In the articles in Science, Dr. Berger’s team describes novel combinations of apelike and humanlike features in the hand, foot and pelvis of the new species. The hand, for instance, is apelike because it has long, strong fingers suitable for climbing trees, yet is also humanlike in having a long thumb that in combination with the fingers could have held tools in a precision grip. A cast of the inside of the skull shows an apelike brain, but one that had taken the first step toward being reorganized on human lines.

This mixture of apelike and humanlike features suggests that the new species was transitional between the australopithecines and humans, the researchers said at a news conference on Wednesday. Given its age, Australopithecus sediba is just old enough to be the ancestor of Homo erectus, the first species that paleoanthropologists agree belonged to the human ancestry and which existed 1.9 million years ago.

But the fossils are significant even if sediba is not a direct human ancestor. They are evidence that a ferment of evolutionary experimentation was going on at the time, out of which the human lineage somehow emerged. “If you take sediba as a metaphor for evolutionary change, it is a whole lot more powerful than the claim for direct ancestry,” Dr. Tattersall said.

A similar view was taken by Bernard Wood, a paleoanthropologist at George Washington University. “I think these are some of the most interesting papers that have been published in recent years,” Dr. Wood said. “But these are probably not the reasons the authors think they are interesting.”

Dr. Wood gave little credence to Dr. Berger’s arguments that Australopithecus is a direct ancestor of the human group, saying there was too little time for the small-brained, tree-climbing ape to evolve into the large-brained Homo erectus. More interesting, in his view, are the strange combinations of apelike and humanlike features that Dr. Berger’s team has described. The new fossils display the modular way in which evolution operates: they have mostly known features but in novel combinations that have never been seen before.