A majority of researchers believe the roots of separate sex roles date far back in human evolution, probably to the time when the size ratio of male to female reached something close to our current differential.

In other animals, this ratio can reveal something about the relationship between the sexes. In species with a big difference, such as gorillas, the males often fight to control harems.

In species with males and females closer to the same size, the sexes are more likely to work in pairs, cooperate, and share the burden of protecting their young.

Modern men and women are similar in body size compared with our closest relatives. So determining how long ago we reached our current ratio should point to when our ancestors stopped organizing themselves like apes and started acting more like people, says Stanford University anthropologist Richard Klein.

Some researchers see evidence placing this date at least 3.2 million years ago, the age of the diminutive australopithicine known as Lucy. Others favor a mere 1.7 million or 1.8 million years.