[jennyp]

I've always had problems telling H. erectus apart from H. sapiens archaic. Except for the brain size - 850cc for erectus vs. 1250cc for sapiens...

The point is, the transitions at this point look very gradual to me, as along a continuum.

[Physicist]

Those skulls look pretty different, IMHO. The brows are pretty different, the zygomatic arches are different, and H. erectus has a pronounced occipital bun.

Throw in a Neanderthal skull and you'll really show how tough it is to cram these fossils into a linear chain. I suspect that hundred years from now the bush of hominid evolution will appear much more overgrown than we'd ever believe now.