While Hominin Males Stayed Home, Females Roamed
New research gives clues to social organization of ancient hominins
The males of two bipedal hominin species that roamed the South African savannah more than a million years ago preferred to stay at home, but their female counterparts wandered the landscape, says a new study led by the University of Colorado Boulder.
Researchers studied teeth from two adjacent cave systems in South Africa that were home to two groups of extinct hominins, Australopithecus africanus and Paranthropus robustus. They found more than half of the female teeth were from outside the local area, whereas only about 10 percent of the male teeth were from elsewhere, suggesting the males likely grew up and died in the same area while the females roamed the countryside.
"One of our goals was to try to find something out about early hominin landscape use," said CU-Boulder adjunct professor and lead study author Sandi Copeland. "Here we have the first direct glimpse of the geographic movements of early hominins, and it appears the females preferentially moved away from their residential groups."
The science journal Nature published a paper on the study today in its June 2 issue.
"This is exciting research," said Physical Anthropology Program Officer Kaye Reed of the National Science Foundation, which in part funded the research. "The research gives us clues to social organization that has been unavailable. We knew that these species are sexually dimorphic, and now we know that the females left their natal groups, leaving the males in place as close kin."
Reed said the study suggests that the males co-operated in some way like primate species today, chimps for example. "You just can't know this from morphology alone."
"It is difficult enough to work out relations between the sexes today, so the challenges in investigating the ways that male and female hominins used the landscape and formed social groups over a million years ago are considerable to say the least," said CU-Boulder anthropology Professor Matt Sponheimer.
"Disembodied skulls and teeth are notoriously poor communicators, so the real difficulty with a study like this is finding new ways to make these old bones speak."
The team used a high-tech analysis known as laser ablation, zapping the hominin teeth with lasers to help them measure isotope ratios of strontium found in tooth enamel in order to identify specific areas of landscape use. A naturally occurring element, strontium is found in rocks and soils and is absorbed by plants and animals.
Since unique strontium signals are tied to specific geological substrates--like granite, basalt, quartzite, sandstone and others--they can be used to reveal landscape conditions where ancient hominins grew up.
"The strontium isotope ratios are a direct reflection of the foods these hominins ate, which in turn are a reflection of the local geology" said Copeland.
"The research suggests that the ancestral condition of modern humans, if A. africanus is an ancient ancestor, is one where females of the group disperse outside of their family group--similar to chimpanzees today," said Reed. "Thus, for Homo sapiens, those cultural groups in which the males disperse outside the family group suggests a differentiation from the ancestral condition."
"This study gets us closer to understanding the social structure of ancient hominins since we now have a better idea about the dispersal patterns," said Copeland.
The new study results were somewhat surprising, she added. "We assumed more of the hominins 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," she said. "Such small home ranges could imply that bipedalism evolved for other reasons."
Sponheimer said the study could be taken as support for the position that bipedalism arose for reasons other than improved locomotion; the data might also indicate that many hominins simply preferred to live on dolomite substrates where caves would have been abundant.
"I've never thought of these early male hominins as the quintessential cavemen," he said. "But the potential use of caves at this early time period is something worth considering."
Additional co-authors for the study include Darryl de Ruiter from Texas A&M University, Julia Lee Thorp from the University of Oxford, Daryl Codron from the University of Zurich, Petrus le Roux from the University of Cape Town, Vaughan Grimes of Memorial University-St. John's campus in Newfoundland and Michael Richards of the University of British Columbia in Vancouver.
The Max Planck Society, a University of Colorado Leadership Education for Advancement and Promotion program Associate Professor Growth Grant and the University of Colorado Dean's Fund for Excellence also provided funding for the research.
More about the study can be found on the University of Colorado website.
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