When anthropology meets evolutionary biology
There are large variations of size among humans but in all populations, men are larger on average than women. For most biologists this fact can be easily explained by the same processes that explain the size dimorphism in large mammals in general and in apes in particular. Due to fights between males for the possession of females, sexual selection has favoured bigger males.
Men and women may be considered equal under law, but there are, of course, differences. Among them is size.
Men, on average, are about 15 percent to 20 percent larger than women. Yet compared with other mammals, that margin is slim. New research suggests it was similarities among the males and females of our early ancestors — not differences — that helped early humans evolve to become the dominant species that we are.
Less difference in size suggests early humans were mostly monogamous, explains Owen Lovejoy, an anthropologist at Kent State University in Ohio and an author of a recently published study in Proceedings of the National Academy of Sciences.
And monogamy, he argues, was early humans’ key to success.
“What monogamy does is eliminates male to male competition for mates,” he said. “It allows for more cooperation and that allows you to take better care of your young.”
How do similar body sizes suggest monogamy? In the field of evolution, exaggerated size differences between the sexes mean that males required their huge physiques to compete with one another for mates. Less pronounced size differences, meanwhile, mean males were spending less time fighting and more time taking care of their mates and their young.
The concept of one-female kind of guys among early humans is a new one in the scientific community.
More than 30 years of research had instead painted a picture similar to gorilla and orangutan social systems, in which hefty males guard large harems of females. These earlier studies had compared fossils from a range of sites and concluded that males of Australopithecus afarensis — the group of early hominids that lived about 3 million to 3.6 million years ago — were much bulkier than their female counterparts.
But Clark Spencer Larsen, an anthropologist at Ohio State University in Columbus, says there are problems with these previous conclusions. Body size among any species can vary greatly over thousands of years, so comparing male and female specimens from different periods can be misleading. A similar problem arises when comparing specimens from very different geographical regions, where general sizes may vary by location.
Lovejoy’s work avoided these issues by testing a series of individuals from one site who all appeared to have died at the same time. He concluded the sexes were more similar than scientists have thought.
“It’s a convincing case,” Larsen said of the work.
Using ‘Lucy’ as a Measure
To test for body size among the early humans, Lovejoy and Kent State graduate student Philip Reno focused on specimens from a unique fossil find known as the “First Family Site” in Ethiopia. Researchers believe a group of individuals all perished here in a flash flood or other kind of catastrophic event 3.2 million years ago.
Reno and Lovejoy analyzed the bones and teeth of nine adults found during the 1970s and used the famous fossil known as “Lucy,” the most complete fossil among the group, as a reference to estimate body size. They compared all of Lucy’s bones to her thigh bone (a good measure of size), came up with a ratio and used that ratio to estimate the size of other individuals in the group. They tested the technique on chimp, gorilla and modern human skeletons and found it was an accurate measure of male and female body sizes.
Based on this analysis, the early humans in Lucy’s group showed even less difference in size between males and females than contemporary people.