Kimbel is not the only paleoanthropologist who thinks the new fossils belong in Homo instead of Australopithecus. Susan Antón of New York University observes that characteristics of the teeth, in particular, align them with our genus. But exactly how the skeletons relate to known Homo species is difficult to establish given the paucity of early Homo remains. "In many cases we don't know what some elements look like at all in some taxa," she says. The absence of any H. habilis hip bones to compare to A. sediba's is one such example. Antón suspects that A. sediba may be a dead-end branch of Homo, rather than an ancestor of later species such as H. erectus.
The level of disagreement about how to classify the Malapa fossils is perhaps to be expected, because the closer paleontologists get to the transition between Australopithecus and Homo, the harder it is going to be to determine which side of the divide a specimen falls on. Either way, the fossils illuminate a very mysterious chapter of human evolution. "They are going to be a remarkable window, a time machine of morphology into the evolutionary processes and evolutionary stresses that were going on at that period between 1.8 and 2 million years ago," Berger said at a press teleconference on April 7.
For instance, the combination of primitive and derived traits evident in A. sediba shows that different parts of the body were changing at different times during this phase of human evolution—the legs changed before the arms; the pelvis changed before the brain.
"A. sediba does give intriguing insights into the co-expression of traits associated with Homo and Australopithecus," remarks Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig. He observes that whereas it has Australopithecus-like brain size and molar shape, it calls to mind Homo in its brain case shape and molar size. These traits of A. sediba show that "brain size and certain features of the brain case are not necessarily linked, and molar size and morphology and not necessarily linked either."
That A. sediba exhibits such a mix of old and new traits both in the skull and throughout the body underscores to Jungers that the primitive features seen in australopithecines were not just evolutionary baggage from an ancestor that lived in the trees, as some scientists have argued. Rather, he says, such characteristics were major contributors to the success of the australopithecines, hence their persistence over millions of years. "It’s the ultimate loss [of these traits] with the emergence of Homo that begs explanation." In his view, selection for traits that enhanced our ability to run long distances can explain the transformation.
The Malapa skeletons seem unlikely to resolve the origin of Homo on their own. But clarification of how they should be classified—Australopithecus or Homo, side branch or main line—may not be far off: Berger told the press teleconference that since submitting his initial findings to Science, he has found at least two more hominin skeletons in the cave. He is currently excavating these, and has assembled a team of about 60 experts to analyze all of the material from the site in detail. Their to-do list includes determining whether the fossils might contain proteins or DNA suitable for sequencing, reconstructing the environment the hominins lived in, and studying the aging process and disease process in this ancient relative. Such efforts will no doubt reveal an incredibly detailed portrait of this newest addition to the human family.