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.