June 2, 2011

Ancient Hominid Females Did All The Roaming

An analysis of two ancient hominid species that roamed southern Africa more than a million years ago suggests that females left their childhood homes while males stayed at home, an international team of researchers said on Wednesday.

The scientists studied teeth from a group of extinct Australopithecus africanus and Paranthropus robustus individuals from two adjacent cave systems, and found that more than half of the female teeth were from outside the local area.

By comparison, just 10 percent of the male hominid teeth were from elsewhere, suggesting they likely grew up and died in the same area, said CU-Boulder adjunct professor and lead study author Sandi Copeland.

"One of our goals was to try to find something out about early hominid landscape use," she said.

"Here we have the first direct glimpse of the geographic movements of early hominids, and it appears the females preferentially moved away from their residential groups."

Copeland said she was somewhat surprised at the findings.

"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," she said.

"Such small home ranges could imply that bipedalism evolved for other reasons."

Copeland and her team used a sophisticated analysis known as laser ablation, zapping the hominid teeth with lasers to help measure isotope ratios of strontium found in tooth enamel.  This allowed them to identify specific areas of landscape use.

Strontium is a naturally occurring element found in rocks and soils and 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 hominids grew up, said Copeland.

"The strontium isotope ratios are a direct reflection of the foods these hominids ate, which in turn are a reflection of the local geology."

"It is difficult enough to work out relations between the sexes today, so the challenges in investigating the ways that male and female hominids used the landscape and formed social groups over a million years ago are considerable, to say the least," said study co-author Matt Sponheimer, an anthropology professor at CU-Boulder.

"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."

Strontium isotope signatures are locked into the molars of mammals by the end of tooth enamel formation -- for the hominids, probably at about eight or nine years old when they were traveling with their mothers.

The Sterkfontein and Swartkans cave systems that yielded the teeth are less than a mile apart, and dominated by a sedimentary carbonate rock known as dolomite, which has a distinct strontium signal, Copeland said.

The researchers tested 19 teeth dating from roughly 2.7 to 1.7 million years ago from both Australopithecus africanus and Paranthropus robustus individuals from the two caves, which are well known for yielding valuable scientific data on hominid evolution.

Because the male hominids, like male humans, were larger than the females, the team used the size of individual molars to determine which were most likely from males or females, said Copeland. They also compared them to teeth and jaw fossils recovered from five early hominid sites in South Africa.

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. While Australopithecus africanus may be a direct ancestor of modern humans, Paranthropus robustus and its close relative, Paranthropus boisei, both dead-ended on a side branch of the hominid family tree for reasons still unknown.

The female dispersal pattern believed seen in the two hominid groups is similar to that of many modern humans, chimpanzees and bonobos, said Copeland.

However, it is a dispersal pattern unlike most other primates, where the females stay with the group they are born into and the males move elsewhere.

"This study gets us closer to understanding the social structure of ancient hominids since we now have a better idea about the dispersal patterns," she said.

The team also used laser ablation to zap 38 fossilized teeth of baboons, antelope, and small, rodent-like creatures known as hyraxes that lived in the same area at about the same time as the two australopithecine species under study.

The results showed nearly all of the mammal teeth were local, implying such groups had relatively small home ranges, much like the australopithecine males, said Copeland.

Although the study could be interpreted as supporting the position that bipedalism arose for reasons other than improved locomotion, Sponheimer said the data might also indicate that many hominids simply preferred to live on dolomite substrates where caves would have been abundant.

"I've never thought of these early male hominids as the quintessential cavemen, but the potential use of caves at this early time period is something worth considering," he said.

The research is published in the June 2 issue of Nature.

The following institutions contributed to this study: Max Planck Institute fr Evolutionary Anthropology, Leipzig, Germany; University of Colorado, Boulder, USA; Texas A&M University, College Station, USA; Oxford University, Oxford, UK; University of Cape Town, Cape Town, South Africa; University of Zurich, Zurich, Switzerland; Memorial University, St. John's, Canada; University of British Columbia, Vancouver, Canada.


Image 1: A new study led by the University of Colorado Boulder indicates that the males of two ancient hominid species, including Paranthropus robustus, pictured here, stayed home while the females roamed. Credit: Illustration courtesy of Walter Voigt/Lee Berger/Brett Hilton-Barber

Image 2: This is a skull of a Paranthropus robustus from Swartkrans Cave in South Africa. Credit: Darryl de Ruiter


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