Thats easy...BOOBS, TITS. Human female are the only animal thats has breasts all the time. Almost all other animals only get Boob when they are expecting. This was the cause of humans evolving so quick. Males would spot likely mates by looking at their butts. By having a butt 'look-a-like'( her breasts ) help to talk to mates.This in-turn lead to 'talking'. Women where then able to put man to work, tell them what to do etc..
This is why men still have trouble looking at a women faces still to this day as it is in their natural instincts.