evidence: The other study finds that humans and some other primates have stuck with being omnivores for a very long time. That's unlike many of our mammal friends, who used the omnivore lifestyle as a mere rest stop on the way from herbivore to carnivore. [...] For instance, carnivores are usually the first to go extinct when times are tough, because they depend on other animals for their food source. [...] They found that all the animals stopped nursing when their brains hit a certain stage of development, regardless of diet. All the meat-eaters, including ferrets, killer whales, and humans, reached that point of brain development earlier than herbivores or omnivores, the researchers found. (They classified humans as carnivores based on the percentage of meat in the typical human diet.)