The discoveries are casting doubt on some long-standing theories about human evolution, including the notion that early human pelvises evolved in response to larger brain sizes. And there is also some new evidence suggesting that Australopithecus 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. The Australopithecus sediba research includes a high-resolution scan of an early hominin's cranium along with work that refines the date of this early hominin site in Malapa, South Africa, to nearly 2.0 million years ago, close to the emergence of Homo.
Lee Berger, the project leader from the University of the Witwatersrand says, "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."
The cranium of the juvenile skeleton of Australopithecus sediba. Credit: Brett Eloff. Provided by Lee Berger and the University of Witwatersrand
The age of these Au. sediba fossils has been constrained 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 these Au. sediba fossils raises the possibility of a separate, older lineage from which Homo erectus may have evolved.
"Science is pleased to be publishing these papers, which add important new information regarding this species, who lived during an important time in human evolution and was first described in the 9 April 2010 issue," according to Brooks Hanson, deputy editor of physical sciences. "Well-preserved and complete early human fossils are so rare, and Au. sediba now provides a detailed look at some key parts of the anatomy, such as the hand and foot which are rarely well-preserved."
The caves of Malapa, nearly 30 miles northwest of Johannesburg, have provided a rich assemblage of early hominin fossils over the years. They are part of the Cradle of Humankind, which has been recognized as a World Heritage Site and set aside 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 that were found together in one of these 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. So, Robyn Pickering from the University of Melbourne in Victoria, Australia, and colleagues used advanced uranium-lead dating techniques and something called palaeo-magnetic dating, which measures how many times the Earth's magnetic field has reversed, on 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," wrote Pickering.
Pelvis of Australopithecus sediba. Credit: Peter Schmid. Courtesy of Lee Berger and the University of Witwatersrand
The old age of these fossils somewhat surprised the researchers, given some of the apparently Homo-like features that Au. sediba was already displaying at the time.
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 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 exceptionally well-preserved cranium of MH-1 was scanned at the European Synchrotron Radiation Facility, revealing internal anatomy with the highest achievable precision and contrast," continued Carlson. "The European Synchrotron Radiation Facility is the most powerful installation worldwide for scanning fossils, setting the standard for what can be achieved during non-destructive studies of internal structures of fossils."
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. Carlson's results cast doubt upon the long-standing theory of gradual brain enlargement during the transition from Australopithecus to Homo. Instead, their findings corroborate the 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.
The right hand skeleton of the adult female MH2 against a modern human hand. The hand, seen here in palmar view, lacks three wrist bones and four terminal phalanges, but is otherwise complete. Credit:Peter Schmid. Courtesy of Lee Berger and the University of Witwatersrand
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 their expanding cranial capacity. In fact, Au. sediba's pelvis was already developing modern, Homo-like features when their brains and skulls 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, 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."
And 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 that there was a strong flexor apparatus, which hints at tree-climbing. But, the hand also had a long thumb and short fingers, which is a sign of precision gripping—a grip that just involves the thumb and fingers, but not the palm. It's even possible that Au. sediba had already started dabbling 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," continued Kivell. "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."
The findings don't mean that Au. sediba was the only hominin around two million years ago who was capable of making stone tools, though. (Homo habilis, or the "handy man," was on the scene, but this hominin had a very different hand structure.) These latest findings do indicate, however, that different hominins with various hand morphologies may have been around at the same time, fashioning simple tools.
Finally, 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, the team may have described them as belonging to different species," said Zipfel.
The ankle joint is largely like a human's, with some evidence for a human-like arch and a well-defined Achilles tendon, according to the researchers. However, its heel and shin bone appear to be mostly ape-like.
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.
"The fossil record for early Homo is a mess," said Churchill. "Many fossils are either questionably attributed to various species or their dating is very poor… Au. sediba has a number of derived characteristics, which it shares with the genus Homo. If you were to make a list of these shared traits—including those seen in habilis, rudolfensis and sediba—the list for sediba would be much longer than the other two, which suggests it's a good ancestor of the first species that everyone recognizes in the Homo genus: H. erectus."