To those who feel that eating animal products is natural--
Look down at your hands. Look at your fingernails. Can they pierce animal flesh? Rip apart muscle tissue?
Walk to the bathroom mirror and open your mouth wide. In the back you'll see molars, which are used to chew plant material. Carnivores don't posess this feature.
Think about your stomach acid. A meat-eater has stomach juices with a pH of about 1 to help dissolve harsh things they may have swallowed, because they can't chew with their sharp pointed teeth. Our stomach acid is far weaker, like that of sheep and cows.
Ever had to use a laxative? Thousands of years ago, when humans settled down to agriculture and adopted a high-fat, low-fiber diet, our digestive systems were shocked by the sudden change from a low-fat, high-fiber diet. This legacy continues today with gas, bloating, and indigestion.
If you're not of European descent, you've most likely got normal human genes that render you lactose-intolerant several years after birth. It's likely that man wasn't intended to drink dairy products through adulthood.
Now, I'm not saying that eating meat and drinking milk is evil, but personally I try to keep my consumption of both low. As for the B12? One theory says that in certain soils it occured naturally due to the death of small insects underground. Over time, the vitamin was pulled into plants and small amounts of B12 were present in vegetables. However, due to today's agricultural practices that kill all of the organisms underneath the soil, there is nothing to add B12 to the plants.