For most creatures, procreation is an emotionally uncomplicated affair. In humans, however, it has a tricky accomplice: romantic love, capable of catapulting us to bliss or consigning us to utmost despair. Yet capricious though it may seem, love is likely to be an adaptive trait, one that arose early in the evolution of our lineage.
Two of the hallmarks of human evolution—upright walking and large brains—may have favored the emergence of love, according to a theory advanced by anthropologist Helen Fisher of Rutgers University. Bipedalism meant that mothers had to carry their babies, rather than letting them ride on their back. Their hands thus occupied, these moms needed a partner to help provision and protect them and their newborns. Ancient bipedal hominids such as Australopithecus afarensis, the species to which the 3.2-million-year-old Lucy fossil belongs, probably formed only short-term pair bonds of a few years, however—just long enough for the babies to be weaned and walking, after which females were ready to mate anew.