Expensive Tissue Hypothesis
Overview[edit]
The Expensive Tissue Hypothesis (ETH) relates brain and gut size in evolution (specifically in human evolution). It suggests that in order for an organism to evolve a large brain without a significant increase in Basal Metabolic Rate (as seen in humans), 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. [1] The ETH has inspired many research projects to test its validity in primates and other organisms.
Original Paper[edit]
The original paper introducing the ETH was written by Dr. Leslie Aiello and Dr. Peter Wheeler. In the paper, they sought to explain how humans managed to have energy for their large and metabolically expensive brains while still maintaining a Basal Metabolic Rate (BMR) comparable to other primates with smaller brains. They found that the humans’ smaller relative gut size almost completely compensated for the metabolic cost of the larger brain. They went on to postulate that a larger brain would allow for more complex foraging behavior, which would result in a higher quality diet, which would then allow the gut to shrink further, freeing up more energy for the brain. This research also presented a case for studying the evolution of organs in a more interconnected manner, rather than in isolation.[2]
Further Research[edit]
The academic debate around the ETH is still active, and has inspired a number of similar tests, all attempting to verify the ETH with another species or group of species by looking at encephalization (a ratio between brain size and body size), gut size, and/or diet quality. Primates, being the closes living relatives to humans, are a natural extension to the hypothesis, and as such are examined by many of these tests. One such study supported the expensive tissue hypothesis and found a positive correlation between diet quality and brain size (as would be expected by the original paper), but it did note that there were exceptions among the species tested [3]. A broader study including primates and other mammals disputed the ETH, finding that there is no negative correlation between brain and gut sizes; it did, however, support the idea of energy trade offs in evolution as it found a negative correlation between encephalization and adipose (fat) deposits[4].
Studies have also been done in species less similar to humans, such as anurans and fish. The study of anurans found that among the 30 species tested, there was a significant negative correlation between gut size and brain size, as Aiello and Wheeler found in humans and primates in their original research [5]. One study of fish used the carnivorous fish Gnathonemus petersii, which has a uniquely large brain, about three times the size expected of a fish of its size. The research found that these fish also had significantly smaller guts than other similar carnivorous fish [6]. These further studies enrich the debate over the ETH.
- ^ Aiello, L., & Wheeler, P. (1995). The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology, 36(2), 199-221. Retrieved from http://www.jstor.org/stable/2744104
- ^ Aiello, L., & Wheeler, P. (1995). The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology, 36(2), 199-221. Retrieved from http://www.jstor.org/stable/2744104
- ^ Fish, J. L. and Lockwood, C. A. (2003), Dietary constraints on encephalization in primates. Am. J. Phys. Anthropol., 120: 171-181. doi:10.1002/ajpa.10136
- ^ Navarrete, A., Schaik, C. P., & Isler, K. (2011). Energetics and the evolution of human brain size. Nature, 480(7375), 91-93. doi:10.1038/nature10629
- ^ Jason A. Kaufman, Claude Marcel Hladik, and Patrick Pasquet, "On the Expensive‐Tissue Hypothesis: Independent Support from Highly Encephalized Fish," Current Anthropology 44, no. 5 (December 2003): 705-707.
- ^ Jason A. Kaufman, Claude Marcel Hladik, and Patrick Pasquet, "On the Expensive‐Tissue Hypothesis: Independent Support from Highly Encephalized Fish," Current Anthropology 44, no. 5 (December 2003): 705-707.