Although nobody knows for certain, it is thought that in human evolution, an upright posture came before a developed brain. This upright posture freed up our ape-ancestors hands to use primitive tools such as rocks and sticks. It also allowed raised their eye level which helped them better see prey and predator. This started happening about 15 million years ago. For over 10 million more years, the brain stayed pretty much the same size. Beginning with Homo erectus, the brain size of our ancestors started growing dramatically. Almost all of this growth was in the expansion of the cerebral cortex which is what gives humans the ability to have language, abstract thoughts and consciousness. In only 3 million years brain sizes had quadrupled, giving humans the largest brain per body size ratio of any species on earth. It is still unknown why or how the brain could grow so much in such a short period of evolutionary time.
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