A new stem-human species, Australopithecus sediba Berger et al. 2010, has just been announced. The paper's supplementary information contains the results of a cladistic analysis of stem-humans. For fun, I thought I'd plug the most parsimonious tree into the in-development version of Names on Nodes:
Stw 53 and SK 847 are specimens that are not readily assignable to named species. (SK 847 might be Homo ergaster). Our own species, sapiens, is presumably descended from the SK 847-erectus node.
The analysis finds sediba as a sister taxon to Homo (which includes habilis, rudolfensis, SK 847, and erectus), and possibly ancestral to it. Which begs the question, why not place it in Homo? If this hypothesis is correct, it shares more ancestry with the type species of Homo (sapiens) than it does with the type species of Australopithecus (africanus). Even Stw 53, which is here placed outside the sediba-Homo clade, has been attributed to Homo in the past.