Watch the full episode of "Holy Cow" online now.

Cattle are at the heart of the Masai's economy and their pastoral lives. Tepiit ole Saitoti, a Masai warrior, once wrote, "Without the land and cattle, there will be no Masai."

(Airs Sunday, May 31) About 8,000 years ago, the relationship between cows and man began with the revolutionary advent of domestication in Mesopotamia, the Indus River Valley, and Africa.

Clips from the NATURE episode "Holy Cow!" show the cow's digestive system and illustrate other naturally evolved and artificially selected traits of cattle.

This video-enhanced lesson focuses on how and why humans have been so successful in selectively breeding cattle to suit our needs, while also exploring the limitations and consequences of this success. (Grades 9-12)

Some ranchers argue that they can raise organic beef using methods that many experts say are friendlier to the environment -- and healthier for beef eaters.

First recognized in the 1980s, mad cow disease -- or bovine spongiform encephalopathy (BSE) -- causes the brains of cattle to waste and become spongy and pockmarked.

John Webster, a former president of the British Society for Animal Science, was asked by the British government to study the causes and consequences of mad cow disease.

Although Hindus follow no single set of rules, reverence for cows can be found throughout the religion's major texts.

Download wallpaper for your desktop from NATURE's "Holy Cow."