The ecosystem of Earth evolved over hundreds of millions of years. It flourishes because it is always in balance. Plants grow in the ground, herbivores eat the plants and carnivores eat the herbivores. This process evolved because it works. We evolved into exactly what we are today because our ancestors ate a largely carnivorous diet for a couple of million years. How can we be sure this is true?
1. Sweat glands. Unlike all other primates, we have lost our body hair and gained a multitude of sweat glands. We have the greatest ability on the planet to run long distances in hot weather because we can lose body heat from the evaporation of sweat on our skin. We developed this ability by chasing large animals across the grasslands of Africa. They struggle to lose heat by panting and eventually collapse and die from heat exhaustion. The Bushmen of the Kalahari still hunt for food this way. (Watch a 7 minute Attenborough video of this here) There is on reason to develop this ability if we were eating plants.
2. Nutrient density. We are closely related to chimpanzees and the other apes. Some of them are entirely herbivores while others are more omnivorous. Gorillas are herbivores and they possess very large intestines and small brains. We have large brains and small intestines. The difference is because we evolved to eat the nutrient and energy dense meat and fat of animals, which are easily absorbed. The Gorilla’s plant diet is difficult to absorb and they need large intestines to extract any nutrition. In fact, they find it necessary to be ‘copraphagus’, which means they eat their own poo in order to improve their diet.
3. Stomach acid. We have exceptionally strong acid in our stomachs. The only other creatures with comparable acidity are all scavengers of dead animals. This suggests that our meat-eating past began by cleaning up the remains of a big cat’s kill. This nutrition helped us to develop the ability to catch our own animals.
4. Vitamin B12. All animals need B12 to form red blood cells and to build the protective Myelin sheath around all our nerves and allow for proper brain development and function. Plants have no blood, nerves or brains and therefore do not need, nor contain, any of this vitamin. We definitely need an adequate supply which get from animal-sourced foods. Herbivores also need B12 but do not eat it. They rely on bacteria in their rumen or caecum to create vitamin B12. In the very distant past, we had a caecum to do this for us, but as we ate more meat the caecum became redundant and shrivelled to what we now call the appendix. This fact alone proves that we evolved into what we are by eating meat.