(EDITOR’S NOTE: The following article is an adaptation of a news release prepared by the Max Planck Institute for Evolutionary Anthropology of Leipzig, Germany and based on a scientific paper published today by the journal Nature. Sandi R. Copeland of the Max Planck Institute and the University of Colorado Department of Anthropology is the lead author and Darryl J. de Ruiter of the Texas A&M University Department of Anthropology is one of the co-authors. He can be contacted at firstname.lastname@example.org.).
COLLEGE STATION June 1, 2011 — Strontium isotope analysis shows that, unlike their male counterparts, females of the extinct hominin species Australopithecus africanus and Paranthropus robustus moved away from the groups that they were born into to join new social groups when they reached reproductive maturity, according to an international team of researchers whose scientific paper was published today by the journal Nature. Such a pattern of female migration is observed in chimpanzees and many human groups, showing that this reproductive strategy was well established in hominins over 2.5 million years ago, the researchers note.
Hominins include modern humans, the closely related African apes, and the fossil ancestors of both groups, they explain.
Until now, the ranging and residence patterns of early hominins have been indirectly inferred from morphology, stone tool sourcing, comparison to living primates, and phylogenetic models. However, the international team of researchers, including Dr. Darryl de Ruiter of the Department of Anthropology at Texas A&M University, announced their discovery that Australopithecus africanus (with fossils from sites dating between 2.8-2.0 million years ago) and Paranthropus robustus (with fossils from sites dating between 1.9-1.4 million years ago) exhibited a social pattern referred to as male philopatry, wherein closely related males remain together throughout life, while females move away from their close relatives to join neighbouring groups once they become capable of producing their own offspring.
The paper’s lead author is Sandi R. Copeland of the Max Planck Institute for Evolutionary Anthropology of Leipzig, Germany, and the University of Colorado at Boulder.
The team examined 19 teeth from the South African sites of Sterkfontein (Australopithecus africanus) and Swartkrans (Paranthropus robustus) using strontium isotope analysis. This method helps identify the geological substrate on which an animal lived while its teeth were growing. The researchers show that a high proportion of the smallest teeth, probably from females, revealed strontium isotopes indicating that they grew up away from the caves that they were ultimately entombed in. At the same time, a high proportion of the largest teeth, probably from males, show strontium isotopes that demonstrate they were born, raised, and died, in the vicinity of the caves from which their fossils were recovered. This is similar to the dispersal pattern found in chimpanzees, bonobos, and many human groups, but dissimilar to that of most gorillas and other primates.
“This is the first time that we have been able to directly document this type of social behavior in australopiths,” says de Ruiter, “providing us a rare opportunity to investigate the reproductive strategies of a human ancestor.”
Funding for the study was provided by the National Science Foundation, the Max Planck Society and the University of Colorado’s Dean’s Fund for Excellence.