Cellulose
Plants use carbon dioxide and energy from the sun to make carbohydrates or sugar. Virtually all of the sugar made by the plant is polymerized into a long chain called cellulose or what we call "fiber". The plant uses cellulose primarily to make the cell wall which provides the plant's structural support. We alternatively metabolize carbohydrates to release the energy stored in the chemical bonds, and use it for moving muscle, or enzymatic reactions like making protein. There are many forms of sugar (glucose, sucrose, fructose, etc., but all of them are converted into glucose before they are used to make most of our ATP energy. Cellulose is simply a long chain of pure glucose, and yet we can not metabolize this most abundant form of energy.
We can not digest cellulose because we lack the necessary genes, and can not make the enzymes; cellulase, lignase, etc. Only the microbial decomposers (bacteria, fungi) possess these genes . From an evolution standpoint the absence of these genes in higher organisms is a mystery if animals truly evolved from microbes since they all have the ability to digest cellulose. The survival advantage of these genes is so great that natural out-selection is inconceivable. If we evolved from microbes, we should also have these same genes.
Cellulose is without a doubt the single most abundant energy source on earth, but no consumer can not digest it. Instead the energy we get from plants comes almost exclusively from reproductive growths where starch and simple sugars are stored. Roots are also frequently rich in starch. However, it is because of our inability to digest cellulose that we must farm and produce massive quantities of plants, and then only harvest a very minor portion for consumption. Cellulose is virtually everywhere we look. It surrounds every plant cell, and yet we can make no use of it.
If we could digest cellulose we would not need to farm to survive. We would be able to live for many days off the energy stored in a small bowl of any part of a plant (grasses, trees, etc.) Instead almost none of the glucose in basic roughage is metabolized, but instead it passes through as undigested fiber. This is a true waste of the stored energy in these foods.
http://nwcreation.net/genetics.html