Monogamy may be cultural, but it is also rooted in our primate physiology. The key question is: When did it start? University of Arkansas anthropologist Mike Plavcan recently reexamined fossils of one of our earliest bipedal ancestors, the 4 million-year-old Australopithecus afarensis, and found hominids may not have been as marriage minded as previously thought.
He drew his conclusions from the size of bones. In species that favor multiple partners, like gorillas or orangutans, males can be more than 50 percent larger than females, apparently to help them fight off other males. But pair-bonding primates, like humans or titi monkeys, tend to show less of a difference. Previous studies on A. afarensis, including the famed Lucy, the most complete example of the species ever found, concluded that the male-to-female size ratio was about 15 percent—on a par with that of humans.
These studies were flawed, Plavcan contends, because they were based mainly on estimates of a single body part, the knee joint or femoral head, and were from a single geographic region. “And that’s dicey in terms of statistical analysis,” Plavcan says. When he compiled data that looked at multiple body parts from a wider selection of A. afarensis fossils, he concluded Lucy’s lovers were 50 percent larger than she was. “That size difference suggests a few big males were monopolizing the mating,” he says.
Still, Plavcan wavers over how much size mattered. “If you look historically at human cultures, and even paternity studies from birth records and DNA, humans start to look a lot less monogamous than we like to think,” he points out. “If we’re not even certain about the relationship between body size and monogamy today, the mating preferences from 3 or 4 million years ago are going to stay a mystery.”