Whether the australopithecines were our ancestors or not, the proper ancestors of men must have been able to stand erect and to walk on their two feet. Three further important things probably were involved, next, before they could become men proper. These are:
1. The increasing size and development of the brain.
2. The increasing usefulness (specialization) of the thumb and hand.
3. The use of tools.
Nobody knows which of these three is most important, or which came first. Most probably the growth of all three things was very much blended together. If you think about each of the things, you will see what I mean. Unless your hand is more flexible than a paw, and your thumb will work against (or oppose) your fingers, you can’t hold a tool very well. But you wouldn’t get the idea of using a tool unless you had enough brain to help you see cause and effect. And it is rather hard to see how your hand and brain would develop unless they had something to practice on--like using tools. In Professor Krogman’s words, “the hand must become the obedient servant of the eye and the brain.” It is the coordination of these things that counts.
Many other things must have been happening to the bodies of the creatures who were the ancestors of men. Our ancestors had to develop organs of speech. More than that, they had to get the idea of letting certain sounds made with these speech organs have certain meanings.
All this must have gone very slowly. Probably everything was developing little by little, all together. Men became men very slowly.