So my not-at-all deep understanding of digestion goes something along the lines of:
We chew up food and swallow it. Various acids and enzymes and bile and whatever have at the mixed glop, in the stomach and small intestines, and there might be some in saliva so it can start in your mouth, and you have loads of bacteria in your gut that chomp away at whatever gets that far, possibly producing side-products that we do benefit from to some degree, and finally your large intestine absorbs most of the water from the remainder so we don’t have constant diarrhea. So the end product is mostly fiber and maybe dead/shed cells from the digestive tract itself?
So what is special about fiber? Presumably it is made out of the same elements as all other foods, probably hooked together in different patterns, so could it potentially be digested to yield stuff we need, at the very least, some energy or useful chemicals?
In the X billion years it has taken to evolve, humans just never came up with an enzyme to disassemble these particular groups of atoms, while we happily rip up almost everything else plants put together?
It happens. This sort of thing is why coal exists. Example: for a long time microogranisms were able to decompose dead plants, but then some new plants figured out how to make lignin. There was a stretch of time (~60M years) during which lignin-rich plant matter accumulated, eventually being turned into coal. Then microorganisms evolved to decompose lignin, after which time coal formation was pretty much over.
Cellulose is really hard to digest and get useful energy out of it. Most animals that get most of their nutrition from cellulose have multiple stomachs where enzymatic digestion and fermentation take place and mechanical crushing through chewing their cud.
Herbivores are the mammals who evolved to digest cellulose. Omnivores and carnivores did not. Presumably, the ability to eat and digest meat gave sufficient nutrients that all the extra innards necessary to digest cellulose was an energy burden that could be jettisoned.
No vertebrate and not even termites can digest cellulose directly. Animals which eat and ‘digest’ cellulose actually do so through symbiotic bacteria in their intestines that produce the enzymes necessary to break down cellulose.
Not every animal evolves to feed off every nutrient. Animals evolve to fill niches in the environment. Being over generalised seems to put you at a disadvantage to a specialist, so your species will be out competed in every niche by specialists. Predators evolve to use other animals to do the hard digestive work. Or as they saying goes - vegetables are what food eats.
Thank you for this! It was far and away the most interesting information I learned that day.
Huh. That opens the possibility that we could actually learn to eat grass, just by adding some new species of bacteria to the the menagerie in our gut. Which would mean that suddenly all our calorie charts would have to be rejiggered.
But apparently it takes even the bacteria a relatively long time to deal with the cellulose, so our guts would have to get a lot longer/bulkier and who wants that?
Actually, maybe it’s really, really lucky that cellulose is so hard to take apart. I mean, if it was easier, would any plant survive long enough to turn into a bush, let alone a tree? Wood is pretty dang useful stuff to have around.
The consortia of bacteria and other microorganisms in our gut change depending on our diet. If what you’re eating favors one type of bacteria, that population will rise. To keep a population of cellulose/lignin eaters happy, you’d have to no only eat a lot of grass, but not eat anything else. If you ate other things, other bacteria would out-compete the C/L eaters.
In fact, you probably have a few C/Ls in your gut now, just not a noticeable population of them.
Point of order, but depending on how you define “human”, we’ve only been around for a few hundred thousand years, not billons.
A good intuitive way to think about evolution is every species has a job, and every species has to do that job in a certain habitats with certain resources. Cows can’t cook, cows can’t really migrate. They graze grassland. That’s their job. They’re good at that, and nothing else.
Humans have much more varied food options. We can hunt, we can gather, we can farm, we can unlock more nutrients by cooking. We never needed a high-effort food source like cellulose, so we never evolved one.
It has seemed to me for a long time that, as StarvingButStrong said, we could make the world’s food supply go farther if we could import bacteria from termites to our own guts. I brought it up in GQ a while back, and the idea was pooh-poohed (no pun intended.) However, since then knowledge about the human biome has advanced, and now some digestive problems are being treated by transplanting gut bacteria from healthy patients.
That’s very much like saying that since a screwdriver is a useful tool if you’re installing a door then it should also to be able to build a moon rocket all by itself.
Whoa, now, Exapno_Mapcase, don’t get ahead of yourself. I’m not talking about eating the morning paper for breakfast. We humans eat and excrete lots of cellulose in a normal diet. Every plant cell wall is made of the stuff. If we could increase our gut efficiency by just 1 or 2%, the food we eat in just the US could feed another 1 or 2 million people.
Cows and other ruminants have multiple stomachs, and rabbits have extra footage for digesting cellulose, but termites? Heck, the whole critter is a little over an inch long. Inner capacity is clearly not the only answer.
I’m not saying I’m sure a little biome transplanting will be a Bob’s-your-uncle quick solution. I’m just saying it bears looking into.
Yeah, a termite’s innards aren’t much of a guide to human digestion.
Here’s the issue. Our gut bacteria live in the colon, the large intestine. Virtually all human digestion, however, takes place in the small intestine. The small intestine is not designed to house bacteria; in fact, excessive bacterial growth there is a serious problem.
So re-enigineering the intestines to digest cellulose is far more complicated than adding some bacteria to the mix. Either the small intestine would have to be made more hospitable to large amounts of bacteria - a job that would entail finding a way for them to stay there despite peristalsis - or changing the large intestine to mimic the villi (or brush border) in the small intestine, the tiny fingers on the inside wall where the enzymes live and the absorption of simple sugars (the byproducts of cellulose digestion) occurs.
I’m not saying that it is impossible to absorb sugars in the colon - I don’t know enough to give a dogmatic answer. I do say that getting to that point would probably involve something like gutting a kitchen to the studs and rebuilding it to serve a previously unthought of service. And what would the result of that be? Adding more sugar to the diet. That’s not expanding the food supply in any useful way.
My personal feeling is that doing this would be like building a bridge across the Atlantic to alleviate crowding in airlines. The challenges are not worth the end or the unintended consequences that would result.
If you have an argument for how and why this is a good idea, I’d like to hear it. But right now it’s not science as I know it.
