CHICAGO Females from two hominid species that roamed the South African savannah more than a million years ago left their families and struck out on their own, while their male counterparts tended the home fires, an international team of researchers said on Wednesday.
The findings, published in the journal Nature, came from an analysis of traces of the isotope strontium that had built up in adult teeth from two extinct groups -- Australopithecus africanus and Paranthropus robustus.
Both Paranthropus robustus and Australopithecus africanus were part of a line of close human relatives known as australopithecines that included the Ethiopian fossil, Lucy, estimated to be some 3.2 million years old and regarded by many as the matriarch of modern humans.
To understand how these hominids used the landscape and formed social groups, the team, led by Sandi Copeland of Colorado University Boulder, turned to the ancient dental record.
Using a high-tech analysis known as laser ablation, the team zapped the hominid teeth with lasers to measure ratios of strontium, a naturally occurring element found in rocks and soils that is absorbed by plants and animals.
The team tested 19 teeth dating from roughly 2.7 to 1.7 million years ago from both Australopithecus africanus and Paranthropus robustus individuals from the the Sterkfontein and Swartkans cave systems in South Africa.
The team found more than half of the smaller, female teeth came from outside the local area, but only about 10 percent of the male hominid teeth were from elsewhere.
That suggests that males likely grew up and died in the same area, Copeland said.
Copeland said the pattern in which the females left home in search of a mate is similar to chimpanzees and some groups of modern humans, in which a woman leaves her family to join her husband's household.
But it is different from most other primates -- including gorillas -- where the females stay with the group they are born into and the males move elsewhere.
"In any mammal group, including primates, where individuals live in groups, either the females or males must eventually leave their birth community. One of the important reasons for this is to prevent inbreeding," Copeland said.
"In most mammals, it is usually the males that leave their home community," she told a news briefing.
She said the findings were a bit of a surprise.
"We assumed more of the hominids would be from non-local areas since it is generally thought the evolution of bipedalism was due in part to allow individuals to range longer distances," Copeland said.
Such small home ranges suggest that bipedalism evolved for other reasons, she said.
(Editing by Cynthia Osterman)