Evolutionary hypothesis proposing that humans reduced the size of metabolically expensive gut tissues to allocate energy toward increased brain size. The hypothesis suggests a trade-off between digestive organ mass and brain mass constrained by total metabolic budget.
Brain tissue is metabolically expensive (20% of basal metabolic rate despite 2% of body mass). The hypothesis proposes that adoption of high-quality, easily digestible diets (cooked foods, animal products) reduced gut length requirements, freeing metabolic resources for encephalization. However, data show weak or absent correlations between brain size and gut size across primates.
While the simple version of ETH is not strongly supported by primate comparative data, it highlights important principles: (1) metabolic allocation trade-offs exist between organ systems, (2) diet quality influences organ energy requirements, (3) brain metabolic demands constrain other physiological processes. Understanding these trade-offs informs cPNI approaches to energy distribution and metabolic stress.