|"Mrs. Ples" is the most famous example |
of Australopithecus africanus from Sterkfontein
cave, South Africa.
Researchers studying the diet of human ancestors who lived two million years ago in southern Africa have unexpectedly come across a crucial clue to their social structure. The males never strayed far from home, and the females dispersed after puberty to neighboring groups.
The pattern of female dispersal is not unexpected, since it is practiced by chimpanzees, the closest living species to humans, and by some hunter-gatherers. But social behavior does not fossilize, and any data on the subject is invaluable in reconstructing how human social structure evolved.
“This is the first direct evidence that we have about the residence pattern of hominins,” said Bernard Chapais, a primatologist and expert on human social evolution at the University of Montreal.
The evidence emerged from study of the fossil teeth of 19 australopithecines, the still apelike ancestors of the human lineage. The makeup of teeth can reflect local geography because some chemicals like strontium are drawn from rocks into plants, and then into tooth enamel when the plants are eaten. In australopithecines, the absorption of strontium continued until the teeth were completed around age 8.
The researchers were measuring different versions, or isotopes, of strontium to see if the australopithecines traveled far from home in search of food. As the data rolled out of the measuring device, “we were at first disappointed,” said Julia Lee-Thorp of Oxford University, a member of the research team, “but we soon realized that we had found another prize.”
This was that half the females had been born far away, whereas all the males had grown up locally, the team reported Wednesday in the journal Nature. The team was led by Sandi R. Copeland of the Max Planck Institute for Evolutionary Anthropology in Germany.
In most mammals, the females stay in the home community and the males disperse after adolescence to avoid inbreeding. But chimpanzees and many human hunter-gatherer groups are unusual in following the opposite pattern. The reason may have to do with the aggressive territoriality of both species: A group of males who have grown up with one another is more cohesive and better at defending a territory against competitors. This obliges the females to be the gender that disperses.
“It’s really nice to see there is biological continuity from chimpanzees to australopithecines,” said Joan B. Silk, an expert on primate social behavior at the University of California, Los Angeles.
The joint ancestor of chimps and humans lived about five million years ago and is often assumed to have had a chimplike social structure, with a male hierarchy, promiscuous mating by the females and all-out war between neighboring bands. The central puzzle of human social evolution, in Dr. Chapais’s view, is to explain how promiscuity was replaced by the pair bond and aggressive relations were pushed up from the band level to that of the tribe, a group of bands tied together by the exchange of women.
The australopithecines studied by Dr. Copeland’s team still had a somewhat chimplike social structure, Dr. Chapais said, because the pair bond did not evolve until the appearance of Homo erectus around 1.8 million years ago. The australopithecine fossils came from the Sterkfontein valley, and one of them has been dated to 2.2 million years ago.
The pair bond, in his view, arose when human ancestors started to develop more dispersed sources of food and males increasingly guarded a group of females to protect both them and their own paternity. With the male around anyway, he could also help rear the children, which allowed for a longer period of juvenile dependence and hence for brain size to grow. The relatively small size of the australopithecine brain suggests that this process had not yet started, Dr. Chapais said.
Another far-reaching consequence of the developing pair bond was that individuals could at last start to recognize their relatives, which chimpanzees mostly cannot do. So when females dispersed to neighboring groups, males from their home community could recognize their daughters or sisters, together with their in-laws, who had an equal interest in their children’s welfare. This transformed the neighboring group from an adversary to an ally, and the human social structure expanded to being that of a tribe. Friendly relations, in Dr. Chapais’s view, then allowed males as well as females to transfer, the pattern typical of hunter-gatherers today.
“It is great to think that we might be in a position to begin dating such major events in our evolution,” he said of the new finding.
The australopithecine fossils are few and precious, and researchers are reluctant to damage them in any way. In a new technique, a barely visible pit is blasted out of a tooth with a laser and the vaporized enamel is passed into a mass spectrometer, which measures the amount of strontium or any other element of interest. The researchers started out looking at carbon isotopes, which showed them that australopithecines had a highly variable diet. To determine whether that was because they ranged far and wide, the researchers then looked at strontium isotopes, the ratio of which varies according to the local geology. And this led to the discovery of the female dispersal pattern.