Mr Caveman stayed close to home
Paris - Scientists on Wednesday unveiled evidence that two species of early cavemen lived and died near their places of birth while most females of the same species settled down after coming from afar.
The study, published in Nature, offers an unprecedented glimpse into the social fabric of australopithecines, an extinct line related to humans that dwelt in southern Africa some two million years ago.
It also challenges the axiomatic idea that our distant forebears began to walk on two legs rather than four in order to cover great distances in search of food or shelter.
If males limited their wanderings to hunt-and-gather forays, then the shift to walking upright might have been driven by other needs, the findings suggest.
Up to now, very little was known of the lifestyle and kinship patterns of our two-legged ancestors.
"Disembodied skulls and teeth are notoriously poor communicators," quipped Matt Sponheimer, an anthropology professor at the University of Colorado at Boulder and a co-author of the study.
For the new study, scientists devised a method worthy of Sherlock Holmes to "make these old bones speak", Sponheimer said.
The behavioural clues were locked inside a handful of two-million-year-old teeth.
Tiny variations in the atoms of a heavy metal element called strontium correspond to various types of soil and rock, thus acting as a telltale of identifiable geographic locations.
Because strontium works its way into tooth enamel only during the first years of life, the element thus shows whether a primate grew up in the same place where she or he dwelt and died.
The scientists examined teeth from 19 individuals who lived 2.4 to 1.7 million years ago, eight Australopithecus africanus and 11 Paranthropus robustus.
Both species lived in woodland savannahs, probably subsisting on a mix of tree fruits, grass, seeds and nuts.
Males and females in the sampling were differentiated on the basis of size.
Laser analysis of the strontium isotopes showed that only 10% of the males originated from outside a range of 30km2, compared to more than half of the females.
The males, in other words, probably strayed only rarely more than a few kilometres from their caves.
Their female partners, however, had often migrated from afar, even if they adopted the same close-to-home lifestyle once settled.
"Here we have the first direct glimpse of the geographic movements of early hominids, and it appears the females preferentially moved from their residential groups," said Sandi Copeland, also of the University of Colorado and lead author of the study.
The practice of females leaving the nest to join the family of their mates has been common in most human cultures across history. Chimpanzees and bonobos also follow the same pattern.
But most other primates, including gorillas, do the opposite: females stay with the group they are born to while males move elsewhere.
The discovery came as a surprise, and calls into question long-held views about how primates came to prefer moving on two feet rather than four, the researchers said.
"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," said Copeland.
"Such small home ranges could imply that bipedalism evolved for other reasons."
The study sheds new light on the "diet, group size, predator avoidance and home-range size" of what may be our direct ancestors, Margaret Schoeninger, a professor at the University of California at San Diego, wrote in a commentary, also in Nature.
But it also raises many vexing problems, she added: "How the australopithecines balanced predator avoidance and the need to compete for food remains an open question."