Well, this explains a lot. (Just kidding.) There is a theory that eating meat is one reason why human beings developed our bigger brains. Now, credence might be added to that hypothesis with a study that shows non meat eating might actually shrink the brain. From the story:
Scientists have discovered that going veggie could be bad for your brain-with those on a meat-free diet six times more likely to suffer brain shrinkage.
Vegans and vegetarians are the most likely to be deficient because the best sources of the vitamin are meat, particularly liver, milk and fish. Vitamin B12 deficiency can also cause anaemia and inflammation of the nervous system. Yeast extracts are one of the few vegetarian foods which provide good levels of the vitamin.
The link was discovered by Oxford University scientists who used memory tests, physical checks and brain scans to examine 107 people between the ages of 61 and 87.
When the volunteers were retested five years later the medics found those with the lowest levels of vitamin B12 were also the most likely to have brain shrinkage. It confirms earlier research showing a link between brain atrophy and low levels of B12.
Although, as the story states, supplements are available, according to The Vegetarian Society, they have not been proved to be effective, which is why the Society recommends lacto/vegetarianism as the best way to not eat meat while maximizing health.
This study would seem to validate that meat is a natural food for humans, and moreover, it is good for us.