The sex contract Book

14 million years ago, the climate changed. Africa was becoming cooler and drier. The trees were thinning, and the apes were forced to come down to the grasslands to find food. This was dangerous. They had to rely more on each other - strength in numbers. A bit of cooperation and coordination was needed for survival.

As the climate change progressed, the woodlands disappeared altogether and it was savanna. 10 million years ago, the apes had to adapt. They couldn't just eat wherever they found food - or they would be eaten themselves. They had to carry off the food to safety, which meant bipedalism was an advantage (hands are more efficient in carrying). As selection pressure pushed them toward walking on two feet, anatomical changes made child birth too dangerous.

Those females who tended to give birth prematurely had an advantage - they lived rather than die in child birth. And their offspring survived. But premature infants meant more work for the mother. She couldn't carry the baby on her back as in the four legged days. Carrying baby and food was tough. She needed help.

The females who could offer sex for longer periods in each cycle got more help. Selection drove the monthly period of heat longer and longer until it pretty much disappeared. She could now copulate almost all month, during pregnancy and quite soon after birthing. She got the males' favors in return - he shared more meat, and helped around.

One thing led to another, and soon they were becoming a bonded pair. This meant a whole lot of new brain functions - communication, planning, feelings like jealousy, kinship.

Let this film run few million years. Bingo! We have modern humans.

The female's ability to have sex pretty much all the time as pivotal to the evolution of homo sapiens, and this is what distinguishes us from all other creatures till today. And oh, we have technology, language, etc. too. But sex made it all happen.