April 24, 2013 | 19
Three years ago researchers added a new branch to the human family tree: Australopithecus sediba, a nearly two-million-year-old relative from South Africa. By all accounts it was a dazzling find—two partial skeletons, an adult female and young male, from a site called Malapa just outside Johannesburg. And it has been making headlines regularly since then whenever scientists release results of new studies of the material, as they did earlier this month. Any time human fossils, especially skeletons, are unearthed it’s a big deal, because such remains are so incredibly rare. But I’m going to go out on a limb here and say that A. sediba may just be the most important hominin (modern humans and their extinct relatives) discovery yet.
Now, I can already hear the protests of more than a few paleoanthropologists. But hear me out–and then if you don’t buy it you can tell me why I’m wrong in the comments.
To appreciate the importance of any given discovery, we must consider it in its historical context. Viewed that way, one might consider the 1856 discovery of Neandertal fossils in western Germany to be the most important, since it marked the beginning of human paleontology as a field of inquiry. The Taung child (Australopithecus africanus), found in South Africa in 1924, was another momentous find, offering up the first convincing evidence that humankind originated in Africa. Then there’s the 3.2-million-year-old Lucy (Australopithecus afarensis)–the most complete hominin skeleton known at the time she was found in Ethiopia in 1974 and still the best known to the public—whose anatomy established that hominins walked upright long before brain size expanded, settling a longstanding debate. More recently, the 18,000-year-old Flores hobbit (Homo floresiensis), announced in 2004, made waves with her diminutive proportions and other traits that challenge longstanding ideas about hominin adaptation and biogeography. And Ardi (Ardipithecus ramidus) was a sensation when scientists unveiled her in 2009, suggesting that some enduring notions about the origin of bipedalism and the last common ancestor of humans and chimpanzees might be wrong.
These fossils and many others are landmark discoveries in paleoanthropology, finds that have filled crucial gaps in scientists’ understanding of human origins. They are all vitally important. And yet the A. sediba fossils manage to stand out from even this elite crowd, because of the sheer volume and quality of information they contain. The finds from Malapa tick pretty much all the boxes on a paleoanthropologist’s wish list. Specimens that preserve multiple skeletal elements? Check. Remains of multiple, coeval individuals (important for understanding variation within a species)? Check. Fossils in near-pristine condition, thus eliminating uncertainties about how pieces fit together? Geological context that allows for precision dating of the fossils? Associated plant and animals remains? Check, check, check.
Since the initial announcement in 2010 the discovery team, led by Lee Berger of the University of the Witwatersrand in Johnannesburg, has published a slew of papers detailing what A. sediba looked like, when it lived, what it ate and how it is related to us, among other insights. The latest analyses, described in six papers published in the April 12 Science, reveal a creature that excelled at climbing trees and also walked upright on the ground with its shoulders shrugged and its arms unswinging, rolling its feet inward with each step—a previously unknown form of bipedalism. Yet in contrast to its alien way of walking, aspects of A. sediba’s teeth and jaws are decidedly familiar, resembling those of our genus, Homo, according to two of the new studies. Indeed A. sediba’s dizzying mosaic of apelike and humanlike traits is a theme the researchers have emphasized with each new round of papers. And it is this mosaic that has researchers debating the central question about the hominins from Malapa: namely, where they belong in our family tree.
Berger and his colleagues have argued from the beginning that A. sediba might well be the long-sought species that gave rise to our genus (or a close relative of that species). Such an arrangement would root Homo in South Africa instead of East Africa and could banish Lucy’s species–traditionally thought to be in our direct line of ancestry—to the evolutionary sidelines. But critics have countered that A. sediba is not particularly Homolike overall and that it probably instead belonged to a South African lineage of hominins that ultimately went extinct—one of many dead-end branches in our family tree.
Because the origin of Homo is perhaps the biggest mystery in paleoanthropology, A. sediba’s perceived importance would get a big boost if new evidence were to strengthen its link to Homo. But I’d go one step farther and argue that regardless of whether it is found to be the ancestor of Homo or a dead-end branch of humanity, the Malapa hominins are now the ones to beat. Because what A. sediba brings to the table is the potential for the most detailed understanding yet of a hominin anywhere near this old.
The Malapa site is an incredibly high-resolution time capsule. The hominin remains include bones that rarely, if ever, turn up at early hominin sites, and bones often preserved only as fragments have survived intact here. Moreover, the hominins represent a range of developmental stages: in addition to the two skeletons, the site has yielded more fragmentary remains of another 4 individuals, including an infant, which will allow the team to study maturation in the species. And the fossilized plants and animals at Malapa are the actual plants and animals the hominins had in their environment, not aggregations of remains over a period of thousands or tens or even hundreds of thousands of years.
Furthermore, conditions at the site, which was once a 30- to 50-meter-deep underground cavern with a shallow freshwater pool at the bottom, apparently allowed for the preservation of some very unusual features. The teeth of the young male were found to have tartar on them, which the research team was able to analyze for clues to what he ate in his final days. Previously the oldest known hominin tartar came from much younger Neandertals and early modern humans. And at the annual meeting of the Paleoanthropology Society in Honolulu earlier this month, Rachelle Keeling of the University of the Witwatersrand reported that molecular imaging of what appears to be skin preserved on some of the bones supports that interpretation. If verified this would be the first evidence of fossil hominin soft tissue, and could conceivably provide insights into A. sediba’s skin color and hair color, and the distribution of hair and sweat glands. Such skin features are themselves clues to the body’s ability to offload excess heat, which became increasingly important as hominins became more active over the course of evolution.
OK, I’m more than 1,000 words into this post and I’ve still barely scratched the surface of what makes the A. sediba find so extraordinary. I can’t hope to be comprehensive here, but I do want to mention two more aspects of this discovery that add to its importance. First, there are more fossils to come, perhaps lots more. CT scanning of some of the many chunks of rock blasted from the site by limestone miners back in the early 1900s has already revealed several bones. And additional hominin bones can be seen sticking out of the ground at Malapa, awaiting excavation. (This blew my mind when I visited the site in November of 2011.)
Second—and this may sound a little insidery, but it’s critical–the way Berger and his collaborators are studying the finds and disseminating what they learn represents a real departure from the cloak-and-dagger manner in which paleoanthropological investigations often proceed. Berger has assembled a huge team of specialists to work on the remains and has made the project open access, with a policy of granting permission to any paleoanthropologist who asks to see the original fossils. He has also sent out scores of replicas to institutions around the world, and routinely brings casts of the bones—even ones that his team has yet to formally describe–to professional meetings to share with other researchers. This can only improve the quality of the science that comes out of the project and may well inspire other teams to be more forthcoming with their own data.
So there you have it. That’s my case. I realize the importance of a fossil depends on the question one is asking of it—e.g. if you want to know about the origin of, say, art, A. sediba is irrelevant. And yes, at the end of the day we need loads of fossils (and artifacts and DNA) from different times and places to piece together the full story of our origins. I’m just awed and delighted by the opportunity this discovery affords to see a human species from so very long ago in such vivid detail—whether it is the elusive ancestor of Homo, or a creature from a parallel lineage that reveals another way of being human and could perhaps elucidate why our line succeeded where others failed.
Think another hominin discovery is more important than this one? I’d love to hear which one and why in the comments. Maybe you’ll change my mind.
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