Humans are hominids, members of the taxonomic family termed Hominidae that includes all of our extinct human-like relatives. But because the fossil record is extremely fragmentary, it is difficult to determine how hominid species are related. Many species that once existed are completely unknown to us, and the remains of discovered species are incomplete and represent only a few individuals. For instance, Australopithecine finds are relatively numerous, but known Australopithecine fossils represent only between .002 to .00002 percent of the estimated living population. Moreover, even the best represented of the Australopithecine species is known from only five or six well preserved crania (the braincase, including the upper facial bones) of which all but one lack anatomically important parts of the skull.
Because the incomplete fossil record supports no one obvious explanation of human origins, different anthropologists have suggested competing theories based on principles of evolutionary change. In oversimplified terms, these principles state that morphological (structural) similarities between species generally imply a common ancestor. For example, most vertebrates have limbs that end in a fivefold spread of digits, indicating that there was a common ancestor before mammals, reptiles, amphibians, birds, and fish evolved into separate groups. However, note that species can also evolve individual similarities based on adaptation to environment. Ducks and frogs have both developed webbed feet in response to wet environments, yet by examining their other characteristics we can see that they are not particularly closely related.
On the left are five different possible hominid phylogenies (species genealogies) arranged chronological order according to when they were proposed. Notice that although the specifics are different, the phylogenies all follow a general pattern with gracile Australopithecines near the base of a genealogy that splits into two separate groups: robust Australopithecines and Homo.
Descriptions of individual species begin on the following page.