Humans are part of a large taxonomic group known as Primates that falls within the phylogenic class that contains all mammals, Mammalia. This group includes our closest living relatives, the great apes, as well as living monkeys, and all extinct early human and non-human primate species. Because we are related, humans and non-human primates share many anatomical and behavioral characteristics. For example, our hands and feet have five digits, we have large brains relative to other mammals, and we have binocular vision in which our eyes face forward on the front of our skulls. Also, some non-human primates can communicate with symbols, occasionally use tools, walk on two legs for short distances, and have complex social interactions. By studying non-human primates, researchers can learn about the ancestry of modern humans and more about the lives of early human ancestors.