Hominins are members of the family Hominidae, subfamily Homininae, and tribe Hominini. The subfamily Homininae is made up of the African forms, of which the living representatives are chimpanzees, gorillas, and humans. Some researchers continue to use the term hominid to represent only the hominins; however, this practice is not supported by the genetic, molecular, or fossil evidence.
The earliest potential fossil hominins come from north-central and eastern Africa and date to approximately 6–7 and 5–6 million years ago, respectively. Among these early forms are at least three species: Ardipithecus ramidus kadabba, Orrorin tugenensis, and Sahelanthropus tchadensis. It is not clear that these species were bipedal, but they show some skeletal indications of bipedal anatomy.
Two subspecies of Ardipithecus ramidus may have lived across a span of nearly 1 million years in east Africa. They combine hominin-like lower anatomy with apelike cranial and dental anatomy.
The species A. anamensis is the first member of the genus Australopithecus and is well known from fossils dating to between 4.2 and 3.9 million years ago. This hominin displays adaptations to bipedality but shares some upper-body arboreal adaptations and dental features with earlier possible hominin forms.
Fossils of A. afarensis date from approximately 3.9 to 3.0 million years ago. This hominin was bipedal but may have used arboreal environments as well. It shares many anatomical similarities with A. anamensis but has smaller canine teeth and a reduced canine/premolar shearing complex relative to previous hominins.
Kenyanthropus platyops and Australopithecus bahrelghazali are two hominin species that lived at the same time as A. afarensis. Kenyanthropus may have lived in the same area as well. We have little information about either of these fossil species.
Although researchers have a good understanding of when and where the fossil hominins lived, there is no full consensus on the evolutionary relationships among these forms.
A. afarensis had a varied but primarily vegetarian diet, probably used wood and other minimally modified tools, displayed significant variation in body size, may have displayed significant sexual dimorphism, and probably lived in social groups consisting of multiple adults and young.
Bipedality is a core adaptation in the hominins. Exactly when, how, and why bipedality arose is the subject of many diverse hypotheses.
