
Subreddit: r/Biology
User: u/LetHimCookPlease
Original Post:
Why do humans miss the cellulose splitting enzyme?
Why did humans not evolve a way of splitting/catalyzing cellulose? Would it not have been useful, as another form of getting simpler sugars? (monosaccharides/shorter polysaccharides)
My Response:
So very few complex multicellular organism produces cellulases, the exception being basidiomycetes (mushrooms), snails, termites, and earthworms. The vast majority rely on large biological tanks of microorganisms to produce them in their gut, which then consume the nutrients in the cellulose-rich material and then in turn are consumed by the animal.
To do this, you need a very large gastrointestinal tract, so there’s enough residence time for the microbes to excrete their cellulases and undergo the long and energy intensive process of digestion. Most animals that do this spend most of their time eating because it requires more biomass to convert into proteins and other biological material. Grass and trees are not known to be particularly nitrogen dense.
Cellulose isn’t the only material that gets in the way of digestion. Lignins also need to be dealt with during digestion, which are far more complex and require many different enzymes to break down. Without a proper way to deal with lignin, cellulases are easily deactivated or run into lower efficiency because of the way cellulose is structurally enmeshed with lignin.
This is actually one of the core challenges in converting cellulose-rich material into sugars for industrial purposes.

On the other hand, we humans lost the ability to consume plant material that evolved in primates because we focused on foraging more biologically available sources of carbohydrates and protein. Hunting helps, as does learning to cook with fire. Being proficient at foraging roots and fruits is another way to get more sources of nutrients faster.
That kind of energy-efficient behavior gave us more time to spend on other activities, such as developing tools, getting better at cooking, and making artwork. We ended up with shorter intestines over time, since all we need are systems to break down starches, proteins, and lipids, so more of our energy could be spent powering our developing brains - intestines are very energetically intensive to maintain. So while in principle, it would be nice to have the ability to consume cellulose, it’s not as easy as it sounds, and we’re better off with our relatively quick and easy digestive system.
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