Anthropologists can extrapolate many things, for example where the hole in the base of the skull is can tell us if an apelike creature walked upright or on all fours, the flexibility of ankles indicate climbing ability, the sizes of vertebrae shows where weight is borne etc. etc. one bone can tell you many things (even a single tooth gives an indication on what was fed on).
>>have they not heard of genetic mutation? there are plenty of humans that dont fit into the neat, tidy box we have set out. i guess they would imagine hundreds of different species because some are slightly different.
This is very true, however, you can either assume that the fossil is typical and probably is a "normal" example, or assume everything is a one off unless you have two or more examples, given fossils are exceptionally rare you wouldn't get very far, so, in reality there probably are some one offs that have wrongly been called "a new species" - but these should be a factor rarer again, i.e. the weight of probability is that most of the fossils we find are not genetic mutants.