Notorious Bones

Almost 2 million years ago in what’s now South Africa, a boy and a woman fell through a hole in the ground into an underground cave, tumbling about 50 meters to their deaths.

Then things got interesting. A storm soon washed the partly decomposed bodies a few meters into a subterranean lake or pool. Much like quick-setting concrete, moistened soil rapidly hardened around the corpses and preserved the unfortunate duo’s bones.

A couple of thousand millennia later, partial skeletons of the roughly 9-year-old boy and 30-year-old woman were discovered where they had rested for so long. Anthropologist Lee Berger of the University of the Witwatersrand spotted the cave at South Africa’s Malapa site by scrutinizing satellite photos on Google Earth. Excavations of the two skeletons and bones from other ancient individuals started in late 2008.

Now the fallen Stone Age kid and his elder have plunged into a long-standing scientific dispute about the evolutionary roots of the Homo genus, a group of upright-walking, large-brained species that led directly to people today. Researchers generally agree that small-brained members of the human evolutionary family, known as australopithecines, evolved into the first representative of the Homo line between 3 million and 2 million years ago.

But so few fossils dating to that stretch of time have been unearthed that the era of early Homo evolution is considered a “muddle in the middle” of the hominid family tree.

Enter the Malapa skeletons, by far the most complete finds from that perplexing period. In 2010, Berger’s team identified these fossil folk as members of a previously unknown australopithecine species, Australopithecus sediba. In six papers published this April in Science, the scientists delivered a complete head-to-toe analysis of the specimens.

Berger takes two big swipes at status quo thinking about Homo evolution with his analysis of the Malapa fossils. First, he nominates A. sediba as the most likely ancestor of the first Homo species. Forget the popular notion that the Homo genus arose in East Africa. Southern Africa was where the evolutionary action was, Berger contends.

Second, he rejects previous contentions that a handful of fragmentary, mainly East African skull and jaw fossils dating to as early as 2.4 million years ago belong to the Homo line. A. sediba features an odd mix of humanlike and apelike skeletal traits. Considering only skull, hand and hip fossils, it would have been easy to misclassify the Malapa discoveries as a Homo species, Berger says. The same danger applies to the East African finds, in his view.

“Australopithecus sediba should be considered as likely a candidate ancestor for the earliest members of the genus Homo as any other presently available fossil specimens, and perhaps the best candidate,” Berger says.

Anthropologists aren’t lining up to endorse A. sediba as a major evolutionary player. But the South African finds have generated new interest in the muddle in the middle.

“For the next decade, questions about the origins of the Homo genus will be in the forefront of hominid research,” says anthropologist Susan Antón of New York University.

A weird mix

Berger and his collaborators never would have predicted that hominids living in southern Africa almost 2 million years ago were put together like the two Malapa individuals. Neither would any other researcher.

A. sediba possessed a brain only slightly larger than a chimpanzee’s. Adult members of the ancient species reached heights intermediate between full-grown people and chimps. Yet the Malapa skulls also display Homo-like traits such as small front teeth, rounded brain cases and narrow faces with slight chins.

Comparisons of tooth measurements known to be largely influenced by genetics show that A. sediba differed greatly from East African hominids, says anthropologist Joel Irish of Liverpool John Moores University in England. That includes Australopithecus afarensis, a species that lived in East Africa from about 4 million to 3 million years ago. The famous partial skeleton of Lucy discovered in 1974 belongs to A. afarensis, which many researchers suspect was a direct ancestor of the Homo line.

Tooth sizes and shapes tie A. sediba most closely to Australopithecus africanus, another southern African hominid that lived from around 3.3 million to 2.1 million years ago, Irish concludes. But the Malapa individuals’ teeth also display similarities to early Homo species.

The same goes for A. sediba’s lower jaws, which in some ways resemble those of A. africanus and in other ways align with fossil chops from Homo habilis and Homo erectus. H. habilis, or handy man, lived in eastern and possibly southern Africa from 2.4 million to 1.4 million years ago. H. erectus inhabited Africa and Asia from about 1.9 million to perhaps 143,000 years ago.

Anthropologist Darryl de Ruiter of Texas A&M University in College Station estimates that A. sediba’s jaws markedly changed shape from childhood to adulthood, in a developmental shift much like one previously calculated for H. erectus.

Unlike early Homo species, A. sediba’s long arms were built for tree climbing and possibly hanging from branches, says anthropologist Steven Churchill of Duke University. Yet the Malapa pair had humanlike hands capable of gripping and manipulating objects.

A relatively narrow, apelike upper rib cage that fanned out like an inverted funnel supported tree scaling by A.sediba, says anthropologist Peter Schmid of the University of Zurich in Switzerland. A cone-shaped chest interferes with arm swinging while walking and running, so the Malapa folk probably didn’t move as adeptly on the ground as early Homo species did.

Still, A. sediba had a relatively narrow, humanlike lower rib cage and pelvis.

Preserved spinal bones indicate that the Malapa hominids had longer and more flexible lower backs than people today do. Inward curving of A. sediba’s lower back recalls that of a 1.5-million-year-old H. erectus skeleton previously found in Kenya, says anthropologist Scott Williams of New York University.

Finally, A. sediba’s leg and foot bones show that the species walked upright, but with an unusual, pigeon-toed gait. Some people walk this way, but they tend to develop problems with their feet, knees, hips and back, says Boston University anthropologist Jeremy DeSilva. Thanks to expanded knee bones and other lower-body adjustments, A. sediba had no such troubles, DeSilva explains. But the hominid wasn’t walking anywhere fast.

“The Malapa fossils look more like Homo erectus than anything else,” says de Ruiter, a coauthor of Berger’s on four of the April Science papers. “A. sediba could be a transitional type of hominid on the way to the Homo genus.”

Bad timing

Many researchers outside of Berger’s group regard A. sediba as an evolutionary bridge to nowhere. Malapa hominids evolved too late to have been ancestors of a Homo genus that, given other fossil finds, must have originated at least 2.3 million years ago, they argue.

By 2 million years ago, several lines of hominids with various humanlike traits had emerged in eastern and southern Africa, says anthropologist Christopher Stringer of the Natural History Museum in London. Only one of those groups could have carved out a path to the Homo genus. He doubts it was A. sediba.

“The Malapa line may have died out as a failed experiment in how to evolve an upright stance and humanlike features,” Stringer says.

His critics are the ones who have failed, Berger responds, by assuming that a few fragmentary fossils represent the earliest members of the Homo genus.

Consider perhaps the crown jewel of early Homo fossils, an upper jaw and palate discovered in Ethiopia in 1994. This fossil was found on the surface of a small hill and may not have eroded out of the 2.3-million-year-old soil layer its discoverers say it came from, Berger contends.

Also, he argues, humanlike features of A. sediba’s jaws and skull illustrate the danger of diagnosing any find as a member of Homo based on such fragmentary evidence. Echoing Stringer’s point but from a different perspective, Berger argues that because different hominids evolved distinctive blends of apelike and humanlike traits, fossils from one body part are unreliable guides to a specimen’s place on the hominid family tree.

Berger asserts that a pair of alleged early Homo species — H. habilis and Homo rudolfensis — possessed large teeth typical of australopithecines and limb bones with more apelike features than those of A. sediba. Lacking more complete skeletons, he suspects that those East African species were actually australopithecines.

If that’s the case, it’s more likely that A. sediba originated somewhere in Africa before 2 million years ago and was a direct ancestor of the first true Homo species, H. erectus, Berger says. Previous fossil discoveries suggest that H. erectus reached western Asia 1.77 million years ago, shortly after appearing in Africa.

That’s the evolutionary story with the strongest fossil support, mainly from the two partial Malapa skeletons and a previously unearthed skeleton of an H. erectus boy, de Ruiter says.

Fossils previously proposed as early Homo representatives are too few and incomplete for his taste. “Every single scrap of fossil evidence for early Homo before 2 million years ago could fit in a shoe box, along with one shoe,” de Ruiter says.

Miffed mentor

In a big way, Berger has the famous anthropologist Donald Johanson to thank for his Malapa discoveries. Johanson, who led the excavation of Lucy’s skeleton at Ethiopia’s Hadar site in 1974, was Berger’s hero and inspired him to pursue anthropology. As an undergraduate in Georgia, Berger invited the famous anthropologist to have breakfast with him when Johanson was in the area to give a talk. Johanson advised the young man to do graduate work at Witwatersrand and investigate South Africa’s rich fossil sites.

Now, 25 years later, Johanson finds himself exasperated at Berger’s rejection of early Homo in East Africa and insistence that A. sediba was an evolutionary skeleton key that opened the door to human evolution.

 “It’s wonderful that Berger found the Malapa fossils, but he wants to sweep evidence for early East African Homo under the rug,” says Johanson, now at Arizona State University in Tempe.

Johanson coauthored a 1996 report on an upper jaw and palate that many researchers outside Berger’s group regard as the oldest known Homo specimen.

That specimen was broken in half along the roof of the mouth when it was discovered on the surface of a low, steep hill at the Hadar site in Ethiopia. The wide, deep palate and relatively small teeth place it squarely within the Homo genus, Johanson asserts.

Soil clinging to the two pieces enabled researchers to identify a section of the hill from which the fossils had eroded, presumably weeks or months before members of a survey team noticed them. A layer of volcanic ash just above the fossil’s presumed original resting spot puts the jaw at about 2.3 million years old, Johanson says.

Fossils from Lucy and other members of her kind, as well as preserved footprints at Tanzania’s Laetoli site, make it clear that upwards of a million years earlier, A. afarensis walked around the same part of Africa on humanlike feet. That evidence makes Lucy’s species a good candidate ancestor for early Homo at Hadar, Johanson says.

In contrast, A. sediba walked on feet and legs that were more apelike and less able to stride efficiently than those of Lucy’s kind. The South African species appears to have been an evolutionary offshoot of A. africanus with a body design unlike that of any other hominid, in Johanson’s view.

“At the moment, it looks like Homo evolved somewhere in East Africa,” he says.

Hominids capable of traveling long distances evolved in Africa by 2.5 million years ago, so it’s hard to know where on the continent Homo first appeared, comments anthropologist Brian Richmond of George Washington University in Washington, D.C.

Like Johanson, Richmond sees A. sediba as a likely descendant of A. africanus in a now-defunct line of southern African hominids.

For now, too few fossils have been found to determine precisely where Berger’s discoveries stand in the grand scheme of human evolution. So Berger and his colleagues will return to Malapa in September 2013. Based on previous observations of fossils poking out of parts of the cave, they suspect Malapa will yield at least three more hominid skeletons.

Even the most momentous discoveries there aren’t likely to explain the origin of the Homo genus to everyone’s satisfaction. Any further finds will be welcomed, nonetheless, to the hominid fossil record’s sparsely populated muddle in the middle.

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Little Foot steps up

Up to a million years before an Australopithecus sediba boy and adult female perished in a cave at South Africa’s Malapa site, one of their evolutionary relatives fell to his death through a narrow shaft into the nearby Sterkfontein Caves. With much less fanfare than that triggered by Lee Berger’s Malapa finds, the Sterkfontein hominid’s nearly complete skeleton has been excavated by University of the Witwatersrand anthropologist Ronald Clarke.

Clarke’s take on hominid evolution in southern Africa differs radically from Berger’s. Clarke’s opinion is informed by his own discovery, which he assigns to a new species: Australopithecus prometheus.

“A. sediba has nothing to do with the origin of the Homo genus,” Clarke says. “I don’t claim A. prometheus does, either.”

Berger and Clarke have a tense relationship. As a Witwatersrand graduate student in anthropology, Berger befriended Phillip Tobias, the head of the university’s human biology and anatomy departments. Tobias then groomed Berger as his academic successor at the university over the more senior Clarke.

Clarke and Tobias realized in 1997 that Sterkfontein held an ancient hominid’s skeleton. A couple of years before, they had uncovered only the lower legs with parts of the feet and a skull. Tobias playfully dubbed the find “Little Foot,” a name that stuck. Clarke has now freed much of the skeleton from its hard shell.

Hundreds of Australopithecus fossils, mainly jaw and skull pieces, have been uncovered at Sterkfontein since 1936. Most researchers lump all of them into the species Australopithecus africanus.

But Little Foot’s relatively large teeth and flat face demonstrate that another hominid species lived alongside A. africanus in southern Africa, Clarke contends.

Soil carried into the cave at various times by rains has complicated attempts to pin down when these hominids lived. Clarke estimates that, based on its position in the cave, Little Foot lived around 3 million years ago. Other fossils of A. prometheus and A. africanus come from a Sterkfontein section dating to around 2.5 to 2.1 million years ago, he says. Some scientists who have worked at Sterkfontein suspect Little Foot lived closer to 2 million years ago.

Where in Africa Homo originated is far more mysterious than Little Foot’s age, Clarke says. Though fossils of 3-million- to 2-million-year-old African hominids come almost exclusively from eastern and southern parts of the continent, he observes, “the Homo genus could have first developed in central Africa for all we know.” — Bruce Bower