The most basic of all operations performed by living creatures are controlled by the brain - animals as simple as ants and earthworms need their brains in order to move their bodies and to tell them when to eat, rest and mate. Very early man also had a very simple brain that allowed him to do the basics - pick plants, to eat and lie down when they were tired. The making of tools and fire, however, requires far more sophisticated thinking - in the case of tools, the brain needs to see the rock and imagine what could be done with it, then co-ordinate the hands to cut and shape the rock, and then know what to do with the finished product. The whole process is, in actual fact, extremely complicated, and is not one that could be carried out by the mind of an ape.
Early Homo erectus fossils have a brain capacity of half of modern day man (1300 - 1350cc), but fossils of this species that appear to have lived later have a brain capacity of two thirds of modern day man. This evolutionary change in brain size seems to have impacted on H erectus' ability to function - his tools seem to become better carved, and he was able to harness and create his own fire. There is in fact some speculation that H erectus was the first talking being, as studies of skulls have shown that the area inside the brain of this species responsible for speech was well developed.
As we dig man out of our soils, it becomes apparent that his brain got bigger and better over time. While the brain size of Australopithecus africanus was only 375 - 485cc, the brain size of Homo habilis, the next hominid on the time line, was 600 - 800cc. There are major differences in the technical advances between the two - A africanus could only manage to use sticks as slight digging tools, while H habilis managed to carve stone into something useful.