In modern times, men explored the New World. But 2 million years earlier, the men among our pre-human forerunners stayed put and it was the women who traveled to start new families, a study of fossil teeth from Africa suggests.
The findings, published in Thursday's journal Nature, indicate females from two pre-human species seemed to move out of their birth homes and journey elsewhere, probably to prevent inbreeding, researchers said. Chimpanzees, our closest living primate relative, also have females that travel to mate and raise families.
That's in contrast to lower primates and most mammals where it is the males that have the wanderlust.
Researchers studied 19 teeth, including eight from Australopithecus africanus individuals, a species considered a probable ancestor from about 2.2 million years ago. The other 11 were from Paranthropus robustus individuals, a dead-end species that was not our direct ancestors but more like prehistoric aunts and uncles from 1.8 million years ago.
They looked for the mineral strontium in the teeth because that element varies by landscape. The idea was to see if they moved to different areas during various seasons. The research didn't show that, but something else popped up: The bigger teeth showed almost no mineral variation while more than half of the smaller teeth indicated they were from individuals that grew up elsewhere.
So the researchers figure this was a male-female difference. Other scientists not involved in the research, though, said the tooth sample may be too small to draw that conclusion.
The study's lead author, Sandi Copeland, a paleoanthropologist at the University of Colorado, said the switch from male to female dispersal may indicate the start of a sense community, which has its evolutionary advantages. And it continues in many societies to this day.
In less evolved animals, it makes evolutionary sense for the male to wander and impregnate many females and show might. In this case, the female moving could show that males in a community have bonded and cooperated, maybe for common defense. So it makes sense for the men to settle, while the females disperse, Copeland said.
"There must be an evolutionary benefit for females to disperse," Copeland said.
University of Oxford archaeologist Michael Petraglia, who wasn't part of the study, said the research was intriguing, showing that our forerunners' "social relations and mating patterns are more in line with ours than with gorillas." But like Richard Potts of the Smithsonian Institution, Petraglia worried that the research may be drawing too many conclusions from a small number of teeth from a large time period.
Study co-author Darryl de Ruiter at Texas A&M University said researchers were limited in how many samples they could use because tests destroyed parts of the fossil and they only used the largest and smallest teeth.