Evolution skeptics like to trot out the argument that if Darwin had been right, scientists would have discovered transitional fossils by now — creatures with a mix of features from earlier and later species. Since they haven’t, the deniers say, evolution must not be true.
The truth is that paleontologists have found transitional species by the score, from many different time periods. But none have materialized from as crucial a point in our evolutionary past as a pair of skeletons whose discovery was announced today by the journal Science.
The fossils, which have been determined to be of a new species, Australopithecus sediba, were found by Matthew Berger, the 9-year-old son of paleontologist Lee Berger, of South Africa’s University of the Witwatersrand. The bones belong to a preteenage boy and a woman estimated to have been in her late 20s or early 30s; the individuals died at about the same time, and before their remains had fully decomposed, they were entombed in an avalanche of sediment and were nearly perfectly preserved deep in the Malapa cave north of Johannesburg.
The Au. sediba bones are important for their vintage — they date back to the moment about 2 million years ago when the genus of human ancestors known as Australopithecus was just giving way to a new group called Homo, which would eventually produce Homo sapiens, or modern humans.
But the new find is perhaps most astonishing for its state of preservation. “These are arguably the most complete hominid skeletons ever discovered,” says Berger.
In the field of human origins, that’s huge: many human ancestors have been identified based on only a few bone fragments, and even Berger’s rivals in the intensely competitive field of human paleontology were quick to praise his discovery. “This is an outstanding find, and Lee should really be congratulated,” says Meave Leakey, a paleontologist with the National Museums of Kenya and a member of the world’s most celebrated fossil-hunting family.
Other paleontologists have confirmed the dating of the skeletons to somewhere between 1.95 million and 1.78 million years ago, which is another crucial clue to figuring out the new species’ place in our evolutionary story. That’s where things get a lot more contentious.
The skeletons show a remarkable mix of primitive and advanced features. The arms, for example, are relatively long and apelike, suggesting Au. sediba was a prodigious tree climber. The hands are apelike in their curvature but surprisingly compact, like a more modern hominid’s. And the longer legs and shortened pelvis are decidedly modern. “They were competent bipeds on the ground,” says Berger.
The boy’s skull is a similar mashup of older and newer features.
