Why don't mammals produce cellulase?

[siroma]

Hi all!

Of all mammals only ruminants can digest wood. Even they use help of simbiotic bacteria/prostists to prepair dinner for them.
Wood is almost everywhere, storing endless amount of energy. Why don't most animals utilize this energy?

I can bring somewhat awkward explaination: "If most animals could eat wood, there would be no wood. If they could, wood would be consisting not of cellulose, but something that animals cannot digest."

Well, but maybe there is some basic principle which, say, mammals cannot bypass to eat wood?

[mith]

very interesting, I haven't thought about this before

[Darby]

Those ruminants can't digest wood - just the less dense cellulose of fiber.

And they use symbionts to do it.

There are almost no animals that digest cellulose with only their own enzymes. It may represent one of the oldest symbioses around.

[siroma]

There are almost no animals that digest cellulose with only their own enzymes.

That's it. But why? The process is ineffective? But to digest cellulose an animal seems to have only one additional enzyme, breaking beta(1-4) links to get free, sweet, tasty glucose.

[MrMistery]

Only earthworms and some inferior insects can digest cellulose, beacuse they have the enzyme commonly named cellulase. But as we know, chordates are not direct descendents of insects, so the enzyme simply wasn't available to them. It is a very simple principle: evolution does not create something on demand, it has to mold things that arrise randomly. And we can only speculate that in our case, it didn't arrise.
By the way, the main constituent of wood is not cellulose, but lignin. Lignin is a branched polymer made out of some phenolic monomers, derived from the aminoacid phenylalanine.

[siroma]

evolution does not create something on demand, it has to mold things that arrise randomly.

That's right. But is absence of cellulase enzyme only due to "bad" luck of gene generation, in which the enzyme didn't arise randomly, or are there some other, more fundamental limitations, for example, ineffectiveness of cellulose digestion process that cannot supply enof energy for, say, mammals?

[Darby]

It may just be the path of least resistance - just as we humans don't make coenzymes already available in our ancestral diet, why would animals derive a huge advantage from developing an enzyme that well-adapted symbionts already have and use to the host's advantage?

[MichaelXY]

Would it be possible for animals to be given a cellulase tablet, thereby enabling one to ingest cellulose?
Kinda like they do for those that are lactose intolerent.

[Ultrashogun]

Because its much easier to just make a sammich than evolve a new enzyme.

[siroma]

Sammich? What do you mean?

[kotoreru]

I think he needs 'sandwich', dear...

[siroma]

Oh! That's profound suggestion.