Scientists working in South Africa have unveiled fossils of a human species new to science that they say could be the direct ancestor of our genus, Homo. Discovered in Malapa cave—just 15 kilometers from the sites of Sterkfontein, Swartkrans and Kromdrai, which have yielded a number of important human fossils—the finds comprise two partial skeletons that are nearly 1.95 million years old. The researchers have given them the name Australopithecus sediba.

The pair—an adult female and juvenile male that may have been mother and son—appear to have fallen into the cave through a hole in the cave ceiling, perhaps while attempting to access a pool of water inside. So exceptional is the preservation of the skeletons, particularly the male, that the discovery is being likened to that of the famous Lucy fossil from Ethiopia and the Turkana boy from Kenya. But the startling mix of primitive and advanced traits evident in the remains is sparking debate over where on the family tree the new species belongs and raising important questions about the dawning of Homo.

The study of human origins has come a considerable way since German quarry workers discovered in 1856 the first fossil to be recognized as an early human (it was a Neandertal). Investigators have traced our roots back to perhaps seven million years ago, close to the point at which the human lineage diverged from that of our closest living relative, the chimpanzee; they have recovered extensive remains of australopithecines such as Lucy and her ilk, creatures that are transitional between apes and us; and they have unearthed fossils representing quite an array of species documenting the evolution of Homo. Considering the virtually nonexistent fossil trails of our cousins the chimps, bonobos and gorillas, the human fossil record is extraordinary.

There are, however, significant gaps in researchers' knowledge of how we came to be. One such blind spot is the origin of Homo. Most experts agree that Homo evolved from a species of Australopithecus—either A. afarensis (Lucy's species) or A. africanus. Connecting the dots between one of these australopithecine species and Homo has been difficult, though, because the oldest known Homo remains are so few and fragmentary.

Scientists’ best guess has been that Homo habilis, which lived between perhaps 2.3 million and 1.5 million years ago, signaled the debut of Homo and subsequently gave rise to H. erectus, the first hominin (member of the human lineage) to spread out from Africa across the globe and the one that is thought to have spawned later human species, including H. sapiens. But H. habilis is a relatively poorly understood species because the known specimens preserve few bones from below the head. In fact, some researchers wonder whether some of the fossils assigned to H. habilis might in fact belong to other species. "H. habilis has been on shaky footing for a long time," observes paleoanthropologist Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research.

Into this morass falls A. sediba. In the paper describing the fossil remains, which will be published in the April 9 issue of Science along with a second paper detailing the geological context and age of the finds, Lee Berger of the University of the Witwatersrand in Johannesburg and his colleagues suggest that A. africanus gave rise to A. sediba, which in turn gave rise to the genus Homo. Intriguingly, the team hints that A. sediba might even be more closely related to H. erectus than H. habilis is, thus potentially relegating H. habilis to a side branch of the family tree, rather than a coveted spot on the line leading to us.

Berger and his collaborators based their conclusions on A. sediba's distinctive amalgam of primitive and derived traits. Features such as its small brain, small body, and very long arms link the creature to the australopithecines, especially A. africanus, whom A. sediba resembles in details of the face and teeth. Yet the new species also exhibits a number of characteristics seen only in Homo, including its flatter face, robust pelvis and long, striding legs. Similarities to H. erectus in particular, according to the team, are evident in details of the skull and pelvis.

Paleontologists not involved in the new work agree that the two South African skeletons represent a major discovery. But they are very much divided on the issue of where this new hominin belongs on the family tree. "It is truly an incredible find," comments Meave Leakey of the National Museums of Kenya, who recently traveled to South Africa and looked at the A. sediba remains. "I found it was hard to believe what we saw in view of the quantity and quality of the fossils. However, I do not think that they are ancestral to Homo or have anything to do with Homo." Rather, she says, "these fossils reinforce my view that the australopithecines in South Africa underwent a separate radiation that had little to do with East African species that have been called Australopithecus, other than that they share a common ancestor." In 2001 Leakey and her colleagues announced their discovery of a hominin they called Kenyanthropus platyops, and suggested that it could be another possible ancestor of Homo.

William Jungers of Stony Brook University concurs with the discovery team's interpretation of the new skeletons as a new species of Australopithecus that is probably descended from A. africanus. But he disputes the ties to Homo. "The proposed link between A. sediba and early Homo is forced and tenuous at best," he asserts, noting that the alleged postcranial similarities between the two groups are not very compelling.

William Kimbel, director of the Institute of Human Origins at Arizona State University, has a different take. Considering the advanced features of the face and pelvis, the new fossils "probably belong in the Homo genus." They do not illuminate its origin, however. Kimbel points out that a site in Hadar, Ethiopia, where he works, has yielded a Homo specimen that, at around 2.3 million years old, predates the A. sediba fossils by hundreds of thousands of years. For their part, Berger and his co-authors contend that the Malapa fossils represent neither the first nor the last instances of A. sediba, and that they could be a late-surviving population of a species that originated rather earlier. "That's a suggestion that further finds would need to clarify," Kimbel counters.

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.