The link below is to a radio programme on human evolution. Some important points are made about the role of the fossil record as the major source of evidence for human evolution. Lot's of information can be extracted from fossils: we can estimate the size of brains from skulls; what hominids ate from the size of teeth, how our ancestors moved from limb bones, and some mitochondrial DNA has been extracted from Neanderthals. These fossils can also be accurately dated. Steve Jones also makes the point that we are all living fossils and our DNA is an important source of evidence.
However, there are limitations to what bones and fossils can tell us about our evolution: not many have been found; fossil DNA is hard to extract and is often corrupted with modern human DNA. Steve Jones also warns against over interpreting the fossil record. We can't tell anything about what our ancestors were thinking, feeling, or doing from a skull fragment.
The final part of the programme focuses on modern humans, Homo Sapiens, and the contributors discuss possible reasons for the development of our big brain. The consensus seems to be that size may not really matter for modern human intelligence, the development of language and the emergence of culture.