Australopithecus sediba fossils dating back nearly 2 million years ago may call for a shift in thinking about the origin of man. Excavated from the Malapa caves near Johannesburg, South Africa, the fossils—including a hand, foot and ankle, a pelvis, and a partial skull—are dated and described in five papers published online today in Science (September 8). The combined findings indicate that Au. sediba had a mosaic of features, some primitive and distinctly of the genus Australopithecus, while others resembled those of Homo.
“If we found [the specimens] as separate parts, we’d probably think they came from different species or maybe even different genera of early human,” said Steven Churchill, an evolutionary anthropologist at Duke University and a co-author on four of the five papers.
The fossils, found in a layer of sediment that dated to 1.98 million years ago, suggest that Au. sediba may have more features in common with modern Homo than Homo habilis or Homo rudolfensis, prompting researcher to rethink the evolutionary relationships among ancient hominins, including the first members of the Homo genus. “The fossil record of early Homo is a mess,” Churchill said. “Sediba provides an opportunity for us to step back and say ok, what do we really know?”
Among the fossils found in the Malapa caves was a remarkably well-preserved skull of an adolescent Au. sediba boy. Although much of back portion of the skull was missing, the area encasing the front of the brain—the seat of abstract thought that is so well-developed in modern humans—was intact. Brain tissue does not fossilize, but its whorls and folds leave impressions on the inner surface of the skull bones, providing clues about a brain’s complexity, as well as its size.
“We can compare patterns in the arrangement of the brain’s bumps and valleys,” in order to place it in evolutionary context, said Kristian Carlson, the anthropologist at University of the Witwatersrand who led the team investigating the brain. To do this, the team produced a digital endocast, recreating the brain imprint left on the Au. sediba boy’s skull using X-ray scans.
Previous work suggested that increasing size and complexity occurred simultaneously, and gradually. But there’s a gap between fossils of the larger-skulled Homo erectus, dating to less than 2 million years ago, and Australopithecus skulls, mostly older than 2.3 million years, which are smaller in size and more chimpanzee-like in design. This interval is littered with contentiously dated fossil fragments that fail to clarify Homo’s origins, said Carlson. Au. sediba’s skull, dated right in the middle at 1.98 million years old, was “surprisingly small,” though already showing signs of reorganization, he said. The structural differences between the newly discovered brain impression and two endocasts of A. africanus (another Homo predecessor), such as the partial projection of the frontal lobe, “foreshadow” characteristics of Homo skulls, and nominate Au. sediba as a transitional species preceding H. erectus, explained Darryl J. de Ruiter, a paleoanthropologist at Texas A&M University, who participated in the skull’s analysis.
Not everyone agrees that Au. sediba was quite so advanced, however. Dean Falk, a paleoanthropologist at the School for Advanced Research in Santa Fe, New Mexico, and at Florida State University, who was not involved in the research, said Au. sediba is an “exciting” find, but she is not convinced that its brain is any closer to human than Au. africanus’s brain is. It’s certainly possible, she said, but more comparisons are necessary to establish that Au. sediba is indeed a transition to Homo.
Hands & Feet
Like the fossilized skull, the hands and feet of the Au. sediba specimens showed a surprising combination of both modern and primitive features, according to the authors. A well-preserved partial foot and ankle hints at a unique form of bipedalism—one of the most critically defining characteristics of the Homo genus—combined with some degree of aboreality.
Furthermore, the nearly complete hand and wrist set recovered from an adult Au. sediba female appears “hyper human,” Churchill said. Its thumb is relatively longer, as compared to the appendages of modern Homo sapiens, which themselves have proportionally longer thumbs than chimpanzees. Like humans, this would allow Au. sediba to exert pressure when manipulating objects.
The orientation of the wrist bone and joints also suggest a structure that is “good at withstanding the forces you get when you’re knocking rocks together,” Churchill said. The researchers think that Au. sediba not only used stone tools, but perhaps made them, too. “We’ve found tools at the site,” Churchill said, “but we haven’t found any tools in situ so we can’t say for sure.”
Other hominins, like Homo habilis, also evolved anatomical structures for tool usage, but not in the same ways. H. habilis had a mobile thumb and robust, curved finger bones, for example, but lacked Au. sediba’s well-developed muscle structure. This suggests that parallel but different tool-oriented morphologies arose in the same period, Churchill said, meaning researchers may need to reevaluate expectations of what the tool-using hand looked like in this early time window.
Analysis of the partial adult female Au. sediba pelvis also revealed features that closely resemble the broader pelvises of modern humans. Anthropologists typically attribute the broadening of the pelvis early in hominin evolution to the advent of bipedal locomotion, Churchill said, with further growth later in evolution to allow for the birthing of babies with larger skulls. Au. sediba, however, is a small-brained hominin, suggesting that the species' broad pelvis is purely a result of a change in the biomechanical forces. Researchers speculate that Au. sediba’s formerly tree-dominated landscape started shifting towards savannah around this time, allowing Au. sediba to expand its range. The increased walking would have created a selection pressure on the pelvis for more efficient bipedal locomotion.
Researchers like Scott Simpson, an anatomist at Case Western Reserve University who was not involved in the study, agree that the fossils are “extraordinary pieces” that will bear on our understanding of ancient human variation, but called for a direct comparison between the Au. sediba pelvis and those of early Pleistocene Homo specimens. “My concern is that [the researchers] have perhaps set the model as an either/or explanation, locomotion or obstetrics,” Simpson said. Because only a handful of specimens have ever been recovered, he said, researchers’ understanding of ancient pelvic anatomy contains many gaps. Therefore, the specimens “absolutely, positively require a very thorough and careful analysis.”
Still, a great fossil like Au. sediba is one that does not meet preconceptions. “Paleontologists are used to being fed scraps of fossils,” Simpson said. “It’s like a feast getting a skeleton of this magnitude. After a lifetime of crumbs, it’s a welcome meal.”
K.J. Carlson, et al., "The endocast of MH 1, Australopithecus sediba," Science, 333:1402-07, 2011.
J.M. Kibii, et al., "A partial pelvis of Australopithecus sediba," Science, 333:1407-11, 2011.
T.L. Kivell, et al., "Australopithecus sediba hand demonstrates mosaic evolution of locomotor and manipulative abilities," Science, 333:1411-17, 2011.
B. Zipfel, et al., “The foot and ankle of Australopithecus sediba," Science, 333:1417-20, 2011.
R. Pickering, et al., “Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo," Science, 333:1421-23, 2011.