Aly:

That's actually an excellent question!

The reason is this: most carnivores have physical adaptations for catching and eating prey. Lions (and other big cats) are big, strong, and have teeth and claws for seizing and killing prey. Spotted hyenas have amazing endurance and the strongest jaws of any large animal. Snakes have venom and/or the ability to coil and crush their prey, and the ability to hide in ambush just about anywhere. Sharks, wolves, crocodiles...the list goes on and on, and it's easy to name the physical adaptations natural selection has produced in each.

In contrast, our australopithecine ancestors had no such physical adaptations! They were smaller, weaker, slower, and inferior in every physical way to the existing predatory species, with two key exceptions:

1. Bipedalism
2. Opposable thumbs

This allowed us to make and use tools, a process which began at least 2.6 million years ago (click for article), and which obviously placed a premium on both intelligence and the capacity to learn. We didn't need big teeth to tear meat from bones: we needed the intelligence to make and use sharp rocks. We didn't need to be large or powerful in order to tackle and bring down prey: we needed to throw rocks accurately, and later spears. And so on.

JS