Science: Australopithecus sediba May Have Paved the Way for Homo
Researchers have revealed new details about the brain, pelvis, hands, and feet of Australopithecus sediba, a primitive hominin that existed around the same time early Homo species first began to appear on Earth. The new Au. sediba findings, unearthed in Malapa, South Africa, make it clear that this ancient relative displayed both primitive characteristics as well as more modern, human-like traits.
Due to the “mosaic” nature of the hominin’s features, researchers are now suggesting that Au. sediba is the best candidate for an ancestor to the Homo genus.
The discoveries are casting doubt on some long-standing theories about human evolution, including the notion that the early human pelvis evolved in response to larger brain sizes. And there is also some new evidence suggesting that Au. sediba may have been a tool-maker.
These new findings, which include the most complete hand ever described in an early hominin, one of the more complete pelvises ever discovered, and brand new pieces of the foot and ankle, are detailed in five separate studies in the 9 September issue of the journal Science.
The papers also narrow the age of the Au. sediba fossils to about 1.977 million years, which predates the earliest appearances of Homo-specific traits in the fossil record. Until now, fossils dated to 1.90 million years ago—and mostly attributed to Homo habilis and Homo rudolfensis—have been considered ancestral to Homo erectus, the earliest undisputed human ancestor. But the older age of the Au. sediba fossils raises the possibility of a separate, older lineage from which Homo erectus may have evolved.
Lee Berger, the project leader from the University of the Witwatersrand in Johannesburg, South Africa, explained what the Au. sediba findings mean for the ancestry of modern humans: “The many advanced features found in the brain and body, along with the earlier date, make it possibly the best candidate ancestor for our genus—the genus Homo—more so than previous discoveries, such as Homo habilis.”
Over the years, the caves of Malapa, located nearly 30 miles northwest of Johannesburg, have provided a rich assemblage of early hominin fossils. They are part of the Cradle of Humankind, which has been recognized as a World Heritage Site for its physical and cultural significance. Last year, Berger and colleagues announced the discovery of the remains of a juvenile male (MH-1) and an adult female (MH-2) Au. sediba found together in one of the Malapa caves.
Since the fossils are too old to be dated themselves, researchers turned to the calcified sediments that have kept the fossils so well-preserved. The fossils hadn’t moved since they were cemented into place, and researchers were able to identify flowstones above and below them. Robyn Pickering from the University of Melbourne in Victoria, Australia, and colleagues used advanced uranium-lead dating techniques and palaeo-magnetic dating, which measures how many times the Earth’s magnetic field has reversed, to determine the age of the surrounding rocks.
“This allowed us to narrow the deposition of the Au. sediba-bearing deposits to one of these short geomagnetic field events, the Pre-Olduvai event at about 1.977 million years ago,” Pickering said.
The old age of these fossils somewhat surprised the researchers, given some of the seemingly modern, Homo-like features that Au. sediba already possessed.
Kristian Carlson from the University of the Witwatersrand and colleagues took a look at the partial skull of MH-1 and made an endocast, or a detailed scan, of the space where the juvenile’s brain would have been. The scan was conducted at the European Synchrotron Radiation Facility in Grenoble, France.
“The actual brain residing within a cranium does not fossilize,” said Carlson. “Rather, by studying the impressions on the inside of a cranium, palaeontologists have an opportunity to estimate what the surface of a brain may have looked like. By quantifying how much volume is contained within a cranium, palaeontologists can estimate the size of a brain.”
According to researchers, the young australopith would have been around 10 to 13 years old, in human developmental terms, at the time of his death.
The researchers found that the brain of the juvenile was human-like in shape, but still much smaller than the brains seen in Homo species. The orbitofrontal region of the brain directly behind the eyes shows some signs of neural reorganization, which perhaps indicates a rewiring toward a more human-like frontal lobe, according to the researchers.
The analysis casts doubt upon the long-standing theory of gradual brain enlargement during the transition from Australopithecus to Homo. Instead, the findings corroborate an alternative hypothesis, which proposes that a reorganization of the neurons in the orbitofrontal region allowed Au. sediba to evolve while keeping its smaller cranium intact.
A separate study of the partial pelvis of MH-2 echoes that sentiment. Job Kibii from the University of the Witwatersrand and colleagues say that Homo pelvises could not have evolved in response to expanding cranial capacity. In fact, the Au. sediba’s pelvis was already developing modern, Homo-like features when its brain and skull were still small.
“It’s clear there could be two things driving the evolution of the pelvis in our Homo lineage,” said Steven Churchill from Duke University in Durham, North Carolina, a co-author of the paper. “One is bipedal locomotion. Between six and two million years ago, we begin to see a lot of it. The other thing is our big brains.”
“Our brains have to pass through the pelvis [during childbirth], so accommodations must be made,” continued Churchill. “What’s cool about sediba is their pelvises are already different from other australopiths, and yet they’re still small-brained…It’s hard to imagine that there’s no change in locomotion behind all this.”
Like most other aspects of Au. sediba, the hominin’s hands and feet display an interesting mix of both primitive and modern features. The wrist and hand of MH-2 are only missing a few bones, making them the most complete hand fossils for an early hominin on record. Tracy Kivell from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues analyzed the female Au. sediba’s hand and found evidence of strong muscles for grasping, which hints at tree-climbing. But the hand also had a long thumb and short fingers, which is a sign of precision gripping that involves the thumb and fingers, but not the palm. It’s even possible that Au. sediba had dabbled with tool-making, the researchers say.
“The hand is one of the very special features of the human lineage, as it’s very different from the hand of the apes,” said Kivell. “Apes have long fingers for grasping branches or for use in locomotion, and thus relatively short thumbs that make it very difficult for them to grasp like a human.”
“Au. sediba has, in contrast, a more human-like hand that has shortened fingers and a very long thumb,” Kivell continued. “Although at the same time, it appears to have possessed very powerful muscles for grasping. Our team interpreted this as a hand, capable of tool manufacture and use, but still in use for climbing and certainly capable of human-like precision grip.”
An analysis of the feet and ankles of MH-1 and MH-2 demonstrate that Au. sediba probably climbed trees sometimes and practiced a unique form of bipedal walking. Bernhard Zipfel from the University of the Witwatersrand and colleagues say that the MH-2 ankle is one of the most complete hominin ankles ever found—and at the same time, no ankle has ever been described with so many primitive and advanced features. “…If the bones had not been found stuck together,” Zipfel said, “the team may have described them as belonging to different species.”
This mix of modern and primitive characteristics evokes the image of a hominin who helped to usher in the various Homo species two million years ago. But only time (and more research) will tell exactly how MH-1 and MH-2 were related to our own human lineage.
“Science is pleased to publish these papers, which add important new information regarding this species, who lived during an important time in human evolution,” said Brooks Hanson, Science’s deputy editor of physical sciences. “Well-preserved and complete early human fossils are so rare, and Australopithecus sediba now provides a detailed look at some key parts of the anatomy, such as the hand and foot which are rarely well-preserved.”
Read the abstract, “Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo,” by Robyn Pickering and colleagues.
Listen to a related Science Podcast about Au. sediba.
Listen to a 7 September teleconference with the Science authors.