Newly Described Hominins Prove Difficult to Classify

Image credit: Brett Eloff and Lee Berger

Three years ago, the nine-year-old son of palaeoanthropologist Lee Berger found an intriguing fossil in a collapsed Malapa cave — the collarbone of a 2 million year-old hominin previously unknown to science. Further searches of the same site revealed 220 more bones of the same species, from infants to adults, with two particularly complete young adults whose combined remains gave researchers a clear image of their anatomy. In addition to the multitude of fragments (including a near-complete skull, shown above), the fossils were remarkably well-preserved — so well that at least one man speculates that they contain preserved soft tissues.

The new species was named Australopithecus sediba (sediba being wellspring, in reference to the Malapa location), as the physical proportions and brain size seemed very similar to Australopithecus afarensis — the species best represented by Lucy. And yet, since A. sediba‘s announcement in 2010, its location on the evolutionary tree has been debated and disputed and remains unsettled. Five papers published in Science this Thursday (Sep 8), which describe the skull, pelvis, hands and feet of the ancient hominin, don’t seem to offer any conclusive answers.

So what’s the big debate, you ask? Well, it seems our friend A. sediba had a few modern features. Though at first glance it appears to be built for an arboreal lifestyle, some interesting discrepancies show up in closer inspection. In addition to the obviously Australopithecine small brain/long limb features, A. sediba sported a human-like pelvis and hand very similar to those of Homo erectus–characteristics of the Homo genus, aka, our branch of the tree. These are the “walking upright, capable of building tools” developments which, interestingly, are not present in Homo habilis and Homo rudolfensis, species long believed to be our early, direct ancestors. So where does A. sediba belong?

“That is exactly what you’d expect when you find a very transitional form: 50% of the field saying they’re right, it’s an Australopithecine, the other half saying, put this in the genus Homo,” said Berger during a conference call announcing the new A. sediba papers.

At the time these fossils were living, breathing mammals, there were many species of humans living at the same time, including the early representatives of our own genus. The similarity of A. afarensis to A. sediba and the relatively immediate chronological appearance of the latter would naturally lead to the assumption that A. sediba is removed from modern human lineage, but the presence of human-like adaptations has us again questioning our ancestry.

[A.] sediba is more similar to us than even Homo habilis, the species that up until now was the front-runner for “earliest human ancestor” status. Sediba could be the bridge between the many australopithecines and Homo erectus.

There’s no consensus yet as to where A. sediba fits, or if or how he was related to modern humans. But the story of human lineage is certainly more interesting, if not any clearer, regardless of where our newest potential relative ends up on the tree.

Further Reading: Check out the source articles for more and detailed information.