diet and evolving a smaller, less energy intensive gut. The ETH has inspired many research projects to test its validity in primates and other organisms.
The human brain stands out among the mammals because its relative size compared to the rest of the body is unusually large compared to other animals. The brain of a homo sapien is about three times larger than that of its closest living relative, the chimpanzee. For a primate of its body size, the relative size of the brain and that of the digestive tract is rather unexpected; the digestive tract is smaller than expected for a primate of a human body size. In 1995, two scientists proposed an attempt to solve this phenomenon of human evolution using the Expensive Tissue Hypothesis.