i see where you are going wrong. you are looking at it as if man was already fully evolved except without one vital part of him in tact. obviously this would mean extinction for his kind if this were actually the case, but its not. before hands what did man do? to put it simply, man didn't exist. the earlier ancestors of primates were quadrupedal mammals who didn't need hands to survive. the advent of the bipedal primate freed up the hands and allowed for more distinct digits with more complex uses, namely the creation and use of tools.