Ancient Female Ancestors Roamed Far and Wide for Mates

From Science
by Ann Gibbons

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.

Reference
Copeland SR, Sponheimer M, de Ruiter DJ, Lee-Thorp JA, Codron D, le Roux PJ, Grimes V, Richards MP (2011) Strontium isotope evidence for landscape use by early hominins. Nature 474: 76–78 doi:10.1038/nature10149

Abstract
Ranging and residence patterns among early hominins have been indirectly inferred from morphol, stone-tool sourcing, referential models and phylogenetic models. However, the highly uncertain nature of such reconstructions limits our understanding of early hominin ecology, biology, social structure and evolution. We investigated landscape use in Australopithecus africanus and Paranthropus robustus from the Sterkfontein and Swartkrans cave sites in South Africa using strontium isotope analysis, a method that can help to identify the geological substrate on which an animal lived during tooth mineralization. Here we show that a higher proportion of small hominins than large hominins had non-local strontium isotope compositions. Given the relatively high levels of sexual dimorphism in early hominins, the smaller teeth are likely to represent female individuals, thus indicating that females were more likely than males to disperse from their natal groups. This is similar to the dispersal pattern found in chimpanzees9
, bonobos and many human groups, but dissimilar from that of most gorillas and other primates12. The small proportion of demonstrably non-local large hominin individuals could indicate that male australopiths had relatively small home ranges, or that they preferred dolomitic landscapes.