The human body evolved over eons, slowly calibrating to the African savanna on which 98 percent of human ancestry lived and died. So, too, did human brains. According to evolutionary psychologists, the mind is shaped by the pressure to survive and reproduce; emotions, communication skills, and language ability are adaptations that enabled our human ancestors to thrive.
Many of the behaviors people exhibit have been tools for self-preservation: Homo sapiens jealously guard their romantic partners because competition for mates has always been harsh. Everyone cherishes their closest kin because it's in one's best interest to preserve genes. Humans also crave social interaction to encourage cooperation, further increasing the chances for survival. Many of these behaviors are innate—how people react and interact with one another is spelled out in DNA.
Juggling our ancestral tendencies with modern-day living can be a struggle. A good example of this is the contemporary diet. Ten thousand years ago, people battled starvation, and high-fat meats, as well as high-sugar foods, were a luxury. Of course, our ancestors had to pile on the necessary calories just to survive lean times. Today, however, fatty foods and processed sugars are readily available at low cost.
This is called evolutionary mismatch—where we find ourselves in an environment inconsistent with our ancestral conditioning. Our pre-determined response to gravitate to that 800-calorie Cinnabon can wreak havoc, but our ancestors made us do it.