Species are classified. A classification system is known as a phylogeny. Species are grouped according to similarities and ancestry. Similar traits, for example three-dimensional eyesight, can be used to group certain animals as primates. Primates are mammals that have front facing eyes allowing them to perceive depth. This trait can be used with others to classify animals, including man, as a primate. Similar traits such as this are known as homologies (similar forms). A whale is a mammal because they bear their young similar to all other mammals. The homology of a placenta birth can be used to classify whales as mammals and not as fish.
Similarities may also be traced using molecular biology and genetics. Chimpanzees and man are 98 percent the same genetically. They are as different from a gorilla as they are to us.