Harry Turtledove 
Different Flesh 

What if when Columbus came to the New World he found, not Indians, but primitive apelike men who were soon dubbed "sims"?These immediate ancestors of modern man were less effective hunters, allowing prehistoric creatures such as mammoths and saber-toothed tigers to survive. Unable to learn human speech or conceptualize at a human level, sims could, however, be trained to do reliable work… as slaves.