About 1.9 million years ago, our ancestors abruptly became a lot more like us. For the first time they paired in long-lasting couples, their jaws and teeth shrank, and females grew larger, approaching the size of males. Why?
Harvard's Richard Wrangham and the University of Minnesota's Greg Laden blame cooked vegetables. Early hominids were surrounded by roots that were difficult to chew or digest raw. When they learned to cook these roots, hominids could have vastly increased their available dietary calories. Cooking would have also created a delay between when food was gathered and when it was eaten, making theft more likely. "Cooperative alliances and a rudimentary form of marriage probably emerged to prevent males from stealing food from females," Wrangham says. "There's a very good case here for the origins of the human family."