Exemplar: KNM-ER-1470 [Koobi Fora, Kenya] - 1.9 million y.a.As an example of the complexity in hominid fossil remains that appears in the earliest stages of the human line, HOMO RUDOLFENSIS is a species proposed in 1986 (on the basis of the skull shown here) as coexisting with habilis 1.8 to 1.9 million years ago.
Despite their assumed coexistence in time, many features separate rudolfensis both from the australopithids and from other species of Homo. The rudolfensis browridges are smaller and more integrated into the skull; the face is flatter, narrower and more vertically sloping; there are no heavy muscle attachments at the top of the skull; the back teeth are smaller and the front teeth are larger than in related hominid species. The skull itself is lighter and more delicate, noticeably more rounded in the back or occipital area. The estimated brain volume of 775cc is clearly larger than any attributed to habilis.
Though it is increasingly accepted that at least two species of Homo -- rudolfensis and habilis -- coexisted around 2 million years ago, there is still considerable uncertainty as to how to connect these fossils to other remains from the same geological era, how all of them are related to the australopithids -- and which of the Homo skulls shows us the true ancestor to subsequent humans.
To my eye, the rudolfensis skull is distinctive: obviously different from habilis, but almost equally distant from Homo ergaster which came later. Indeed, it seems in many respects easier to draw a direct connection from habilis to ergaster that bypasses rudolfensis altogether. In this scenario rudolfensis becomes a remarkable evolutionary offshoot without known descendants -- or perhaps is the earliest specimen of the "round skulls" that lead to Homo heidelbergensis, in contrast to the "football skulls" that characterize ergaster and its undisputed descendant, erectus.
All these complications make it clear we will see further revisions to the human story at its origins.