Randolph E. Schmid of AP wrote in 2011:“An analysis of 2 million-year-old bones found in South Africa offers the most powerful case so far in identifying the transitional figure that came before modern humans — findings some are calling a potential game-changer in understanding evolution. The bones are from Australopithecus sediba. The research places that pre-human branch of the evolutionary tree as the best candidate to be the ancestor of the human line, said Lee R. Berger of the University of Witwatersrand in South Africa. [Source: Randolph E. Schmid, AP, September 8 2011]
The bones, found in 2008 in the fossil-rich cave region of Malapa, 40 kilometers northwest of Johannesburg, show a head-to-foot combination of features of Australopithecus and the human genus, Homo. "It's as if evolution is caught in one vital moment, a stop-action snapshot of evolution in action," said Richard Potts, director of the human origins program at the Smithsonian Institution. He was not among the international research team, led by South African scientists. Their research was published online in the journal Science. The journal published five papers detailing the findings, including separate reports on the foot, hand, pelvis and brain of A. sediba.
Scientists have long considered the Australopithecus family, which includes the famous fossil Lucy, to be a primitive candidate for a human ancestor. The new research establishes a creature that combines features of both groups. Berger said the brain, hand and foot have characteristics of both modern and early pre-human forms that show a transition under way. It represents a bona fide model that could lead to the genus Homo, Berger said.
Berger has argued that Australopithecus sediba may represent an intermediate form between the primitive australopiths and our genus, Homo. Josh Fischman wrote in National Geographic magazine, “The evidence they point to includes an australopith's little brain (with some curiously modern features), apelike shoulders, and arms adapted to climbing in trees—attached to a bizarrely modern hand with the precision grip of a toolmaker. According to the researchers, the adult female's foot presents an even odder melange; her mostly modern ankle is connected to a heel bone more primitive than that of A. afarensis—Lucy's species—which is at least a million years older. In a science known for its contentiousness, such a claim will surely not go unchallenged. But no one disputes that the Malapa fossils are unprecedented. [Source: Josh Fischman, National Geographic , August 2011]
"It really is a jaw-dropping find," says Carol Ward, a paleoanthropologist at the University of Missouri who studies the evolution of apes and early hominins (a term for humans and other nonape primates; some researchers prefer the older term, hominid). "We have no other collection of fossil skeletons, until the Neanderthals just over 100,000 years ago, that are so articulated, so complete."
Discovery and Significance of Australopithecus sediba
The fossils of Australopithecus sediba were accidentally discovered by the 9-year-old son of a scientist in the remains of a cave in South Africa in 2008, findings detailed by researchers last year. Two key specimens were discovered — a juvenile male as developed as a 10- to 13-year-old human and an adult female maybe in her late 20s or early 30s. The species is both a hominid and a hominin — hominids include humans, chimpanzees, gorillas and their extinct ancestors, while hominins include those species after Homo, the human lineage, split from that of chimpanzees. Australopithecus means "southern ape," and is a group that includes the iconic fossil Lucy, while sediba means "wellspring" in the South African language Sotho.
When asked if Australopithecus was a missing link, Darryl J. DeRuiter of Texas A&M University said that if scientists prefer the terms "transition form" or "intermediary form": "This is what evolutionary theory would predict, this mixture of Australopithecene and Homo," DeRuiter said. "It's strong confirmation of evolutionary theory." But it's not yet an example of the genus Homo, he said, though it could have led to several early human forms including Homo habilis, Homo rudolfensis or Homo erectus. [Source: Randolph E. Schmid, AP, September 8 2011]
These articles "force a rethinking of how traits are coupled together in human evolution," the Smithsonian's Potts said. "For example, in previous definitions of our genus, the leading edge in the emergence of Homo has been brain enlargement. The sediba bones show, however, that reorganization of the brain and pelvis typically connected with the evolution of Homo need not have involved brain enlargement," he noted.
"The more we learn about human evolution, the more we see that traits" that must have happened together could occur separately, Potts said. The study of the hand shows that major changes in the thumb usually associated with stone toolmaking "did not imply abandoning life in the trees. In the foot article, we're introduced to a unique and previously unknown combination of archaic and advanced traits in sediba," Potts explained.
Unusual Mix of Primitive and Advanced Features in Australopithecus sediba
In September 2011, LiveScience.com reported: “A startling mix of human and primitive traits found in the brains, hips, feet and hands of Australopithecus sediba make a strong case for it being the immediate ancestor to the human lineage, scientists have announced. These new findings could rewrite long-standing theories about the precise steps human evolution took, they added, including the notion that early human female hips changed shape to accommodate larger-brained offspring. There is also new evidence suggesting that this species had the hands of a toolmaker. The scientists detailed their findings in the Sept. 9 2011 issue of the journal Science.
The heel bone seems primitive, the researchers said. Yet its front is angled, suggesting an arched foot for walking on the ground, and there is a large attachment for an Achilles tendon as in modern humans, they said. [Source: Randolph E. Schmid, AP, September 8 2011]
The pelvis is short and broad like a human pelvis, creating more of a bowl shape than in earlier australopith fossils like the famous Lucy, explained Job Kibii of the University of the Witwatersrand. That find may force a re-evaluation of the process of evolution because many researchers had previously associated development of a humanlike pelvis with enlargement of the brain, but in A. sediba the brain was still small.
The subjects of the research were the bones of an adult female and a child. After the discovery, the children of South Africa were invited to name the child, which they called "Karabo," meaning "answer" in the local Tswana language. The older skeleton has not yet been given a nickname, Berger said. The juvenile would have been aged 10 to 13 in terms of human development; the female was in her 20s and there are indications that she may have given birth once. The researchers are not sure if the two were related.
Australopithecus sediba Brain
Kristian J. Carlson, a paleoanthropologist at Witwatersrand Wits who is reconstructing A. sediba's brain, told AP, the brain of A. sediba is small—420 cubic centimeters—like that of a chimpanzee, but with a configuration more human, particularly with an expansion behind and above the eyes.This seems to be evidence that the brain was reorganizing along more modern lines before it began its expansion to the current larger size, Carlson said in a teleconference."It will take a lot of scrutiny of the papers and of the fossils by more and more researchers over the coming months and years, but these analyses could well be 'game-changers' in understanding human evolution," according to the Smithsonian's Potts. [Source: Randolph E. Schmid, AP, September 8 2011]
"The frontal lobes on the two halves appear to be different sizes," Carlson told National Geographic. Pronounced asymmetry between right and left brain hemispheres is a hallmark of humans, because our cerebrum has become specialized, with the left side more involved in language. On that side Carlson sees hints of a protrusion in the region of Broca's area—a part of the brain linked to language processing in modern humans. But Dean Falk from the School for Advanced Research in Santa Fe, an expert on fossil endocasts, adds the caution that Broca's area is defined by specific creases in the brain, and "it would be quite a reach" to identify it based only on a bulge. [Source: Josh Fischman, National Geographic , August 2011]
"The fossils demonstrate a surprisingly advanced but small brain, a very evolved hand with a long thumb like a human's, a very modern pelvis, but a foot and ankle shape never seen in any hominin species that combines features of both apes and humans in one anatomical package," Berger said. "The many very advanced features found in the brain and body and 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." [LiveScience.com, September 2011]
The juvenile specimen of Au. sediba had an exceptionally well-preserved skull that could shed light on the pace of brain evolution in early hominins. To find out more, the researchers scanned the space in the skull where its brain would have been using the European Synchrotron Radiation Facility in Grenoble, France; the result is the most accurate scan ever produced for an early human ancestor, with a level of detail of up to 90 microns, or just below the size of a human hair. The series of ultrahigh-resolution images create a virtual endocast: an impression of the boy's skull showing the general contours of the outer brain layer.
The scan revealed Au. sediba had a much smaller brain than seen in human species, with an adult version maybe only as large as a medium-size grapefruit. However, it was humanlike in several ways — for instance, its orbitofrontal region directly behind the eyes apparently expanded in ways that make it more like a human's frontal lobe in shape. This area is linked in humans with higher mental functions such as multitasking, an ability that may contribute to human capacities for long-term planning and innovative behavior. "We could be seeing the beginnings of those capabilities," researcher Kristian Carlson at the University of Witwatersrand told LiveScience.
These new findings cast doubt on the long-standing theory that brains gradually increased in size and complexity from Australopithecus to Homo. Instead, their findings corroborate an alternative idea — that Australopithecus brains did increase in complexity gradually, becoming more like Homo, and later increased in size relatively quickly.
Australopithecus sediba Hips
In September 2011, LiveScience.com reported: “An analysis of the partial pelvis of the female Au. sediba revealed that it had modern, humanlike features. "It is surprising to discover such an advanced pelvis in such a small-brained creature," said researcher Job Kibii at the University of the Witwatersrand. "It is short and broad like a human pelvis ... parts of the pelvis are indistinguishable from that of humans." [LiveScience.com, September 2011]
Scientists had thought the human-like pelvis evolved to accommodate larger-brained offspring. The new findings of humanlike hips in Au. sediba despite small-brained offspring suggests these pelvises may have instead initially evolved to help this hominin better wander across the landscape, perhaps as grasslands began to expand across its habitat.
When it came to walking, investigating the feet and ankles of the fossils revealed surprises about how Au. sediba might have strode across the world. No hominin 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 researcher Bernhard Zipfel at the University of the Witwatersrand.
The researchers discovered that its ankle joint is mostly like a human's, with some evidence for a humanlike arch and a well--efined Achilles tendon, but its heel and shin bones appear to be mostly ape-like. This suggested the hominid probably climbed trees yet also halkid in a unique way not exactly like that of humans. Altogether, such anatomical traits would have allowed Au. sediba to walk in perhaps a more energy-efficient way, with tendons storing energy and returning that energy to the next step, said researcher Steve Churchill from Duke University in Durham, N.C. "These are the kinds of things that we see with the genus Homo," he explained.
Australopithecus sediba Hands
In September 2011, LiveScience.com reported: “An analysis of Au. sediba's hands suggests it might have been a toolmaker. The fossils — including the most complete hand known in an early hominin, which is missing only a few bones and belonged to the mature female specimen — showed its hand was capable of the strong grasping needed for tree-climbing, but that it also had a long thumb and short fingers. These would have allowed it a precision grip useful for tools, one involving just the thumb and fingers, where the palm does not play an active part. [LiveScience.com, September 2011]
One hand specimen lacks three wrist bones and four terminal phalanges but is otherwise complete. Altogether, the hand of Au. sediba has more features related to tool-making than that of the first human species thought of as a tool user, the "handy man" Homo habilis, said researcher Tracy Kivell at the Max Planck Institute for Evolutionary Anthropology in Germany. "This suggests to us that sediba may also have been a toolmaker." Though the scientists haven't excavated the site in search of stone tools, "the hand and brain morphology suggest that Au. sediba may have had the capacity to manufacture and use complex tools," Kivell added.
The researchers do caution that although they suggest that Au. sediba was ancestral to the human lineage, all these apparent resemblances between it and us could just be coincidences, with this extinct species evolving similar traits to our lineages due, perhaps, to similar circumstances.
In fact, it might be just as interesting to imagine that Au. sediba was not directly ancestral to Homo, because it opens up the possibility "of independent evolution of the same sorts of features," Carlson said. "Whether or not it's on the same lineage as leading to Homo, I think there are interesting questions and implications."
The fossil provides the first chance for researchers to evaluate the function of a full hand this old, Tracy Kivell of the Max Planck Institute for Evolutionary Anthropology in Germany, told AP. Previously, hand bones older than Neanderthals have been isolated pieces rather that full sets. The researchers reported that the fingers of A. sediba were curved, as might be seen in a creature that climbed in trees. But they were also slim and the thumb was long, more like a Homo thumb, so the hand was potentially capable of using tools. No tools were found at the site, however.
Australopithecus sediba and the Evolution of Man
Josh Fischman wrote in National Geographic magazine, “ A. sediba's greatest promise may lie in its power to illuminate the murky origins of Homo. The birth of our genus has long been a conundrum for paleoanthropologists, to say the least. Only a few scattered and fragmentary fossils older than two million years have been argued to belong to the genus. Then, around 1.8 million years ago, not one but two or possibly even three Homo species appear, mostly in East Africa. The smaller brained, more primitive ones are called Homo habilis, or "handy man," a name given by Louis Leakey and colleagues in 1964 to specimens from Olduvai Gorge because of their association with the first crude stone tools. Some researchers group a few H. habilis specimens into a separate species, Homo rudolfensis. Then there is Homo erectus (the early African forms are sometimes called Homo ergaster)—larger brained, bigger bodied, more advanced, yet contemporaneous with little H. habilis. [Source: Josh Fischman, National Geographic , August 2011]
Where did all these characters come from? Attempts to look deeper into the past only increase the frustration, says William Kimbel, a paleoanthropologist at Arizona State University and director of the Institute of Human Origins there. "There are only a handful of specimens. You could put them all into a small shoe box and still have room for a good pair of shoes," he says. An upper jaw from Hadar in Ethiopia, found by Kimbel himself, is 2.3 million years old. A lower jaw from Malawi may be 100,000 years older, though the dating is uncertain. Some researchers would include a skull piece from Kenya of about the same age. That's about it.
Enter the skeletons of A. sediba—as resplendently well preserved as those shoe box fossils are not. Anatomically, the species shows a mix of primitive and advanced traits. In addition to its long upper limbs, small brains, and primitive heel bone, its small body size and the shape of its molar cusps and cheekbones hark back to earlier australopiths, such as A. africanus, that lived in southern Africa between two and three million years ago. (Indeed, some researchers suggest that it might be a late form of that species.)
The long legs and that modern ankle are key elements on the human side of the ledger, says Darryl de Ruiter, a paleoanthropologist at Texas A&M University and part of the Malapa team. He also cites the surprisingly humanlike pelvis built for a fully bipedal stride; smaller teeth and chewing muscles; a projecting nose and some other features of the face; and that remarkable, precision-grip hand. These traits are enough for the team to propose it as the australopith species most likely to have given rise to Homo.
But which Homo? The team leans very cautiously toward Homo erectus, the species generally seen as the immediate forerunner of Homo sapiens. If this is so, then the smaller, mostly East African forms now attributed to Homo, including Louis Leakey's original toolmaker H. habilis, would become a branch of the family tree that simply petered out. It is not the first time scientists have suggested these species could be evolutionary dead ends. But the Malapa fossils bring more clout to the debate. "Sediba casts everything called Homo before erectus into question," says de Ruiter.
The biggest obstacle facing this challenge to the establishment view is the timing. If two-million-year-old A. sediba is indeed the true ancestor of Homo, how could it give rise to those even older fossils assigned to Homo in Bill Kimbel's shoe box? A fossil cannot be ancestral to something older than itself any more than a daughter can give birth to her own mother. One possibility is that the Malapa specimens represent a late stage of an enduring species that gave rise to Homo at an earlier date. But Berger's team questions whether that shoe box really contains any Homo fossils in the first place—after all, they're just fragments. Kimbel doesn't buy it.
"It's nonsensical to dismiss fragments, because fragments do tell you something," he says. He points out that the upper jaw from Hadar has a short, broad, humanlike dental arch and flat snout, placing it firmly in the Homo genus—and it is at least 300,000 years older than A. sediba. Berger's team, however, insists that Malapa changes the game. Articulated skeletons are far more than the sum of their parts: They prove that parts in isolation can be misleading. Think of the bits of A. sediba that look primitive, and the other bits that look modern, he says. The Hadar jaw, in the same way, might not accurately represent the rest of the creature.
"How can the Hadar jaw be misleading?" says Kimbel. "Either it shares features with later Homo, or it does not. Nothing in sediba can change that." If the Hadar jaw really is Homo, says Berger, then perhaps its dating is wrong—a contention Kimbel disputes as vigorously as he supports the validity of his fossil.
The truth about A. sediba's place in our ancestry may still be lying in the ground. "The beauty of a place like Malapa is that there are many more bones, and more individuals to come," Berger says. Ultimately the fossils, not the arguments, will carry the day.
Malapa Caves: Australopithecus sediba Discovery Site and Maybe a Source of Hominid Skin
Describing the site where the Australopithecus sediba fossils were found, Josh Fischman wrote in National Geographic magazine, “It is a hole in the ground about 25 miles northwest of Johannesburg, in a ridged brown valley where herds of giraffes occasionally parade between stands of trees. The red-rock walls of the pit are higher than Berger's head, and steep enough in spots to make a scramble up, or down, rather daunting. Some two million years ago, the hole was a great deal deeper, with no possibility of escape for any creature that fell in. This accounts for the trove of fossils Berger is finding, which in turn accounts for his upbeat mood. He leans over a red boulder near the pit bottom, tracing a white-colored protrusion with his fingers. "It looks like part of an arm," he says. "That means we've found another individual." [Source: Josh Fischman, National Geographic , August 2011]
The first two skeletons removed from the pit were a young adolescent male, 12 or 13 years old, and an adult female... The site, an eroded limestone cave called Malapa, is in a region already so famous for its ancient human fossils that it is often referred to as the Cradle of Humankind. Much of that reputation rests on finds from the early 1900s, back when South Africa harbored the best evidence for early human evolution, including Australopithecus africanus, at the time our oldest known ancestor. Beginning in the late 1950s, the epochal finds of the Leakey family in Tanzania and Kenya, followed later by Donald Johanson's celebrated discovery of the 3.2-million-year-old Lucy skeleton in Ethiopia, shifted cradle-bragging rights to East Africa, where they have remained ever since. Lee Berger thinks the cradle is about to rock again. He believes Malapa may hold the key to one of the most significant, least understood chapters in the human evolutionary journey: the origin of the first species enough like us to be called human—a member of the genus Homo. "This is where that story may have begun," he says, as he starts the climb out of the pit.
The age of the fossils was determined by dating the calcified sediments surrounding them with advanced uranium-lead dating techniques and a method called paleomagnetic dating, which measures how many times the Earth's magnetic field has reversed. They discovered the fossils were approximately 1.977 million years old, which predates the earliest appearances of traits specific to the human lineage Homo in the fossil record. This places Au. sediba in roughly the same age category as hominids such as Homo habilis and Homo rudolfensis, which were thought to be potential ancestors to Homo erectus, the earliest undisputed predecessor of modern humans.
Malapa Caves: A Water Source for Australopithecus sediba
Josh Fischman wrote in National Geographic magazine, “The abundance and spectacular condition of the fossils have much to do with the peculiar geography of the place. Malapa, it seems, was both a water source that gave life and a trap that snuffed it out. Two million years ago, a cave-studded aquifer lay beneath an undulating plain of shallow, wooded valleys and rolling hills. Some of the caves were open to the surface through steep entryways or vertical shafts stretching up to 160 feet. In wet periods, when the water table was high, animals could easily drink from seepage ponds near the surface. During drier times they would venture into the darkness of a hole, following the sound or scent of water—and risking a plunge down a hidden shaft. (The boy's upper arm bones show fractures typical of a headfirst fall from a great height.) [Source: Josh Fischman, National Geographic , August 2011]
"These animals had no choice. They needed water to survive," says Brian Kuhn, a zoologist from the Johannesburg university—called Wits for short—who works at the Malapa site. After death, their bodies would wash down even deeper in the cave system, becoming entombed within days or weeks in a single, thick layer of sand and clay, rather than a succession of thin layers, as would have happened had the sediments accumulated over months or years.
This raises the possibility, says Berger, that all the hominins—at least four are now known from the site—died weeks or even days apart, and therefore may have known each other in life. The rapid burial also caused their flesh to take longer to decompose, packaging the skeletons in death as they were arranged in life, right down to tiny bones of the hands and feet. Indeed, the rapid entombment may have preserved some of the skin itself, on top of the boy's skull and on the woman's jaw near the chin—something never before seen in a hominin fossil.
"Wow!" says Nina Jablonski, an anthropologist at Pennsylvania State University and author of the book, Skin: A Natural History. "The possibility of preserved australopithecine skin is massively cool." What makes it so cool is the possibility of determining how these near humans reacted to heat. She is particularly interested in whether the alleged skin (or a fossilized impression of the skin, if that's what it is) might contain evidence of scalp and facial hair, and a high density of sweat glands.
Jablonski thinks such glands could be a precondition to the bigger brains long seen as a defining attribute of Homo. Chimpanzees, our closest living relatives, spend most of their time sheltered from the sun's heat by forest cover and have a limited ability to sweat. Our earliest ancestors also typically occupied woodland environments. But as the environment became drier around two million years ago, they began to forage in more-open grasslands—a problem for brains, which are notoriously vulnerable to heat. Bigger brains require even more cooling. A marked increase in the number of sweat glands and a reduction in body hair could have provided that, Jablonski speculates, in turn allowing for further brain growth as Homo began to use those bigger brains for toolmaking, planning, and other cognitively challenging activities.
Image Sources: Wikimedia Commons
Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
© 2008 Jeffrey Hays
Last updated January 2012