Why Australopithecus sediba could rewrite our evolutionary history

Just under two million years old, Australopithecus sediba has attracted attention ever since its 2008 discovery because of its mix of ancient and modern traits. It's been hailed as the direct ancestor of the Homo genus...but that might be impossible.

What makes sediba's placement in our evolutionary family tree so difficult is where it came from. Paleoanthropologist Lee Berger and his team at Johannesburg's University of the Witwatersrand found the specimen in the caves of South Africa, which places it very far away from the East African locations of most key Homo discoveries - including a few isolated finds that seem to predate Australopithecus sediba.

Scientific American has an excellent, lengthy overview of both the discovery of sediba and the fierce, ongoing anthropological debate about whether it really is the ancestor of Homo or essentially just an evolutionary dead end that just happens to have some Homo-like features. This passage gives some sense of just how complicated it is to sort out this mystery, and how these anthropologists do not mince their words in questioning their opponents' arguments:

Maybe the East [Africa] story of human origins is wrong. The traditional view of South Africa's oldest hominin fossils is that they represent a separate evolutionary experiment that ultimately fizzled out. A. sediba could turn the tables and reveal, in South Africa, another lineage, the one that ultimately gave rise to humankind as we know it (indeed, sediba is the Sesotho word for "fountain" or "wellspring").

William Kimbel of Arizona State University, who led the team that found the 2.3-million-year-old jawbone in Ethiopia, is having none of it. The idea that one needs a skeleton to classify a specimen is a "nonsensical argument," he retorts. The key is to find pieces of anatomy that contain diagnostic traits, he says, and the Hadar jaw has features clearly linking it to Homo, such as the parabolic shape formed by its tooth rows. Kimbel, who has seen the Malapa fossils but not studied them in depth, finds their Homo-like traits intriguing, although he is not sure what to make of them. He scoffs at the suggestion that they are directly ancestral to H. erectus, however. "I don't see how a taxon with a few characteristics that look like Homo in South Africa can be the ancestor [of Homo] when there's something in East Africa that is clearly Homo 300,000 years earlier," he declares, referring to the jaw.

There are a bunch more fascinating arguments - both for and against Australopithecus sediba - over at Scientific American.

Photo by Brett Eloff, courtesy Profberger and Wits University via Wikimedia.