Australopithecus sediba lower jaws

Lower jaws, or mandibles, attributed to early human ancestor Australopithecus sediba differ in the shape of the so-called mandibular notch--the indentation in the top portion of the bone. Two paleoanthropologists contend that the difference signals that the top mandible actually comes from a member of our own genus, Homo, whereas the bottom one comes from Australopithecus. Image: Kate Wong; casts of A. sediba mandibles courtesy of Darryl de Ruiter

CALGARY—In 2010 paleoanthropologists announced to great fanfare that they had recovered from a South African cave two partial skeletons of a previously unknown member of the human family that lived nearly two million years ago. The skeletons—a young male and an adult female referred to as MH1 and MH2, respectively--were said to exhibit a striking melange of traits associated with our own genus, Homo, and traits characteristic of the older, more apelike genus Australopithecus, from which Homo presumably evolved. Its discoverers thus proposed that the creature, which they named Australopithecus sediba, could be a transitional species between the two groups, and thus the long-sought ancestor of Homo or a close relative of that ancestor. Critics countered that the fossils were too young to be ancestral to Homo, among other objections to that evolutionary scenario. Now two researchers have put forth an entirely different interpretation of the remains.

Addressing the Paleoanthropology Society in Calgary last week, Yoel Rak and Ella Been of Tel Aviv University made the case that the bones do not represent a transitional species but instead come from two different genera. In his presentation, Rak focused on the form of the lower jaw, or mandible. Specifically he looked at the dimensions of the so-called mandibular notch--an indentation in the uppermost portion of the bone. According to Rak, this aspect of the mandible differs reliably between Australopithecus and Homo, and the two lower jaws of the A. sediba individuals differ along those lines. MH1 has a mandibular notch like that of Australopithecus; MH2 has one like Homo’s.

Been, for her part, assessed the four lumbar vertebrae of A. sediba and arrived at a similar conclusion. For each of the specimens she considered the front-to-back length of the central portion of the bone (the vertebral body) against the height of the entire vertebra. Comparing the resulting ratios to those obtained for lumbar vertebrae from modern Homo, early Homo and Australopithecus, she determined that the MH1 lumbar vertebrae align with Homo yet the MH2 lumbars group with Australopithecus.

That MH1 seems to have an Australopithecus-like mandibular notch and Homo-like lumbars, whereas MH2 combines a Homo-like mandibular notch with Australopithecus-like lumbars indicates to Rak and Been that each alleged skeleton is actually a mix of bones from Homo and Australopithecus, for a total of four individuals in all.

Australopithecus sediba skeletons MH1 (left) and MH2 (right) flank the famous Australopithecus afarensis skeleton known as Lucy (center). Image: Compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand/Via Wikimedia Commons

That interpretation does not sit well with members of the Malapa team. Darryl de Ruiter of Texas A&M University acknowledges that the shape of the mandibular notch differs in MH1 and MH2. He disputes the idea that this single feature can be used to diagnose genera, however. “The pattern is real,” he says, “but the meaning [Rak] attaches to it is where I disagree.” Likewise Marc Meyer of Chaffey College notes that he and others tasked with evaluating the A. sediba vertebrae take 70 measurements per bone and generate dozens of indices. “I can cherry-pick one index and turn sediba into a chimp; another turns it into a human,” he comments. “Generating a taxon-level conclusion from one index is pre-mature.”

One of the challenges paleontologists face in dealing with the imperfect fossil record is that of distinguishing variation within a species from variation between species. Complicating matters in the case of A. sediba, MH1 is a juvenile and MH2 is an adult, which means that some of the differences between them may simply reflect the incomplete growth and development of MH1.

But most problematic for the argument that MH1 and MH2 comprise two genera and four individuals is the fact that the specimens were found as articulated skeletons, with the bones in anatomical alignment, and without any duplication of parts to indicate that more than two individuals are represented. “There are not two genera,” states Lee Berger of the University of the Witwatersrand, who led the recovery of the fossils. He points out that MH1 and MH2 share a number of characteristics with each other that are not seen in any other member of the human family. “The time has come to stop looking for silver bullet traits for questions of higher taxonomy,” Berger asserts. “When you have fantastic partial skeletons you need to look across the entire skeleton.”