Paranthropus robustus. This specimen known as SK 48 is one of the best examples of Paranthropus robustus from Swartkrans cave, South Africa.
Credit: Darryl de Ruiter
Australopithecus africanus. 'Mrs. Ples' is the most famous example of Australopithecus africanus from Sterkfontein cave, South Africa.
Credit: Darryl de Ruiter
Australopithecus africanus third molar (M3) after laser ablation analysis (specimen Sts 28). You can see the faint marks left by laser ablation along the right side of the tooth crown.
Credit: Sandi Copeland
LONDON: A study of fossilised teeth from 19 hominins - early human ancestors that lived 2 million years ago -has suggested that the females tended to move away from their natal groups while the males stayed at home.
Teeth from eight Australopithecus africanus individuals and 11 Paranthropus robustus individuals thought to have lived around 1.8 to 2.2 million years ago were found in the Sterkfontein and Swartkrans cave sites in South Africa.
Based on the movements of the individuals, the new research published in the current issue of Nature provides a rare insight into the lives of our early ancestors by showing the first evidence of their social structure.
"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," said co-author of the study, Sandi Copeland, a palaeoanthropologist at the University of Colorado in the U.S.
Males had the bigger teeth
The researchers used a new technique called strontium isotope analysis to analyse tooth enamel and map the movement of the hominins across the landscape. Strontium, an earth metal found in soil, is taken up by plants, which would have been consumed by the hominins.
It would then have been stored in trace amounts in their teeth up to the age of eight, so can provide a means of finding out where the hominins were born and raised before they moved on.
The largest canines and third molars were attributed to the males, and the smallest to the females. The male and female teeth were tested and compared.
"The interpretation of differences in behaviour of males and females is dependent upon the assumption that smaller teeth represent females. This assumption is likely the best explanation for the data," said Jay Stock from the Leverhulme Centre for Human Evolutionary Studies at the University of Cambridge, UK.
Avoiding close inbreeding
The strontium isotope ratios of these 19 hominins were then compared to 170 plants and animals that currently live near the caves.
"The comparison to contemporaneous species known to have relatively restricted movement provides excellent corroboration of the data suggesting that small-bodied hominins have significantly different ratios," Stock added.
The researchers found that around 90% of the larger teeth, thought to be from the males, were local to their natal group compared to less than half of the smaller, female teeth. The dispersal pattern of females moving away from their natal groups is common in chimpanzees and bonobos but unlike that of gorillas and other primates. The movement of female hominins away from the natal group was significant as it would have avoided inbreeding with family members.
"By virtue of the fact that the males choose to remain, the females are indirectly forced to leave their communities in order to avoid close inbreeding. It could be that among these early hominins, female dispersal has some correlation to close cooperative behaviour between males," Copeland said.
