Ancient Female Ancestors Roamed Far and Wide for Mates

by Ann Gibbons 

Women’s movement. Teeth (inset) of australopithecines, such as this skull ofParanthropus robustus, show females moved farther away from their birthplace than males.

Credit: Darryl de Ruiter; (tooth, inset) Sandi Copeland

When it came time for members of the human family to find a mate in South Africa about 2 million years ago, it was the females, not the males, who made the first move. A new study of the teeth of 19 australopithecines from cave sites in South Africa suggests that females moved away from their birthplaces far more often than the larger males, who stayed surprisingly close to home and kin.

For several decades, researchers have debated whether early human ancestors lived in close-knit social groups made up of related brothers and fathers, with new genes introduced by female mates gathered from other groups. Chimpanzees follow this pattern too, but in most primates, new males move into groups of related females; in gorillas, for example, one male lives in a harem with many related females. It has been a “monumental task” to test models of the social organization of early members of the human family, known as hominins, says Matt Sponheimer, a co-author of the new study and a paleoanthropologist at the University of Colorado, Boulder. “It’s as if someone told you to investigate the ecology of a giraffe but you were forbidden to observe them in the wild and confined to a room that contained a relatively small number of giraffe bones.”

In recent years, though, scientists have refined a nondestructive method that can reveal what ancient hominins ate and what type of ecological zones they inhabited. Researchers use lasers to ablate tiny sections of tooth enamel, less than 1 millimeter long, which releases two isotopes (or two forms) of the element strontium, so-called ⁸⁷SR and ⁸⁶SR. The ratio of those two isotopes reflects the region where individuals lived as children—when their teeth were forming—because the strontium from soil is incorporated into the plants and animals that they ate.

Lead author Sandi Copeland, a paleoanthropologist at the University of Colorado, Boulder, along with Sponheimer and international colleagues, analyzed tooth enamel in 19 individuals from two species,Australopithecus africanus and Paranthropus robustus, two members of the human family that were closely related to our direct australopithecine ancestor. The teeth came from two famous South African caves, Sterkfontein and Swartkrans. When they measured the ratio in the larger molars and canines, presumably from males, they found that before age 8, males fed primarily on dolomite soils around the caves. But at least half of the individuals with smaller teeth, presumably females, fed elsewhere, away from the local dolomite soils, when they were young. The pattern held for both species.

“This is the first direct evidence that exists for dispersal patterns among early hominins,” Copeland says. The findings, reported online today in Nature, suggest that such patrilocal organization of social groups is ancient in human ancestors, perhaps dating back to the common ancestor we shared with chimpanzees, as some researchers have proposed.

Copeland isn’t sure why males would move less than females in a region where there were no natural barriers. She plans to see if the pattern holds in australopithecines in other parts of Africa to see if this was the usual way australopithecines organized their clans. Regardless, “Copeland and her colleagues have come up with an innovative way to test this model, and in the process, they have developed the very first direct evidence of early hominin social organization,” says paleoanthropologist Peter Ungar of the University of Arkansas, Fayetteville. The new method should also reveal other ancient behaviors. “These results have implications for understanding australopithecine diet, group size, predator avoidance, and home-range size,” paleoanthropologist Margaret Schoeninger writes in a commentary published online today in Nature.