Males within two human ancestral species that existed roughly 2.7 to 1.7 million years ago were stay-at-home fellows, while females of these same species traveled, according to a new Nature paper.
The finding not only suggests that homebody males today may have a genetic predisposition for their lifestyle choice, but that certain female dispersal patterns among humans may mirror those of chimpanzees and bonobos. These two other primates also have stay-put males and traveling females.
"In any primate society, the females, the males, or some of both must eventually leave their birth community and join or form other communities," lead author Sandi Copeland, an adjunct professor in the Department of Anthropology at the University of Colorado at Boulder, told Discovery News. "One important reason for this is to prevent inbreeding."
For the study, Copeland and her team analyzed 19 teeth from both Australopithecus africanus and Paranthropus robustus individuals. These early human relatives lived at different time periods, but in two adjacent South African cave systems: Sterkfontein and Swartkans.
The researchers used a technique known as laser ablation, which zaps the teeth with lasers, measuring isotope ratios of the metallic element strontium. Unique strontium signals are tied to specific geological substrates, such as granite and sandstone, and therefore "strontium isotope ratios are a direct reflection of the foods these hominids ate, which in turn are a reflection of the local geology," Copeland explained.
The strontium "signatures" lock into the molars of humans probably when they are about 8 or 9 years old. The measurements revealed males tended to not stray far from home. The majority of the females, on the other hand, had moved from the place where they were born.
Copeland said, "It is possible that female hominins chose to leave their natal groups in order to mate with unrelated males, an indirect result of the males in their natal group choosing not to leave." But, she added, "We cannot exclude the possibility that female hominins did not move of their own free will, as abduction of females is known to occur in modern humans, rarely in chimpanzees, and often in Hamadryas baboons."
Chimpanzees have actually been observed taking females away from their home communities and attacking them if they resist leaving. Whether or not this occurred among the early human relatives remains unclear.
The findings, however, suggest that our ancestors did not live as gorillas do today, with males traveling and females staying put and living in harems. The fact that early human ancestral males did not travel, however, does not mean that they helped to raise children. Chimpanzees, which exhibit the same dispersal patterns, have males that stay at home but yet "don't participate in childcare," Copeland says.
Another possible implication is that two-legged walking emerged in humans for reasons other than improved locomotion.
"If one interprets our results as indicating that male australopiths rarely moved long distances, then one is left to wonder if the need for energetic efficiency was sufficient to drive the origins of bipedalism," co-author Matt Sponheimer explained.
Margaret Schoeninger, a University of California at San Diego anthropologist, authored a commentary in Nature about the new findings.
Schoeninger echoed Copeland's reasoning for why females dispersed, saying "it eliminates the potential genetic problems that can appear due to inbreeding." Based on the new research, and prior determinations, she told Discovery News that we now know the australopithecines lived within small ranges, were relatively stationary (with perhaps even the traveling females not moving very far away) and that they "lived in areas with lots of large predators."
Speaking of A. africanus, whose most famous representative is Ethiopian "Lucy," she said, "This is one weird ape-like primate," mentioning that many questions remain about it, such as if it sat to eat.
Sponheimer agrees that important questions remain.
"This study is one example of how we can sometimes, if we are lucky, coax old bones and teeth to relinquish a few of their secrets," he said. "And I don't doubt that we are getting better and better and getting more from less and less, but I think we have a long road before us. Much about our forebears continues to be resolutely mysterious."