Definition
Early hominins is an informal grouping of the oldest taxa proposed as hominins — Sahelanthropus tchadensis (~7 mya, Chad), Orrorin tugenensis (~6 mya, Kenya), and the two species of Ardipithecus (~5.8–4.4 mya, Ethiopia). Together they span the interval between the inferred chimp-human last common ancestor and the appearance of Australopithecus anamensis around 4.2 mya.
These taxa share an ambiguous, mosaic anatomy: enough hominin-like features to support membership in the clade, but enough primitive ape-like features to make the classification fragile. Each rests on a small, often fragmentary fossil sample, and each has been the subject of vigorous classification debate.
Why it matters
How they fit
Each early-hominin taxon occupies a different patch of Africa and a slightly different time slice. Sahelanthropus is the oldest and westernmost; Orrorin is intermediate in age and known mainly from femora that suggest bipedalism; Ardipithecus is the youngest and the most completely known, especially through Ardi's near-complete skeleton.
By 4.2 mya, the early-hominin period transitions into the Australopithecus radiation, which provides much richer samples and clearer anatomy. The early hominins set the stage; the australopiths consolidate bipedalism and diversify into the lineages that lead to Homo.