by Melissa McEwen.
The study found that Africans were more likely to have the long-narrow type and the least likely to have the classic type. Indians and whites were more likely to have the classic type and less likely to have the long-narrow type.
Are these differences a result of genes or environment? It is hard to tease out in the moment. There are a few animal studies showing gut size can be affected by diet, but none in humans. (Topping et al., 1997). The authors of the autopsy study noted that the difference was also found in children and that unlike in other animals, including the other great apes, human colons are not known to enlarge with age. (Madiba & Haffajee, 2011b). A limit to this particular study was that “African” represents the most genetically diverse population on Earth. Some preliminary work in Uganda showed that colon size varies between different African tribes, with the Baganda having larger colons than the other tribes studied (Katsarski & Singh, 1977). There is a strong possibility that colonic variation is connected with adaptations to diet and ongoing evolution of the digestive tract. The digestive implications of these anatomical differences remains to be studied, but there is a strong possibility the colon has continued to decrease in size in populations less and less reliant on colonic fermentation for nutrients and more and more able to acquire high-quality food.
It is also possible that gut size calculations based on a broader population would alter the equations Aiello and Wheeler used in the Expensive Tissue Hypothesis, either reducing the trade-off between brain and gut or finding that the tradeoff is present and variable between human populations. More study is needed on the matter, but it underscores the major importance of the colon in human evolution. The colon’s microbiome and anatomy hold much promise in illuminating our evolutionary past and teaching us about the importance of a healthy colon for overall health. Current data suggests the colon may be more variable in our species than previously thought, calling into question whether the representative colon used in medical and scientific textbooks and anatomy studies represents recent adaptations. Clues point to the adaptations being related to both the type and amount of fiber, as well as dietary constituents like butyrate.