Cajun, its a gradual process. And btw, humans have far, far less in common (genetically) with gorillas than with "monkeys".
I'll take the small monkey vs tall monkey a bit further.
Lessay we now have mr. tall monkey who was once a small monkey. He is a big, tough, very muscular monkey (you know, arboreal muscles and such).
Now mr. tall monkey has prospered and multiplied. Call this tall (in comparison with other monkey species) prosperous monkey the australopithecus (sp?) ancestor of humanity.
Mr tall monkey, after thousands and thousands of years, through trial and error and luck begins to use tools like a sharp stone or long sticks.
More thousands of years pass. Tall monkey becomes more adept at using his tools...something that other monkeys of the time apparently did not catch on to.. but something happens to this tall monkey!
The use of tools begins to change him. First it improves his longevity and survival chances. Bone and stone tools now let him skin animals they kill, as time passes the sharp stones allows the species to CUT meat into snack size, making the need for large canines and massive jaws no longer be a survival trait.
Along the way some nutcase begins to play with fire. A primitive "industrial revolution" begins. Fire allows better tools, better food, PROTECTION, they dont freeze in the winter, reduction of calories needed to survive per day (less food needed to fight off the cold), etc, etc.
More thousands of years, these tools, now used to make clothing and basic hunting tools make our ancestors shed their animal like body hair, the jaw becomes smaller, the body leaner and less muscular and taller (height=more field of vision) and begin to walk upright. Homo erectus, homo abilis, etc etc. Cranial size is the one and only thing that seems to INCREASE in this species while the rest of its body becomes more agile,dexterous and precise, and less strong.
And you get humans. Nothing more than some stupid monkey that through thousands and thousands of years did trial and error and changed itself by changing its enviroment and its way of life through tools which ended up in turn, changing the species.
And Humans werent the only monkey to do it apparently. Cromagnon humans seem to have "split" from the tall monkey tree at the start and evolved its own way... this monkey kept its brute strength and big jaws, but its head did not grow as much as the other tall monkey. The result? The 2 met, the smarter monkey beat the crap out of them, or simply were better equipped to survive in a changing world (ice ages).
Trial, error and luck. That about sums evolution & natural selection.
