If you look at the animal kingdom you will see a bewildering array of life forms, with an even more astounding variety of no longer extant species in the past. Among these life forms we find the class of mammals in which the relation of brain size to body mass is greater than in other groups of animals. This is particularly true for cetaceans (sea mammals like whales and dolphins) and for elephants. However, there is one species which stands out from all others: the genus Homo, specifically the species Homo sapiens, the only surviving species of this genus. We are characterised by our large brains in proportion to our body mass and the prominent cortex (outer layer of the brain), especially at the front of the head.