Posted by Hornpipe2 13 years ago
I've often been curious about why humans don't simply eat grass. We're surrounded by the stuff, yet people starve to death anyway. Why don't rich people employ the homeless as lawnmowers?  
I found my answer here:  
Why cant humans eat grass?  
Cows can eat grass, so why can't we? Well, plant cells contain something called cellulose. Chemically, it's almost identical to a lot of the carbohydrate molcules (like glucose and fructose) in the food we eat. But cellulose has one bond that's different, and we can't break that bond to get the energy out of the cellulose the way we get it from other molecules. So we could eat grass, but we wouldn't be able to digest it to get the energy we need. Cows and other mammals can eat grass because they have special bacteria living in their stomachs called symbiont bacteria. These symbionts can break the bond in cellulose for the animal its living in. That way cows can get energy from grass. We don't have any of these symbiont bacteria, so we can't digest the cellulose in grass.  
The more you know.  
deathburger: We don't have any of these symbiont bacteria  
But could we?
Kassi42: Why don't rich people employ the homeless as lawnmowers?  
But could we?
Fluffy: "We don't have any of these symbiont bacteria"  
No, but we have plenty of other gastrointestinal symbionts, two of which are responsible for production of vitamin K via the Krebs cycle.  
I think that digesting cellulose is somewhat of a one-way evolutionarypath. Once you go there, introducing high-protein foods into the mix becomes problematic. This is probably why eating grass would not have been a good niche for humans to fall limits your diet to cellulose.
deathburger: And meat is where we get our intelligence from, basically.
bear: I was wondering something similar the other day while watching survivorman
FoolProof: He doesn't eat grass but he'll chew on it and swallow the juices. ...and spit out the grass.