As it is really beneficial for all organisms, why are animals, especially mammals (I don't know about other vertebrates) unable to synthesize cellulase enzyme in their body? Is it linked to some DNA defects or have we not evolved from cellulase producing bacteria?
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Doesn't the bacteria flora in some higher vertebrates (ie: cows) allow for the celluase, and therefore the digestion of cellulose? We lack the bacteria and therefore the enzyme. I am not sure I understand the last part of your question, " or have we not evolved from celluase producing bacteria". – Ro Siv Jan 01 '16 at 13:14
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i actually meant 'why cant mammals synthesize their own cellulase?' or 'why are cows dependent on bacteria for cellulose digestion?' and, by the last part i just meant that if we lack cellulase producing gene, then we must not have received it from our ancestors, which could thus not be cellulase producing bacteria ;-) – another 'Homo sapien' Jan 01 '16 at 13:41
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I think its a symbiotic relationship, so the bacteria bet something benifital, as do we(the calories). Bacteria flora are often passed down from mother to child anyway, so why would we even need to have it in our genome? Couldn't we extend your logic to why we don't have any gene "x" that provides benifit "y"; for evolution tinkers and not engineers right? – Ro Siv Jan 01 '16 at 14:21
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thanks, and maybe its because of those bacteria only that mammals never needed to develop that gene (if i got you right) – another 'Homo sapien' Jan 01 '16 at 14:33
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Or they lost the gene. That is usually the better assumption. Think about gene duplications. Once a gene is duplicated, only one copy need be under selection, so the other one can be mutate, either becoming a nonfunctional gene or a newly functional gene. Also remember that there is a cost to maintaining and expressing a gene in the genome. If a symbiont is producing something for you, then it costs you less to maintain your own genome if you lose the gene you don't need to use. – AMR Jan 01 '16 at 15:47
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2Duplication of If a trait would be advantageous to an organism why hasn't it evolved. – AMR Jan 01 '16 at 15:53
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@AMR your last line "If a symbiont is producing something for you, then it costs you less to maintain your own genome if you lose the gene you don't need to use." is confusing me now. How can a gene be expelled or lost just because a symbiont is already providing the resulting protein to you. Its just like you cut off your legs just because a horse is there to carry you. So I doubt "Or they lost the gene.". Maybe that gene gradually became non-functional with time or is still present, but works on feedback mechanism. – another 'Homo sapien' Jan 01 '16 at 15:54
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You cannot call it duplicate as that question says "This is a general question that would be applicable for any kind of trait." – another 'Homo sapien' Jan 01 '16 at 15:56
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These are molecules, not body parts. If you do not have a symbiont that produces the product, then you have to maintain the gene or a mutation that causes the gene to be non-functional would be deleterious and could lead to the organisms extinction. A mutation that makes the gene non-functional can occur if there is a symbiont producing the gene product and so you remove selection for that gene and it can mutate to non-function without hurting the host organism. – AMR Jan 01 '16 at 16:01
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Substitute cellulase for any trait and that is your question which is why it is a duplicate of that question. – AMR Jan 01 '16 at 16:03
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Why substitute? (don't mind) This question is specifically about a single trait which, according to me, has probably evolved but never showed up. Anyways, this discussion is now going off-topic, so thanks everyone for your help. – another 'Homo sapien' Jan 01 '16 at 16:07
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Histidine is one of the most important amino acids for proteins, but humans cannot produce it. So if I asked, Why did humans not evolve (or retain) the gene to produce histidine? then it is a duplicate of that question. That is why we have it as a community wiki. – AMR Jan 01 '16 at 16:07
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But thats different. Humans can eat proteins and breakdown them to get histidine, but humans cannot eat cellulase and become able to digest grass. – another 'Homo sapien' Jan 01 '16 at 16:20
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AMR is absoltely right. The question is opinion based and we can go on debating for hours upon hours on this topic – Jan 02 '16 at 07:39
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The answer to questions like this is just a guess (Primarily Opinion Based), and I've voted to close. But some food for thought too long for a comment:
As it is really beneficial for all organisms...
Why do you think that cellulase is so very beneficial to mammals? Animals did have cellulase (some ancient invertebrates still do, like sea squirts, abalone; crayfish have it); we may well have faulty copies of the gene (as @AMR suggested in comments.) But you presume to know we'd be better off with it. That's a belief without any objective evidence.
What might happen if we could convert cellulose into sugar? This is also important to think about. If some of that surfeit glucose in out guts were fermented into alcohol (this condition does rarely occur in humans, and the living people afflicted with this are truly afflicted), because we have a gut microbiome, we would walk around drunk much of the time (ok, maybe even only some of the time), which would make escaping predators less drunk than we were difficult. Maybe the body would be incapable of handling that level of glucose ingestion, and we'd all be T2 diabetics by the time we were just out of toddlerhood.
Humans need fiber in our GI tracts. I don't know all the reasons why we developed that way, but we do. Cellulase would do away with that. Do you know for sure that this would be good for us?
The thing is, you've made an assumption (we'd all be better off with cellulase) that has no evidence to support it at all (or, at least you have not provided any.)
It's always tricky to assume that you know what's more beneficial for a mammal in the presence of billions of mammals that seem to be doing OK. There are millions of reasons that they are doing OK. We don't know all of them, but the absence of the very thing you presume would be a benefit may be what allowed vertebrates to develop at all.
Ancient Origin of Glycosyl Hydrolase Family 9 Cellulase Genes
Structure and function of a cellulase gene in redclaw crayfish, Cherax quadricarinatus
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"I don't know all the reasons why we developed that way, but we do" lack of cellulase might be the reason for it. As you've said, answers to such questions are, and will be, opinion based and also, as you said "It's always tricky to assume that you know what's more beneficial for a mammal in the presence of billions of mammals that seem to be doing OK" one can never estimate how would something have affected our evolution. My point was that maybe having cellulase would have enabled us to live more easily in food scarcity. But, again I cannot assume this to be perfect solution. – another 'Homo sapien' Jan 02 '16 at 04:28
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