ABSTRACT: 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. By comparing the modern human colon with those of our nearest ape relatives, we can infer much about the uniqueness of the human dietary niche, which may be characterized by reliance on high-quality (lower fiber) cooked foods and starch. Further variation between human populations provides clues on more modern adaptations to diet.
COMMEMORATIVE ESSAY: 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. 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.
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 .8 grams larger.It is hard to know when this change started, as guts do not fossilize. However, it is possible to infer some information from post-cranial anatomy. Living apes with big guts 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 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.
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.
In the colon, bacteria digest otherwise useless dietary constituents into important nutrients and other chemical byproducts. These include short-chain fatty acids (SCFA). 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. Upper estimates for human caloric use of SCFA range from seven to nine percent.
Suggestions that humans may have obtained more calories from SCFA in the past are rooted in estimates of fiber consumption from the Paleolithic. Evidence is rather sparse and limited to coprolites, showing evidence for fiber intakes as high as 150 grams as day, well over what any known human culture currently consumes. Even if the method for estimating fiber consumption from coprolites is accurate, they may not support the conclusion that they represent some species level optimal.
Some of the issue is also overemphasis on fiber, when other food constituents that play a similar role may have been more important in human evolution. Early optimism that high fiber could prevent many diseases of civilization spurred many studies on the matter, which had mixed results. Focus on fiber in the past was on its abilities as indigestible bulking matter to increase digestive transit time and bind up certain food constituents.
The fact that humans cannot digest certain fibers and starches in the diet does not mean they are nothing but bulking matter. In the scientific world, more and more research focus has been on the fact that these seemingly indigestible ingredients actually are often digested in the human body, just not by human enzymes. Instead, they are digested by human gut bacteria.
The colonic microbiome remains of vital importance to human health. Scientists are just discovering how the bacterial population and its byproducts play important roles in human nutrition, the immune system, and other vital bodily processes. The gut flora is currently under investigation for its role on hundreds of diseases.
Borne out of this are several new paradigms for studying fiber, not as bulk, but as an interaction agent with gut bacteria. The importance of the species mix, population level, and products has been emphasized. One new term for some fibers is “prebiotic.” A prebiotic fiber is indigestible by human enzymes, but stimulates the growth of certain beneficial gut bacteria such as Bifidobacterium and Lactobactillus.
Another hypothesis is that lack of SCFAs is behind such diseases of civilization. A SCFA called butyrate provides some insight into this. Butyrate is the preferred fuel of the colonic epithelial cells and also plays a major role in the regulation of cell proliferation and differentiation. Lower than normal levels have been found in patients with several diseases, notably types of colitis and inflammatory bowel disorder. Studies show such diseases can be treated through application of butyrate in the colon.
Bacteria affect butyrate production, but so do dietary inputs. Certain fibers produce more butyrate than others in humans. Interestingly, one of the top producers is something known as “resistant starch.” Resistant starch represents the growing nuance in understanding of fiber, since it is a starch that acts like a fiber in terms of acting as a bacterial substrate.
Richard Wrangham has suggested that utilization of cooked starches was one of the dietary quality innovations that fed our rapidly expanding expensive brain tissue as it evolved towards hominid size. The burgeoning field of archeological starch grain analysis has transformed our view of hominids once thought to be mostly carnivorous. Microfossils on Neanderthal teeth from around 44,000 years ago show evidence of the consumption of many roots and tubers, some of which show evidence of cooking. The full impact of the adoption of cooked starches on the human body has not been fully elucidated. One promising adaptation is the starch-digesting salivary amylase gene, AMY 1. Chimpanzees and bonobos have only two copies of this gene, humans have as many as 10 copies, though it varies quite heavily by population from 2 to 10 correlated with the importance of starch in the diet. Molecular genetic evidence places the origin of divergence on this gene at about 200,000 years, about the time when habitual fire use became common.
Some humans may be better at fermenting than others. Recent studies of human gut variation have revealed possible genetic variations as well as those caused by environment and lifestyle.
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.