My view is that the evolution of humans was preceded and driven by the development of technology. Chimps have a basic technology. Individuals in the wild, without human influence have been observed chewing sticks to make points, and then using them as short spears to hunt monkeys. Others have been seen using short sticks for extracting termits, and yet others have been seen pounding nuts between two rocks to open them.

 

If chimpanzees have basic technology, then we can assume our remote ancestors did also. Once technology, the use of tools and weapons, began, then the growth of the human brain makes sense. Better brains mean better technology and more imaginative use of technology.

 

I once saw a "Walking with Cavemen" TV program. It tried to suggest that our ancestors, the one metre high Australopithecus walked the savannah of Africa, and somehow fended off predators due to their social behaviour. Yeah, right. To a Smilodon (contemporary sabre tooth cat) a bunch of sociable Australopithecines would be so much smorgasbord on the hoof. My view is that they went loaded for Smilodon. Each young male carrying a long stick with a point on the end, and the knowledge of how to use it!

 

If the use of tools began very early in our evolution, it explains the evolution of upright stance, and long, strong hind legs. If carrying a spear, then walking upright with two hands available is a major evolutionary advantage.

I agree. It seems like tool use would have had to preceed higher intelligence. Which in turn would be preceeded by opposable thumbs, the ability to stand upright, and enough intelligence to use basic tools in the first place. Lots of factors had to come together before intelligence was possible and worth it. I remember reading somewhere the idea that climate change led to diet change which led to a different, less powerful jaw, which meant the jaw muscles didn't need as much anchorage on the top of the skull, which meant the skull was free to get larger and have more room for brains. So what might at the time have seemed like "de-evolution" from disuse into a less straightforwardly formidable animal actually was one of the necessary conditions for the development of our super-ape brains.