It's a busy time in the study of human origins. Hot on the heels of a potential new species found entirely via DNA sequence, we have the announcement of a new species of Australopithecus, discovered in a cave in South Africa. The authors of a paper describing the species, Australopithecus sediba, make the bold claim that it's likely to be the direct ancestor of the entire Homo genus, placing the species on a direct line to modern humans. We'll describe the fossils themselves before returning to these claims.

The site is called Malapa, and is located in South Africa. Other sites nearby have yielded significant early hominin finds, and the area is preserved as the Cradle of Humankind World Heritage site. Miners had evaluated the caves earlier, and had dumped some of their contents outside. It was apparently in one of these waste piles that an author's son found a hominin clavicle. Excavations of the cave revealed two partial skeletons: a juvenile male, and a mature female.

The fossils appear to have been preserved when water created a sedimentary deposit in an earlier version of the cave. The area was inaccessible enough that none of the skeletons present seem to have suffered bite marks indicative of scavenging, which explains their excellent preservation. The young male sample includes a largely complete skull, along with pieces of the arms, legs, and hips; the female has a large portion of the jaw, a nearly complete shoulder and arm, and fragments of the legs and hips.

The deposits that hold the fossils were dated in three different ways. Species that were preserved along with the hominins provide a rough estimate, based on when these species went extinct. More exact methods include the timing of magnetic reversals, which place it as between 1.95 and 1.78 million years ago, while uranium/lead dating places the age at just over 2 million years. In any case, it's fair to say that the fossils are approximately 2 million years old, right about the time of the origins of Homo.

What are we looking at?

It has become clear over recent decades that our ancestors evolved the features we now consider human feet first. Most of the species that precede Homo seem to have a lower body well adapted to upright walking, while retaining a relatively small brain and a combination of long arms and robust shoulders that would enable them to move easily in trees. The new skeletons share these features, which has compelled the authors to place them into the Australopithecus genus.

A comparison of features to other species in this genus indicates to the authors that the skeletons group most closely with Australopithecus africanus—so closely, in fact, that they suggest they descend from that species. But, they argue, there are significant features—greater adaptation to walking upright, smaller teeth and cheek bones—that are shared with Homo that it warrants a new species designation: Australopithecus sediba ("sediba" meaning "wellspring" in a local language).

Because of the features it shares with early Homo species, the authors also argue that the Homo genus evolved from sediba ancestors. Obviously, that's the sort of bold claim that's likely to fire up a scientific controversy. But, it appears that there will be arguments over just about every aspect of these fossils.

Let the rumble begin

Science has produced an accompanying news article that includes thoughts from a number of people who study human origins, and it seems there's disagreement over whether the authors are even correct to assign this species to Australopithecus; a significant group of researchers are quoted as saying it actually belongs among the Homo. Others accept the authors' opinion here, while at least one points out that the Homo-like features are largely apparent in the juvenile skeleton, and suggests the similarities would be less pronounced as it matured. If that were the case, the skeletons might represent the well known Australopithecus africanus.

The challenge of figuring out relationships here is illustrated by some of the extensive tables present in the paper. All together, 10 species and dozens of skeletal features show up in these tables. The net result, the authors conclude, supports their placement of the species. But it's easy to see how someone could focus on a subset of these—presumably because they view these features as more significant or diagnostic—and reach a different one.

Three other issues complicate matters. The first is the matter of age. A given fossil isn't going to be the first or last of its kind. So, although fossils that are considered to be Homo date from the same time that A. sediba, the authors' argument hinges on the assumption that their new species was around much earlier—we just don't have fossils from earlier examples. Of course, the same argument could be applied to essentially any of the early Homo species, and doing so would leave A. sediba off on a (relatively) uninteresting side-branch of human evolution.

The other issue is the erratic and generally fragmentary nature of many of the fossils of this era. We do have some remarkably intact skeletons (including the ones described here), but few of them are anywhere near complete, so the full suite of a species' features, and the variability within a species, has to be inferred by grouping samples that are thought to be the same species in order to construct a complete picture. Of course, if there are basic disagreements among paleontologists over which samples to group, all that becomes complicated.

The final issue is that both of the above are especially acute when it comes to the earliest Homo species. Homo erectus, the most successful among the genus in terms of longevity, appears very early in the fossil record relative to species that are thought to be its ancestors, like (possibly) Homo habilis. There are arguments over whether H. rudolfensis and H. ergaster represent distinct species. And, there are many fragmentary skeletons that, much like A. sediba, seem to contain a mix of features from both genuses.

All in all, it's a recipe for arguments.

The one thing that just about everyone agrees on is that Australopithecus sediba is a fantastic find. They're reasonably complete skeletons with lots of interesting features, and there are indications that at least two other skeletons have been found, but aren't described in this paper. The new species is unlikely to settle any arguments, but it will allow scientists to argue from a more informed perspective, which is all anyone can ask for.

Science, 2010. DOI: 10.1126/science.1184944

DOI: 10.1126/science.1184950 (About DOIs).

Listing image by Brett Eloff/Lee Berger