In the wake of the discovery of the oldest human skeleton, the 4.4-million-year-old Ardipithecus ramidus, nicknamed â€œArdi,â€ comes this interesting, dinner-party-friendly theory of the circumstances that led humans to walk on two legs, while still living in trees.
According to Owen Lovejoy of Kent State University, it all comes down to food, and sex.
In apesâ€”both modern apes and, presumably, the ancient ancestors of Ardipithecusâ€”males find mates the good old-fashioned apish way: by fighting with other males for access to fertile females. Success, measured in number of offspring, goes to macho males with big sharp canine teeth who try to mate with as many ovulating females as possible. Sex is best done quicklyâ€”hence those penis bristles, which accelerate ejaculationâ€”with the advantage to the male with big testicles carrying a heavy load of sperm. Among females, the winners are those who flaunt their fertility with swollen genitals or some other prominent display of ovulation, so those big alpha dudes will take notice and give them a tumble, providing a baby with his big alpha genes.
Letâ€™s suppose that some lesser male, with poor little stubby canines, figures out that he can entice a fertile female into mating by bringing her some food. That sometimes happens among living chimpanzees, for instance when a female rewards a male for presenting her with a tasty gift of colobus monkey.
Among Ardipithecusâ€™s ancestors, such a strategy could catch on if searching for food required a lot of time and exposure to predators. Males would be far more successful food-providers if they had their hands free to carry home loads of fruits and tubersâ€”which would favor walking on two legs. Females would come to prefer good, steady providers with smaller canines over the big fierce-toothed ones who left as soon as they spot another fertile female. The results, says Lovejoy, are visible in Ardipithecus, which had small canines even in males and walked upright.
Lovejoyâ€™s explanation for the origin of bipedality thus comes down to the monogamous pair bond. Far from being a recent evolutionary innovation, as many people assume, he believes the behavior goes back all the way to near the beginning of our lineage some six million years ago.