> It would be much more efficient to eat the plants straight instead of feeding them to animals to get their meat.

The problem is although plants have nutrients, most of them are inaccessible to the human digestion system.

Cellulose = indigestible fiber, yet forms the bulk of the structural component of green plants. C6H10O5, a polysaccharide which would be very valuable if we could use it. The most abundant organic polymer on earth. But we can't eat grass; or we can try, it is non-toxic but no nutrients to speak of will be acquired from chewing and swallowing it.

Grasslands cover about 30% of the earth's surface (more before the rise of agriculture), an abundant resource which cannot be ignored. So how can we derive nutrition from it?

Ruminants have found an answer: bacteria. More specifically, 200 trillion bacteria, 4 billion protozoans, millions of yeast and fungi present in the rumen of a cow, the first of the cow's two stomachs. The cow can't directly digest grass either, but the contents of her rumen can do it for her, then she absorbs the output.

This large complicated digestive system however is very expensive. Is there a more efficient mechanism of acquiring nutrients? Fortunately, early primates discovered a way. "Eating meat led to smaller stomachs, bigger brains" (Harvard Gazette, 2008). Encephalization, the 3rd stage that led humans to civilization (1st: terrestriality, going down from the trees, 2nd: bipedalism), trading off a larger brain for a smaller gut:

> But growing brain size presented a metabolic problem. A gram of brain tissue takes 20 times more energy to grow and maintain than a gram of tissue from the kidney, heart, or liver, she said. Gut tissue is metabolically expensive too — so as brains grew gut sizes shrank.

> It’s likely that meat eating “made it possible for humans to evolve a larger brain size,” said Aiello. Early human ancestors probably consumed more animal foods — termites and small mammals – than the 2 percent of carnivorous caloric intake associated with chimpanzees.

There are quite a few essential nutrients not present in plants. Of those that are, they are often less bioavailable or are found along side anti-nutrients. I'll mention just one: docosahexaenoic acid, an essential omega-3 fatty acid important for normal brain function. Has been with us for a while, Michael A. Crawford calls it "nutritional armor in evolution", and curiously it is photosensitive; photoreceptor cells contains high levels of DHA.

The human body can make DHA itself (being an essential nutrient after all, you would hope so), by converting from ALA, but this process is inefficient so vegetarians often have lower levels of DHA than meat-eaters. Apparently it is also found in microalgaes so some vegetarians supplement. But what else are they missing? What are they missing that we don't even know about yet?

An example of an anti-nutrient in plants: phytic acid in legumes, interfering with the absorption of iron, zinc, and calcium. Some plant foods do contain iron, but it is less easily absorbed than the type of iron only found in meat: heme-iron. This makes vegetarians especially women more prone to anemia. I've even heard of women who stopped having their periods after a vegan diet for many years, which promptly resumed when eating meat. Other animals with larger stomachs are more equipped at processing these substances in plants than humans.

Granted there are other mechanisms for unlocking the nutrients caged away in plants. The Aztecs invented grinding corn soaked in limewater, now known as nixtamalization, to increase nutritional value (converting bound niacin to free nicin, helping prevent pellagra) and decrease mycotoxins. But having ruminants eat grass, which humans then in turn eat in the form of meat and milk, is a very efficient and effective process.

This process repeats itself in other systems: for example, small crustaceans/plankton eaten by feeder fish like herrings, eaten by larger fish like salmon, eaten by mammals like bears. The "Food Chain".