Expert: John Shea - 12/8/2006

Question
How can we justify scientifically that the Homo erectus, Homo heidelberensis and Neanderthal and modern humans are seperate species?
How/wher can I find the answer to this question? Thank you.

Answer
Barbara
The main argument for treating these hominins as separate species is that each differs from the others in terms of skeletal morphology.  
Some of these morphologies are thought to arise from genetically-programmed differences in growth.  Thus, morphological differences are treated as proxies for genetic differences.
Most of these fossil taxa also have non-overlapping distributions in time and space.  This further suggests they were reproductively isolated from each other.
This combination of morpho-genetic difference and inferred reproductive isolation are basic criteria for identifying a species.
Now, you are correct to be skeptical.  We do not know the specific genetic underpinnings of many aspects of skeletal growth.  Nor are we ever likely to know the precise geographic and chronological range of all hominin species.
Treating these hominin morphological taxa as biological species is, in essence, accepting an hypothesis that cannot be proven conclusively wrong.  
This being said, however, there are (for me at least) compellling reasons to accept such hypotheses.
1. It is vastly more common for there to be significant reproductive barriers among mammal species with similar skeletal morphology than it is for there to be no such barriers among species that do differ significantly in their skeletal morphology.
2. Where we do have fossil DNA (from Neandertals) the consistent pattern of there being wide genetic differences between them and ourselves tends to confirm the species-level difference inferred from their morphological differences.
A third consideration is that equating morphological taxa with reproductive species, while an hypothesis, is an hypothesis that can be falsified by the discovery of new evidence.  The alternative, not equating morphological taxa with biological species simply stops the whole enterprise of human origins research, creating a classic unfalsifiable negative hypothesis (i.e., that we cannot idenfy species in the fossil record -kind of like asserting you cannot fly but then refusing to board an airplane).
For good overviews of the morphological contrasts among these taxa, see.
Ian Tattersall,  Extinct Humans.
Richard Klein,The Human Career.
John Fleagle, Primate Adaptation and Evolution
The Smithsonian Institutions' Human Origins website also has some useful information, as does the website of the American Museum of Natural History.
Sincerely
John Shea