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One of our newest relatives, a juvenile male Australopithecus sediba (photo by Brett Eloff courtesy of Lee Berger and the University of Witwatersrand)

Poor Rick Potts. He just put the finishing touches on the National Museum of Natural History’s new Hall of Human Origins a few weeks ago, and it’s already out of date. Now there’s a new branch on the human family tree—Australopithecus sediba—and we can thank a 9-year-old kid for its discovery.

Throughout 2008, University of the Witwatersrand paleoanthropologist Lee Berger mapped cave sites near Johannesburg, South Africa, and searched for fossils within them. Nearly a third of the fossils that provide evidence for humans’ African origins have come from this region, so mapping new sites should lead to new discoveries. One of the caves he was searching was Malapa, about nine miles northwest of the city. In August 2008, Berger brought his young son Matthew and a post-doctoral student to the cave. Within minutes, Matthew spotted a rock with a bone sticking out of it. That bone was the collar bone of an ancient hominid, and the rock also contained a lower jaw.

Berger and his colleagues would soon collect more bones from the cave site (their find is described in this week’s issue of Science), including the nearly complete skull and partial skeleton of boy 11 or 12 years old and the skeleton of an adult female in her late twenties or early thirties. The bones, which are nearly two million years old, aren’t quite like anything that’s been found before: the boy’s brain and both skeletons are similar in size to australopithecines (like Lucy) but the teeth are smaller, the cheekbones less pronounced and the nose more pronounced. The legs were also longer than australopithecines, and the pelvis was more similar to one from another human relative, Homo erectus. Berger’s group concluded that the new skeletons belonged to a new species, A. sediba (sediba means “fountain” in the local Sesotho language).

But scientists aren’t quite sure where A. sediba fits in the human family tree. Berger and his colleagues believe that the new species is a descendant of A. africanus and may be the immediate ancestor of our own genus Homo. Some have even suggested that A. sediba may belong in the Homo genus. Others, however, contend that the new bones must belong to an australopithecine, and perhaps not even a new species within that genus.

It is not unusual for anthropologists to argue over new finds. But in the coming years, as new bones are found and analyzed (Berger found the bones of an infant and another adult female but has yet to describe them scientifically), A. sediba‘s place in our evolutionary history may be fully realized. If nothing else, it will keep Potts and the rest of curators of the Hall of Human Origins busy for a long time to come.