Two million-year-old bones belonging to a creature with both apelike and human traits provide the clearest evidence of evolution's first major step toward modern humans — findings some are calling a potential game-changer.

An analysis of the bones found in South Africa suggests Australopithecus sediba is the most likely candidate to be the ancestor of humans, said lead researcher Lee R. Berger of the University of Witwatersrand in South Africa.

The fossils, belonging to a male child and an adult female, show a novel combination of features, almost as though nature were experimenting. Some resemble pre-human creatures while others suggest the genus Homo, which includes Homo sapiens, modern people.

"It's as if evolution is caught in one vital moment, a stop-action snapshot of evolution in action," said Richard Potts, director of the human origins program at the Smithsonian Institution. He was not among the team, led by South African scientists, whose research was published online Thursday in the journal Science.

Scientists have long considered the Australopithecus family, which includes the famous fossil Lucy, to be a primitive candidate for a human ancestor. The new research establishes a creature that combines features of both groups.

The newly studied bones were found in 2008 in the fossil-rich cave region of Malapa near Johannesberg. Berger's then 9-year-old son, Matthew, found a bone that was determined to belong to the child. Two weeks later Berger uncovered the fossils of the female.

The journal published five papers detailing the findings, including separate reports on the foot, hand, pelvis and brain of A. sediba.

Berger said the brain, hand and foot have characteristics of both modern and early pre-human forms that show a transition under way. It represents a bona fide model that could lead to the human genus Homo, Berger said.

Kristian J. Carlson, also at Witwatersrand, said the brain of A. sediba is small, like that of a chimpanzee, but with a configuration more human, particularly with an expansion behind and above the eyes.