The term hominin refers to modern humans plus all extinct species more closely related to us than to chimpanzees, our closest living relatives. This definition frames human evolution as a branching bush rather than a single ladder, because many hominin species coexisted without being our direct ancestors.
The fossil record shows that upright walking appeared in early hominins millions of years before any substantial increase in brain size. This ordering refutes the once-dominant assumption that a large brain drove the other distinctive human traits.
Reconstructing human origins combines morphological evidence from fossils with molecular evidence from DNA, each constraining and dating the other. Genetic data can estimate when lineages split while fossils reveal what the organisms looked like, so neither source alone settles the history.
Most hominin species are known from scraps: a jaw, a few teeth, a partial limb, so every claim about behaviour or relationship rests on inference from incomplete material. Treating the record as fragmentary forces one to state which traits are observed and which are reconstructed, keeping confidence proportional to evidence.
Similar features can evolve independently in unrelated lineages (homoplasy) rather than being inherited from a shared ancestor. Because look-alike traits may mislead, grouping species by resemblance alone can produce false evolutionary trees.
Deciding whether two fossils belong to one variable species or two distinct species is a judgment call, since the biological test of interbreeding cannot be applied to the dead. This ambiguity means the number of recognized hominin species reflects the interpreter's splitting or lumping tendencies as much as the bones themselves.