SUMMARY: The gut microbiome and anatomy promise much promise in illuminating our evolutionary past and teach us about the importance of a healthy gut and colon to overall health. Comparing the modern human colon with those of our closest ape relatives, we can conclude much about the uniqueness of the human dietary niche, which can be characterized by the hope of high quality (low fiber) cooked foods and starch. Further variation between human populations provides guidance for more modern dietary adaptations.
COMMEMORATIVE RESEARCH: In 1995, anthropologists Leslie K. Ayello and Peter Wheeler published a book on the theory they called the Expensive Fabric Hypothesis (ETH). It is expensive to refer to our brain tissue, which is uniquely metabolic compared to other primate brains. According to ETH, humans offset the increased metabolic costs of the brain by developing less metabolically expensive splanchnic organs, which include the intestine and liver. Humans were able to feed their brains using only a relatively small intestine, since increased dietary quality reduced the need for the gut. The hypothesis was that the main driver of this increased nutritional quality was the increased use of animal products.
Just how unusual is the modern human gut? Based on a reduced basic axis equation calculated for taller primates, human intestines should be about .8 grams larger. It is difficult to know when this change began, as the intestines do not fossilize. However, it is possible to extract some information from the postcranial anatomy. Live monkeys with large intestines have a rounded abdomen, continuous with the lower ribs, which gives it a funnel shape, as well as a wide pelvis with open upper edges. In contrast, the size of the human pelvis is reduced and the abdomen has a defined area of the waist. Hominids began to show this in the fossil record, beginning with Homo erectus, about 1.5 million years ago.
In humans, compared to primates, the gut is reorganized. The size of the colon is very reduced and the size of the small intestine increases.
In the large intestine, bacteria absorb otherwise useless nutrients into important nutrients and other chemical by-products. These include short-chain fatty acids (SCFA). The main difference between humans and other great apes is that apes such as gorillas are able to use their larger columns to get up to 60% of their calories from SCFA alone. Higher estimates of the human caloric intake of SCFA range from seven to nine percent.
The suggestion that humans may have received more calories from SCFA in the past is rooted in estimates of Paleolithic fiber consumption. The evidence is rather sparse and is limited to coprolites, showing evidence of fiber intake of up to 150 grams per day, much more than any human culture currently consumes. Even if the method for estimating the consumption of coprolite fibers is accurate, they may not support the conclusion that they represent the optimal level of species.
Part of the problem is over-emphasizing fiber, when other nutrients that play a similar role may have been more important in human evolution. The early optimism that a high amount of fiber can prevent many diseases of civilization has prompted many studies on the subject that have yielded mixed results. The focus on fiber in the past was on its ability as an indigestible bulky substance to increase digestion time and to bind certain nutrients.
The fact that people cannot absorb certain fibers and starch in their diets does not mean that they are anything but bulky substances. In the scientific world, there is a growing research focus on the fact that these seemingly indigestible ingredients are actually often digested in the human body rather than by human enzymes. Instead, they are digested by human bacteria in the gut.
The colon microbiome remains vital for human health. Scientists are just discovering how the bacterial population and its by-products play an important role in human nutrition, the immune system and other vital bodily processes. The gut flora is currently being tested for its role in hundreds of diseases.
Several new paradigms for the study of fiber were born, not as bulk, but as a means of interacting with bacteria in the gut. The importance of species mix, population level and products is emphasized. One new term for some fibers is "prebiotic". Prebiotic fibers cannot be digested by human enzymes but stimulate the growth of some beneficial gut bacteria, such as Bifidobacterium and Lactobactillus.
Another hypothesis is that the lack of SCFA is behind such ailments of civilization. SCFA, called butyrate, provides some insight into this. Butyrate is the preferred fuel of colon epithelial cells and also plays a major role in regulating cell proliferation and differentiation. Patients with multiple diseases, in particular colitis and inflammatory bowel disorders, have been found to be lower than normal. Studies show that such diseases can be treated by administering butyrate to the colon.
Bacteria affect the production of butyrate, but also nutritional supplements. Certain fibers produce more butyrate than other fibers. Interestingly, one of the best manufacturers is something known as “persistent starch.” Sustainable starch is a growing nuance in the understanding of fiber, as it is a starch that acts as a fiber in that it acts as a bacterial substrate.
Richard Wrangham suggested that the use of cooked starch has been one of the innovations in diet quality that nourishes rapidly expanding expensive brain tissue as it develops to hominid size. The growing field of analysis for archeological starch grains has transformed our notion of hominids, which were once considered predominantly carnivorous. Microfossils on Neanderthal teeth from about 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 starch on the human body is not fully understood. One promising adaptation is the salivary amylase gene that absorbs starch, AMY 1. Chimpanzees and bonobos have only two copies of this gene, humans have as many as 10 copies, though it varies quite strongly from the population of 2 to 10, depending on the importance of the starch in the diet. Molecular genetic evidence dates to the divergence of this gene around 200,000 years ago, around the time when conventional use of fire became commonplace.
Some people can ferment better than others. Recent studies on human intestinal changes have revealed possible genetic alterations, as well as environmental and lifestyle changes.
More research is needed on this issue, but this highlights the major importance of the colon in human evolution. The gut microbiome and anatomy promise much promise in illuminating our evolutionary past and teach us about the importance of a healthy gut and colon to overall health. Current data suggest that the colon may be more variable in our species than previously thought, questioning whether the representative colon used in medical and scientific textbooks and anatomy studies represents the latest adaptations. The guidelines indicate that adaptations are related to both the type and amount of fiber and nutrients such as butyrate.