This will be one of the few series posts I'll actually finish since it's already written :) I'd like to thank Stephan Guyenet, Chris Masterjohn, and Professor Holloway for their tips, critiques, and inspiration! I welcome more such educated thoughts in the comments. Full disclosure: yes, I did write this for a class, but I thought some people might enjoy it and then I could also kill two birds with one stone. Haha.
In 1995, anthropologists Leslie C. Aiello and Peter Wheeler published a paper on a theory they termed The Expensive Tissue Hypothesis (ETH). Expensive refers to our brain tissue, which is uniquely metabolically demanding compared to other primate brains (Aiello & Wheeler, 1995). However, our total metabolic rate is close to what would be predicted for a primate our size, so according to the ETH, humans compensated for the increased metabolic costs of the brain by evolving less metabolically expensive splanchnic organs, which include the gut and liver. Humans were able to fuel their large brains using only a relatively small gut because increased dietary quality reduced the need for gut mass. The hypothesis was that the main driver of this increased dietary quality was the increased use of animal products.
Aiello and Wheeler
This hypothesis rests on assuming that reduced gut size coincided with the major jump in encephalization experienced by hominids millions of years ago. In their calculations, Aiello and Wheeler used the modern human gut to demonstrate its uniquely small size. Unfortunately, using the modern human gut as a hallmark has some problems, as there is some evidence that it has been reduced in size due to dietary innovations that may have taken place long after encephalization and since these innovations it has possibly continued to evolve. The trend in human innovation has been towards a diet of increased quality and this innovation continues even today. In response to these dietary changes, the human population shows variation in dietary adaptations. The reorganization and variation of the human colon provides important clues about this process.
Exactly how unusual is the modern human gut? Based on a reduced major axis equation computed for higher primates, the human gut should be about 781 grams larger (Aiello & Wheeler, 1995).
It is hard to know when this change started, as guts do not fossilize nor do they leave their impressions as brains do in endocasts. However, it is possible to infer some information from post-cranial anatomy. Living apes with big guts have protuberant abdomens to accommodate them.
Skeletally, they have a rounded abdomen continuous with the lower portion of the rib cage, giving it a funnel shape, as well as a wide pelvis with flared upper margins. In the fossil record we can see that Australopithecus afarensis had skeleton anatomy that would indicate a large gut if this pattern holds.
Figure 3: Chimpanzee, human, and Australopithecus afarensis, from Aiello and Wheeler
In contrast, the human pelvis size is reduced and the abdomen has a defined waist region. Hominids start exhibiting this in the fossil record starting with Homo erectus, about 1.5 million years ago. However, there is some evidence that this anatomical change may not have to do with gut size. For one, it is not entirely a consistent pattern among hominids. Reconstructions of a post-cranial Neanderthal skeleton based on the 70,000 year old La Ferrassie 1 and 60,000 year old Kebara 2 specimens shows a wider trunk showing up again (Sawyer & Maley, 2005).
It is possible that the trunk and pelvis size represented adaptations to cold, a type of hunting, or some other lifestyle variable (Bramble & Lieberman, 2004). Until more data is collected and analyzed tying post-cranial anatomy to gut mass, it is hard to tell if the inference is valid.
In response to the ETH paper in 1995, Katherine Milton questioned whether the data presented was really representative of our species. She stated that our guts may have played a larger role before the relatively recent invention of agriculture when fiber consumption was much greater and our guts might have been larger then because of “gut plasticity.” She mentioned that what really sets us apart from our primate relatives is the reorganization of the gut morphologically rather than the size.
In humans compared to primates, the gut is reorganized. The size of the colon is much reduced and the size of the small intestine is increased. The human colon takes up 17-23% of the digestive tract. In chimpanzees, orangutans, and gorillas it occupies 52-54%. Instead of a large colon, humans have a small intestine that represents 56-67% of the gut (Milton, 1989).
These are important to note because of their role in digesting food. The small intestine is where primate enzymes digest and absorb nutrients immediately available in food. In contrast, the colon can be thought of as a bioreactor, where bacteria digest otherwise useless dietary constituents into important nutrients and other chemical byproducts. These include short-chain fatty acids (SCFA), organic fatty acids with 1-6 carbon atoms created by the fermentation of polysaccharides, oligosaccharides, protein, peptides, and glycoprotein precursors in the colon. The major source of these in primates is through the fermentation of fiber and some types of starch. The major difference in this matter between humans and the other great apes is that apes such as the gorilla are able to use their larger colons to obtain as much as 60% of their caloric intake from SCFA alone (Popovich et al., 1997). Upper estimates for human caloric use of SCFA range from seven to nine percent. (McNeil, 1984).
Figure 6: The contribution of SCFA to metabolism in gorillas from Popovich, et al.
Aiello, L. C., & Wheeler, P. (1995). The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology, 36(2), 199. doi: 10.1086/204350.
Bramble, D. M., & Lieberman, D. E. (2004). Endurance running and the evolution of Homo. Nature, 432(7015), 345-52. Nature Publishing Group. doi: 10.1038/nature03052.
McNeil, N. (1984). The contribution of the large intestine to energy supplies in man. Am J Clin Nutr, 39(2), 338-342. Retrieved May 2, 2011, from http://www.ajcn.org/cgi/content/abstract/39/2/338.
Milton, K. (1989). Primate diets and gut morphology: implications for hominid evolution. In M. Harris & E. B. Ross (Eds.), Food and Evolution: Toward a Theory of Human Food Habits (p. 93). Temple University Press. Retrieved May 8, 2011, from http://books.google.com/books?hl=en&lr=&id=xHYxSHr86T8C&pgis=1.
Popovich, D. G., Jenkins, D. J. A., Kendall, C. W. C., Dierenfeld, E. S., Carroll, R. W., Tariq, N., et al. (1997). The Western Lowland Gorilla Diet Has Implications for the Health of Humans and Other Hominoids. J. Nutr., 127(10), 2000-2005. Retrieved April 28, 2011, from http://jn.nutrition.org/cgi/content/abstract/127/10/2000.
Sawyer, G. J., & Maley, B. (2005). Neanderthal reconstructed. Anatomical record. Part B, New anatomist, 283(1), 23-31. doi: 10.1002/ar.b.20057.