As we know, cellulose is the most abundant polysaccharide in nature. Why don't we have an enzyme to digest cellulose?

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    $\begingroup$ Evolution doesn't make everything perfect. $\endgroup$
    – Armatus
    Commented Sep 4, 2013 at 18:00
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    $\begingroup$ I removed the line about evolution making things perfect. It is obviously wrong. Try watching an albatross land for example and tell me that they're perfect. $\endgroup$
    – terdon
    Commented Sep 4, 2013 at 18:22
  • $\begingroup$ @terdon or take off for that matter ... $\endgroup$
    – user3970
    Commented Jan 12, 2014 at 2:08
  • $\begingroup$ Bcoz human body doesn't have appropriate enzymes to breakdown beta acetal linkages. $\endgroup$
    – user16988
    Commented Jul 26, 2015 at 12:10
  • $\begingroup$ If we were able to digest cellulose which is the fibre which protects us from colon cancer, don't you think many of us would die of colon cancer? Probably we lost the gene for producing cellulase, the enzyme that digests cellulose, during evolution to our advantage. $\endgroup$
    – user19081
    Commented Sep 29, 2015 at 13:18

4 Answers 4


While it's true that cellulose is full of calories, it's very difficult to get the calories out.

Symbiotic bacteria take ages to digest cellulose, and as a result animals that digest cellulose with specialized symbiotic bacteria have a huge gut to house them in.

It's likely that the reason humans can't digest cellulose is because mammals generally can't. And mammals generally can't because it's way too much work, and we don't need to.

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    $\begingroup$ "mammals generally can't"? $\endgroup$ Commented Sep 6, 2013 at 18:18
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    $\begingroup$ Even ruminants that eat grass and so on mostly just let bacteria eat the cellulose then digest the bacteria. $\endgroup$
    – Resonating
    Commented Sep 8, 2013 at 23:02
  • $\begingroup$ Are there animals who can digest cellulose without the help of bacteria? $\endgroup$
    – Remi.b
    Commented Aug 7, 2015 at 0:00
  • $\begingroup$ Some termites, but I think that's it. $\endgroup$
    – Resonating
    Commented Aug 7, 2015 at 17:18
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    $\begingroup$ @Resonating imho your answer would improve, if you incorporated your comments. $\endgroup$
    – Arsak
    Commented Nov 9, 2016 at 8:40

This seems one form of a common question about evolution.

That being: "Trait X would seem to be an advantage, so why can't organisms adapt to X?

That is, why don't we have all traits that are advantageous at all? Why not just digest cellulose, but maybe why not lignin as well? Why shouldn't we photosynthesize our own food? What about sonar as well as eyes? We know that some living thing can do all these things.

The answer, somewhat mentioned elsewhere here, is that some advantages are not enough for the adaptation to take hold. You have to consider that there is a cost to each such ability as well.

Cellulose is energy poor and any animal which relied on cellulose would have to specialize in cellulose metabolism to the point that they might not compete against fellow animals that got their food from other sources like meat or vegetables.

thanks for the comment below: addendum:

energy poor in this case is a combination of how common the material is, how advantageous it is to digest (woody plants don't grow back very quickly once you eat them up). i don't know why i wrote that there are not cellulases. there are plenty of them and we use them a lot but they tend to be from slow growing fungi...

if you want to follow your question to the conclusion : "celluose digestion is a huge advantage in all respects" then the question is why aren't they in every bacteria that grows around plants? because evolution is not how cellulases are created?

in fact woody plants include other materials supporting their structure to prevent this eventuality. lignin is a non carbohydrate semirandom fiber based on phenols that is indeed very slow to digest and difficult to get at with an enzyme... at the moment it does seem to be enough to prevent plants and paper celluose from dissolving and being eaten...

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    $\begingroup$ You make a good point. I think these questions also come from the wrong perception (which was evident in the original version of the question) that evolution is finalistic towards making a species "perfect". $\endgroup$
    – nico
    Commented Jan 13, 2014 at 7:55
  • $\begingroup$ we make the same mistake when we buy cars too - a V8 2 ton pickup with back seats and i'll have the perfect car - it can do anything! But in biology a tiny fly is perfect and so is a rock clinging piece of lichen...they just do what they need to do well enough $\endgroup$
    – shigeta
    Commented Jan 13, 2014 at 15:34
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    $\begingroup$ shigeta, I agree with nico, evolution does not make things "perfect". Not by a long shot. Living organisms are a mess, riddled with hopelessly inefficient mechanisms. Think Rube Goldberg machines. It's nowhere close to what a decent designer would have come up with, but it's what you get from random mutation + selection. This more general question probably deserves an answer in itself ... $\endgroup$
    – Roland
    Commented Jul 9, 2015 at 6:37
  • $\begingroup$ there's lots of shades of meaning here, which is definitely interesting to talk about. Organisms are so optimal for selected traits that they amaze and astound us - better than we can design ourselves in many cases. Where selection has not had as much pressure (e.g. long life, resistance to damage and disease, empathy, speed, intelligence) it can seem that they are strangely vulnerable. $\endgroup$
    – shigeta
    Commented Jul 9, 2015 at 15:23
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    $\begingroup$ "Cellulose is energy poor" Excuse me? You're basically saying glucose is energy poor. You should specify that it's "energy poor" in the sense that it's resistant to enzymatic breakdown or something (I don't know if that's the actual reason, it's just what I'm assuming based on other answers in this thread.) $\endgroup$
    – Inhibitor
    Commented Aug 7, 2015 at 1:37

Why don't we have any enzyme to digest cellulose?

Why should we? We don't use it as a source of energy so why bother? Even animals that do "digest" cellulose, like ruminants, only do so because of symbiotic bacteria; it would be a poor system indeed in which every organism utilized the same resource. We occupy enough of the food chain as it is.


The question forgets that organisms consume materials for two reasons: To capture chemical energy (which it focuses on) and to gain molecules that the organism cannot otherwise produce. Cellulose effectively serves only one purpose in organisms that digest it: energy production. Eventually, it is broken down into H2O and CO2, both of which are excreted.

Throughout the millennia, getting calories was not that difficult, but getting enough essential micronutrients (vitamins, minerals and amino acids) as humans moved into different environments has been where there was selective pressure. Selective pressure for other calorie sources, such as cellulose, has never really happened. Beyond this, there is a ratio of optimal caloric intake to optimal nutrition intake a human should get from his diet for that human and humanity in general. A mutation or sudden symbiosis with gut bacteria that allowed for cellulose digestion would make it even more difficult to keep that ratio, but it is non-optimal situations where selective pressure occurs. The issue here, in my opinion, is that that would be too great a selective pressure on decreasing the micronutritional requirements. The human would far too easily get calories and need to eat many times more in calories what he needed to get the necessary nutrition. So actually, there is selective pressure against cellulose digestion. Further, because of the way humans are built and the other great primates, this sudden mutation or symbiosis seems very unlikely.

Also, cellulose already serves a vital purpose in the gut. It is commonly called fiber, and allows the feces in the colon to remain moist enough to be passed. Some mammals have adapted colons that can pass very dry feces (the Camel's feces is so dry that you can start a fire with it), but they are typically arid climate species, where water is by far the most precious resource. So the selective pressure was on water intake, not calorie intake.

So there has been no selective pressures on humans to digest cellulose, nor will there ever be, so long as there is such a high need for vitamins, minerals and essential amino acids. Such selective pressure would take the form of there first being super nutrient rich foods (relative to humans), then that same food becoming calorie poor. Basically, humans would have to start replacing a meal or two per day with only vitamin supplements. Even with that, the only mammals I know of that digest cellulose are actually utilizing symbiotic gut bacteria to do so. I can't imagine the clever way humans might get that bacteria into their own guts. Actually, I can.

  • $\begingroup$ I don't want to bump with an edit, but I recall a criticism of this answer that accused the line about nutrition to calorie intake ratio hinging on that common error that evolution makes perfect. I want to clarify that's not what I mean. I mean to point out that currently, because of known evolutionary pressures, there is an optimal ratio and an adaptation to digest cellulose would gravely affect that, therefore that would have to adapt as well. $\endgroup$
    – user3970
    Commented Nov 19, 2018 at 18:10

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