By Ewen Callaway of Nature magazine

Fossilized teeth of early human ancestors bear signs that females left their families when they came of age, whereas males stayed close to home.

A chemical analysis of australopithecine fossils ranging between roughly 1.8 million and 2.2 million years old from two South African caves finds that teeth thought to belong to females are more likely to have incorporated minerals from a distant region during formation than those from males.

"What that's telling us is that the females grew up somewhere else and they died in the caves," says Julia Lee-Thorp, an archaeological scientist at the University of Oxford, UK, and a co-author on the study, published today in Nature. "It's a very small clue, but it's something that is at least hard evidence for what we really didn't have before."

The shape of ancient human families has been the subject of speculation, based mainly on differences in the relative size of male and female fossils, and the behavioral patterns of our primate relatives. Female chimpanzees, for instance, typically leave their social group once they hit maturity. Among gorilla groups, which are dominated by one large male 'silverback', both males and females tend to strike out.

Modern humans, who are influenced by relatively recent cultural practices such as marriage and property ownership, are difficult to compare to our early ancestors, lead author Sandi Copeland of the University of Colorado at Boulder said in a press briefing.

Forensic dentistry

Lee-Thorp and her colleagues measured the levels of two isotopes of strontium, an element found in soil. This is taken up by plants and then snakes its way up the food chain into the growing bones of animals. The ratio of two strontium isotopes in bones or teeth provides a signature of the local environment in which an animal grew up, Lee-Thorp says. "It's a kind of forensic tool."

Her team measured the strontium isotope ratios in canine and third molar teeth--which are formed by about the age of eight--in eleven Paranthropus robustus individuals from the Swartkrans cave, as well as in teeth from eight Australopithecus africanus individuals from the nearby Sterkfontein cave, about 50 kilometers northwest of Johannesburg. The researchers also measured the strontium in 170 plants and animals currently living near the caves to get a sense of the different strontium signatures of the region, including the thin Malmani dolomite formation that includes both caves.

They discovered that larger teeth--ostensibly from bigger-bodied males--of both species were much more likely to share the strontium signature of dolomite-dwellers than the smaller teeth of female australopiths. About 90% of the larger teeth looked local, compared with less than half of the smaller teeth. The best explanation for this pattern is one in which females left their clan once they reached maturity, say Lee-Thorp and her colleagues.

Alternatively, males could also have left their home groups, but stuck to the band of dolomite that extends to the northeast and southwest of the two caves, she says.

Gorillas in the mists of time

"I must say that the hypothesis is interesting," says Jacopo Moggi-Cecchi, a palaeoanthropologist at the University of Florence in Italy, "but I think the numbers and the samples that they have are not large enough to prove it." Moggi-Cecchi's team reported in 2007 that P. robustus males develop over a longer period than females, a pattern echoing that seen in gorillas, in which large males fight for near-exclusive access to females.

However, Owen Lovejoy, an anatomist at Kent State University in Ohio, says that a gorilla-like society, in which both males and females join new social groups, wouldn't have worked for early human ancestors. New males are likely to turn aggressive once they join a new group, risking the lives of infants and juveniles, he says. A society in which females left their homes is much more likely for early human ancestors, he says.

Lee-Thorp says that her team would like to get its hands on more australopithecine fossils from South Africa, but the destructive nature of the strontium tests and the paucity of fossils make that unlikely. Her team is also interested in asking similar questions about human ancestors from East Africa, as well as later species of hominin, such as Homo erectus.

This article is reproduced with permission from the magazine Nature. The article was first published on June 1, 2011.