Saturday 4 June 2011
Source: Science Magazine.
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.
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 mm 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.
After Copeland, this is the first direct evidence that exists for dispersal patterns among early hominins. The findings, reported online 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.