You may have read the recent press coverage about the discovery of Homo erectus and habilis remains that suggest that these species coexisted for much longer than scientists had previously thought. The coverage was accompanied by headlines implying that the new finds overturned our current understanding of human evolution. But just how accurate is this?
Combined with other material, the new fossils – a habilis upper jaw from around 1.44mya, and an older erectus skull cap – suggest that the two species coexisted for up to half a million years. The team that made the discoveries have interpreted this to mean that erectus could not have descended from habilis, but that the two species share an earlier, undiscovered common ancestor (Ann Gibbons (2007) "New fossils challenge line of descent in human family tree" Science 317: 733). Not all scientists agree with this interpretation, with some commenting that having two species overlap in time does not mean that one couldn't evolve from the other.
So there seem to be two possible scenarios. Either Homo habilis is ancestral to erectus, but the two species diverged much earlier than previously thought. Or erectus did not evolve from habilis and instead the two share a common ancestor. But this second possibility does not overturn our current model of human evolution – it simply adds another branch to an already bushy family tree.