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Cellulose metabolism

  1. Apr 9, 2010 #1
    As a species evolves away from maladaptive forms, there may be occasional reversions. A well-known example is the tail in humans. A very small percentage of humans are born with a tail. My question is this. The appendix is possibly a vestige of an era when human ancestors were able to digest cellulose. True or not, we might expect that a tiny percentage of humans would be born with the ability (to varying degrees) to metabolize cellulose. Does anyone know if this is the case? Are there any documented cases of people who could metabolize cellulose? Thank you.
  2. jcsd
  3. Apr 9, 2010 #2
    I believe that newest mainstream research suggests that the appendix is part of the immune system. I'm not exactly up to date on this particular area of research but the last I checked this was the belief.

    Anyways to you question. I do not believe there have been any recorded instances of a human being able to digest cellulose using the appendix. At least I haven't read of any such instance and a quick search on some databases hasn't given me anything promising.
  4. Apr 9, 2010 #3
    My question is, whether the appendix is involved or not, are there any known instances of humans with the ability to digest cellulose? The answer is probably "no" but I doubt that there would be any occasion to test for this, and the test might be somewhat involved. After all, someone who had this ability in small degree would probably not even know it.

    Two ancillary questions.

    1) If human ancestors had the ability to digest cellulose, does the theory of evolution not suggest an occasional atavism, provided we haven't evolved too far beyond that point?
    2) From another angle, if the ability existed, would it tend to support the idea that humans evolved from cellulose-metabolizing ancestors?
  5. Apr 9, 2010 #4


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    Please elaborate on what you mean by "metabolizing cellulose". From what I remember most higher animals leave cellusose digestion to symbiotic prokaryotic organisms.
  6. Apr 9, 2010 #5
    I gave my answer as: No, I do not know of any recorded instance.
    Yes the theory of evolution does include atavism. However you can not know specifically could still function or show up. Just because human ancestors may have eaten plants does not suggest that some humans must also be able to digest cellulose in a similar fashion. It is possible but not necessary. It also depends on what the actual function the appendix has. Your entire point is based on the premise that the appendix was in fact 100% used for cellulose metabolism.
    Yes, of course at some point down human ancestory there will have been an organism which could metabolize cellulose. Not that it really needs any more support (In my opinion).
  7. Apr 9, 2010 #6
    I think it's clear that the OP is saying that the organism itself produces the enzymes required to break down cellulose. An organism which depends on bacteria to produce the enzymes (in humans cellulase I believe) are not cellulose-metabolizing organisms themselves.

    In short, humans are not cellulose-metabolizing but cellulose is broken down and digested in a normal, healthy human.
  8. Apr 9, 2010 #7
    Thanks for both sets of responses.

    I did qualify, "correct or not." My interest is, regardless of whether appendix is implicated, the possibility of atavistic use of cellulose akin to growth of atavistic tail. The answers to the supplementary questions suggest that prehistoric use of cellulose may but need not imply atavistic cases, and that atavistic cases would imply prehistoric use (i.e., proto-hominid ability to use cellulose). This sounds right. With all we know about nutrition and dietary disorders, I'm a little surprised we don't know of any instances of this condition.

    Thanks again. Daniel
  9. Apr 9, 2010 #8
    Not only are you right... you're REALLY right. Beavers, Ruminants (cows, sheep, etc) depend on bacteria to either produce the enzymes, or for rough grasses multi-staging of digestion with bacterial aid is still the way.

    It's just fiber for us, and even Beavers only eat a very specfic portion of the wood. I don't know of anything except bacteria which truly metabolize cellulose.

    @Daniel6874: There are plenty of instances, it's called Pica. EDIT CLARIFY {Instances of humans eating dirt, and other "non-nutritrive" substances. The cause of Pica is still a bit mysterious, but it could relate to a time when the appendix did serve as a human "gizzard" EDIT ENDS} As for the rest, the appendix, or any other endogenous bacteria etc... don't metabolize. The appendix is more akin to a gizzard than intestines or stomachs; a place to mechanically break down fiber along with various other detritus (sand for intance).

    Our diet changed radically with the advent of agriculture and the storage of grain, along with domesticated livestock (especially milk-producing), and frankly our little appendix was rendered not only useless, but potentially harmful.

    It makes one consider if anything short of a return to near-barbarism could induce the kind of selection that would eliminate the appendix... or if it would rise to (well not exactly) prominance again.
    Last edited: Apr 9, 2010
  10. Apr 9, 2010 #9
    Thanks for the pica note. That's interesting.

    There is a good article--Cellulose and the Human Gut, Cummings, Gut, 1984, 25: 805-110. It complicates the picture. Apparently humans do digest the cellulose that occurs in normal diet somewhat. Pure cellulose is a different matter. Of course the cellulose is broken down by bacteria in the gut, and the quality of digestion is in part a function of time, with older persons showing an advantage due to the slowness of their metabolism.

    I don't think that there was ever a time when proto-humans digested cellulose without the aid of bacteria. The appendix hypothesis I noted (I forget the source) suggested that the appendix was a repository for these bacteria (in an earlier era). Even ruminants use bacteria to digest cellulose. So I think this moots my original question. We all metabolize naturally occuring cellulose to some extent, but perhaps less efficiently than creatures with a guts containing optimal flora (our forbears?) and/or added digestive capacity for fermentation.

  11. Apr 9, 2010 #10
    Hmmm, if you consider bacteria-aided metabolism to be essentially an endogenous feature, you could be correct. I think we're all clear on the basic facts here, and now it's a matter of historical exploration. It seems highly unlikely that we had any capacity for endogenous fermentation... we just don't descend from that kind of animal.

    If you look at chimpanzees. their diet is primarily figs (tiny wasps and all, heh), and the occasional monkey. Fiber in their diet is primarily, well... passed. Speaking of "passed" it woudl be easy to detect metabolites associated with cellulose in urine, and that just isn't there either.

    Now, the "bacterial resevouir" notion is one I can't say is wrong, but I don't personally believe in it. I see it as a somewhat ad hoc explanation for infenction of the appendix (leading to appendicitis)... because the cause is rarely clear. When an easy surgery presents an obvious solution, the drive to find a cause diminishes. Here is one view I'm quoting from emedecinehealth.com:

    To me, the lack of heredity, and the many causes tends to rule out a single favourable species of bacteria as "ideal" for the appendix. The problem here really isn't medicine, or biology, but Archaeology, but even so I can't think of a single species in our presumed line that ever derived nutritional value from cellulose. It just takes a lifestyle that is incompatible with our lineage.

    Again, that is my view, not hard fact.

    As for Pica... hey, that's why we have a diverse crowd here. :smile: A lot of physics, some biology, philosphy, but of course, psychology is not a science, is changeable, and often culture-bound. Pica... is not culture-bound, or racially, or in any other way bound to one group. That, like Schizophrenia (.5% across the board, roughly) makes it interesting as fundamentally human, compared to something which is clearly an artifact of culture, or a genetic mutation/fetal insult/etc... etc...
  12. Apr 9, 2010 #11
    The mountain gorilla eats mostly foliage. Our larger brains have enriched our diet and lessened our dependence on leaves. My guess is that the common ancestor was more like the gorilla than us in terms of digestion, but who knows. I guess Lucy's teeth would be an indication.

    My original question was ill-posed. The article cited above makes clear that we all digest cellulose to some extent, especially when it occurs naturally in plants. The author recovered marked carbon (C_14) from the breath of subjects. So it's not as if there is a cellulose gene that can be switched on and off. We and our ancestors have long had the ability to digest cellulose using flora in the gut--just a question of degree.