Bovines are thought to have first appeared on the landscape millions of years ago and were domesticated by humans about 10,000 years ago. Rumen microbes evolved to produce molecular machines in the form of enzymes able to efficiently deconstruct plant cell wall polysaccharides such as cellulose and hemicellulose into their constituent small sugar molecules. Another way of looking at it is that in exchange for housing in the cow rumen, these microbes pay rent by efficiently converting fiber that the cow cannot utilize into small sugar molecules that serve as substrates for fermentation into end-products that provide energy for the cow.
Read more on PhysOrg.