Many people have asked me if our guts have adjusted to the diet of the past 300,000 years. Basically since fire was discovered, humans began cooking their food (we also ferment our food, such as tofu, yogurt, sour herring, etc. and we marinate our foods that also breaks down protein and carbohydrates).(i) The act of cooking breaks down a lot of nutrients and makes digestion easier. The answer to the question is that with a high likelihood: yes! Our guts probably have adjusted.
Obviously we have to rely on circumstantial evidence, since no one was around to measure the intestines of our ancestors before and after the advent of fire.
One way to look at it is to compare our digestive system with our closest living relatives (apes). Compared to Gibbons, Orangutans, Gorillas, and Chimpanzees (in order of genetic closeness), humans have a markedly different gut. In the other apes, the colon makes up about 50% of the total volume of the intestines and in the case of humans about 20% (our small intestine on the other hand makes up about 50% of the volume of our digestive tract, whereas in apes it makes up about 30% of the total volume).(ii) All non human apes eat a mostly vegetarian diet, with small amounts of meat, insects etc. added as delicacies (e.g., less than 5% of a Chimp's diet is meat or insects).
One way to understand this is that our cooked and fermented foods are primarily absorbed in the small intestine and we do not have as much a need to extract energy from fermenting things in our large intestine. So the fermentation products in the large intestine are primarily used for feeding the lining of the gut rather than being used as an energy source.
Looking forward to the next 300,000 years
Over the past 150 years, there has been a fundamentally new shift in our food consumption.
We are eating increasing amounts of processed foods (e.g., over-eating simple carbs)
We are eating more meat (e.g., over-eating saturated fat and protein)
We are highly deficiency in fiber (not enough plant matter)
How will our gut evolve given the above? Your guess is as good as mine (evolution has a mind of its own). The main reason why such a guess is highly speculative is that it is not at all clear what the evolutionary pressure might be. Evolutionary pressures that allow better survival and procreation are difficult to determine in our current world where there are no isolated populations and our long lifespan is way beyond the procreation point. However, lets speculate:
Given the discussion above, point 1 (more processed foods) should lead to a further relative shortening of the large intestine.
In general, among animals, meat eaters (relative to body size) have the shortest small intestine, followed by omnivores (eat everything), followed by herbivores (plant eaters). (iii) So this should lead to a shortening of our small intestine. (iv)
How fiber deficiency will impact the future of the gut is anyone's guess. It could be that:
People who do eat more fiber will be healthier and on average will dominate in the population by having more and healthier children, who then in turn will continue the trend. Highly unlikely (v)
The bacteria in our colon will become more efficient at fermenting the little fiber that passes through. Not likely but possible, for example Koalas have evolved to be able to eat Eucalyptus leaves and have unique gut flora to help them accomplish this.
Our colon will adapt and possibly lengthen or increase its surface area to make the most of the little fiber that passes through.
Our colon will just shrink further as it has less to do
Genetic engineering will compensate for some of our deficiencies
So in total, my close-to-random-guess would be a further overall shortening of our gut (large and small intestine).
(i) Anatomically modern humans have been around for about 300,000 years, but the discovery of fire seems to be older than 1m years and used by many early hominids such as Homo Erectus.
(ii)Furness, John & Cottrell, Jeremy & Bravo, David. (2015). Comparative physiology of digestion. Journal of Animal Science. 93. 10.2527/jas2014-8481.
(iii) O'Grady, Shannon & Morando, Mariana & Avila, L.J. & Dearing, Maria Denise. (2005). Correlating diet and digestive tract specialization: Examples from the lizard family Liolaemidae. Zoology (Jena, Germany). 108. 201-10. 10.1016/j.zool.2005.06.002.
(iv) Note the large intestine follows the same pattern. In other words both large and small intestines are shorter in carnivores versus herbivores with omnivores in between
(v) Note: from an evolutionary perspective living long enough to ensure survival of the progeny is the main driver, so dying 10 years earlier (say at 70 versus 80) is not a significant driver of natural selection.