I am now going to describe the history of the human species on Earth. All mammals were born onto the Earth about 60 mya (million years ago). The salient features of these early mammals were four-limbs, good eyes, and a sense of smell that detects both natural smells and inter-species communication smells.
The earliest main branch, a mere 6 million years later, has an important lesson for later human development, the hippos. The hippos are aquatic mammals. Aquatic mammals can live on the border between land and water, and can avoid predators in either realm (land or water) by moving to the other. Since their 1. aren't "amphibious" predators (hippos used to be smaller, but I am having trouble picturing a frog that big) and 2. no co-ordination between land and sea predators (Lions and Sharks Unite!), this was quite successful.
The group of animals that eventually gave rise to humans are now called proto-simians, and are represented by the lemurs, tarsiers, and a few other rare cute, fuzzy animals. The main feature that this group had over the run-of-the-savannah mammals was the whole climbing thing. Up and down, in the branches, skipping away from lots of enemies if there is a nearby tree and the chaser isn't agile enough to leap from one tree to the next (since most predator cats can climb trees pretty well). This worked pretty well, but there were CONSEQUENCES (and repercussions).
One, and I think this one was the beginning of the end for any chance that any other species was going to take over the planet, we started thinking about things NOT ONLY in terms of the grassy plains, and patches of grassland, bodies of water, and forests around, BUT ALSO in terms of up and down (3D!). Birds, of course, do this, but why would a lion? Of course, we still were pretty much small fry in the whole scheme of things, although some of them looked a little fearsome.
Two, and this gets relevant more and more later, is that branches break off in our hands. What happens when this happens? Well, it might have happened a million or a trillion times before anyone did anything with it, but a broken branch has many potential uses. It is, foremost, captivating. "How come that monkey is making that rustling noise with that leafy, funny appendage, and how come the monkey all of a sudden seems so much bigger?" Spearing must have come much later, but, simply throwing sticks might prove to be a small advantage sometimes, if any early monkeys figured out to do it.
Next: We Get Big!