A just-published study in Nature suggests that our ancient female human ancestors were the ones who, on coming of age, left their social group for newer pastures. That is, ancient females were more likely to roam while ancient males stay closed to home, in contrast to the image we often have of females being the homebodies and males going out into the world.
Archaeological scientists analyzed the fossilized teeth ranging between roughly 1.8 million and 2.2 million years old from two South African caves. The teeth of the females were found to contain minerals from a different area than those of the males (which were distinguished from the females’ teeth due to their larger size).
Julia Lee-Thorp, an archaeological scientist at the University of Oxford, UK, and a co-author on the study, says that this is indeed “a very small clue,” but it is “at least hard evidence for what we really didn’t have before.” Nature describes the chemical analysis the scientists performed:
Lee-Thorp and her colleagues measured the levels of two isotopes of strontium, an element found in soil. This is taken up by plants and then snakes its way up the food chain into the growing bones of animals. The ratio of two strontium isotopes in bones or teeth provides a signature of the local environment in which an animal grew up, Lee-Thorp says. “It’s a kind of forensic tool.”
Her team measured the strontium isotope ratios in canine and third molar teeth — which are formed by about the age of eight — in eleven Paranthropus robustus individuals from the Swartkrans cave, as well as in teeth from eight Australopithecus africanus individuals from the nearby Sterkfontein cave, about 50 kilometres northwest of Johannesburg. The researchers also measured the strontium in 170 plants and animals currently living near the caves to get a sense of the different strontium signatures of the region, including the thin Malmani dolomite formation that includes both caves.
They discovered that larger teeth — ostensibly from bigger-bodied males — of both species were much more likely to share the strontium signature of dolomite-dwellers than the smaller teeth of female australopiths. About 90% of the larger teeth looked local, compared with less than half of the smaller teeth. The best explanation for this pattern is one in which females left their clan once they reached maturity, say Lee-Thorp and her colleagues.
While noting that the results of the study are fascinating, palaeoanthropologists and other scientists expressed caution, noting that the number of samples was not very large.
Supporting the study, Owen Lovejoy, an anatomist at Kent State University in Ohio, points out that “new males are likely to turn aggressive once they join a new group, risking the lives of infants and juveniles.” Thus, it would make more sense for males to stick with their social group of origin while females left home.
Another lead author of the study, University of Colorado at Boulder professor Sandi Copeland, notes in Science Daily that such a “female dispersal pattern,” with the females joining new social groups, is thought to exist in “two hominid groups is similar to that of many modern humans, chimpanzees and bonobos.” It is, though, unlike a pattern found in most other primates including gorillas, where “females stay with the group they are born into and the males move elsewhere.”
Lee-Thorp says that she is hoping to ask similar questions about early human ancestors, including our ancestors from East Africa and also later species of hominin, such as Homo erectus.
Finding out that males are the “original homebodies” does call into question assumptions such as a “woman’s place is in the home.” Wouldn’t it be something if our female ancestors were found to be the pioneering, adventuring spirits?
Photo of Australopithecus africanus from Wikimedia Commons.