highwaystar101 on 18 June 2010
Fitness and adaptability to an environment.
Using our hands was more advantageous in the environment that our early ancestors lived in, therefore they were more likely to survive and pass on their trait. Eventually we got better and better adapted to using our hands in this way as our need to use them increased using crude tools, etc...), until the point that we didn't need to use our front limbs for walking any more, they were much more important for other manipulating objects and carrying out manual tasks... A few thousand generations later you have us upright walking homo sapiens. Simple.
Well, I may have missed out a few details, but that's a pretty broad overview of what happened.
