But clearly it a close relation, closer to us then the chimp/human common ancestor, more distant than Homo erectus. The ordered skulls were intended to show increasing relatedness through time, not illustrate a proven direct progression.
Darwin's discussion on this topic was actually quite good:
"In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition."--Charles Darwin, Origin of Species
There is simply no way of knowing if any fossil has a living descendant, short of extracting DNA from that fossil. What we can do is look at how morphology changed over time and use this to infer the changes that occurred in the lineage of a living species.
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