April 17, 2013 | 9
New research challenges the story of human evolution, revealing a more complex picture than anyone imagined.
Studying the bones of our ancestors does more than connect past with present. When Hamlet held aloft the skull of poor Yorick or when the Boston Puritan Thomas Smith sat for America’s first self-portrait posed with two crania, they were participating in a meditation on the inevitability of human mortality. Throughout the Christian world from the Medieval to the Victorian era, the tradition known as memento mori (Latin for “remember that you must die”) was a common theme in artistic and literary works reflecting a worldview that looked beyond our individual lives to the shared fate that awaits us all. Looking at the big picture comes quite naturally when we contemplate the bones of those who preceded us. The dead carry meaning for those who are alive today by revealing our connection in the larger mosaic of existence.
In the spring of 1999 I turned away from the life that was blossoming all around me and descended underground to learn first hand what the dead had to teach. I saw femurs stacked like cord wood as high as the domed ceiling, disarticulated ribs arranged into ornate candelabra, and skulls placed with exquisite care so as to completely surround robed figures of emaciated dead.
The church of Santa Maria della Concezione dei Cappuccini in Rome, known more popularly as the Capuchin Crypt, is an early modern chapel that has been entirely decorated with the skeletal remains of some 4,000 friars, members of the holy order who had died since the first group arrived in 1631. However, for this particular meditation on memento mori the emphasis wasn’t so much on death as on the life of the world to come. The Capuchin monks believed that each individual was part of a larger whole that lay just beyond this vale of tears. By constructing a mosaic with the once living bodies of their brothers in Christ, and reassembling anatomical features into entirely new configurations, they were reaffirming the divine plan to which they had all committed their lives.
The science of human paleontology is the modern secular equivalent of memento mori, for in no other field is the contemplation of life through the origin of species tied so explicitly to the ravages of death. The first time I walked into a paleoanthropology laboratory that contained drawers and cabinets filled with ancestral remains I felt the same kind of visceral response I experienced while touring the Capuchin Crypt years earlier.
Of course, instead of the aesthetic arrest pursued by the friars, the scientific mosaic we are creating from the bones of our forebears today requires precise cataloguing and measurement in order to understand the pattern that connects individuals across continents and epochs. It was Charles Darwin’s keen attention to the facts of natural history that allowed him to predict that the fossil ancestors of Homo sapiens would most likely be found in Africa. In the last 150 years paleoanthropologists have demonstrated the grandeur in his view of life with a wealth of evidence on more than 6,000 individuals representing at least 18 different species.
The latest species to join our hominin clade, however, has upended the pattern constructed over the last fifty years that placed human origins in East Africa. In 2008 a paleoanthropologist at the University of Witwatersrand in Johannesburg, South Africa, by the name of Lee Berger unearthed the fossils of a juvenile male and adult female on the Malapa Nature Reserve after his son accidentally stumbled across their remains while chasing his dog. Berger classified the species as Australopithecus sediba—meaning “southern ape” in a mixture of Latin and Greek combined with the local South African SeSotho word for “wellspring.” Both specimens were remarkably complete given their advanced age of 1.977 million years and placed the species as a possible descendant of Australopithecus africanus, which lived in the same region from about 3.03 to 2.04 million years ago (mya). But once Berger’s team of researchers published their analysis in the journal Science in 2010 suggesting that Au. sediba was the direct ancestor of Homo—and ultimately humans—their interpretation has been enormously controversial within the field.
“There are a couple of reasons for this,” explained Steve Churchill, associate professor of evolutionary anthropology at Duke University and a co-author on the original study of Au. sediba as well as five of the six new papers published in Science on Friday. Two large African tribal masks loomed over each shoulder on the office wall as I asked him about the now three-year old controversy. [Full disclosure: I attended Duke’s Evolutionary Anthropology department for my initial PhD research but have never worked with Dr. Churchill and have no connection with this current study.] “One reason is just the historical precedent for researchers to see East Africa as the place where all the evolutionary action is,” Churchill said.
In fact, nearly every hominin species ever found has been unearthed in that region with most of the material discovered in Ethiopia, Kenya, and Tanzania. These include Au. afarensis (3.9 – 2.9 mya), famously known as Lucy, Homo habilis (2.33 – 1.4 mya), the earliest species to be morphologically similar enough to be included in our genus, and our direct ancestor Homo erectus (1.8 mya to about 140,000), the first hominin to leave our ancestral home and radiate west to Spain and east as far as Indonesia. Given the diversity of species and the adaptive radiation that occurred from East Africa, this is a historical precedent that is hard to dismiss.
“Secondly there is the issue of time,” Churchill continued. “Most people would say that Homo originates somewhere on the order of 2.3 million years ago.” This predates Au. sediba by more than 300,000 years, meaning that unless you plan to argue from the point of view of Wordsworth that “the child is father of the man,” it is a difficult issue to reconcile. However, this early date for the genus Homo is based on a single maxilla fossil (the two bones that form the roof of the mouth), designated AL 666, and was originally discovered smashed into fragments.
“AL 666 does share a few features with Homo,” Churchill admits, “but it was found in approximately thirty-five pieces and they had to excavate down into the sediment upon which it was lying. That suggests to me that it moved and then broke when it moved down sequence.” In other words, the stratigraphy that was ultimately used to establish the date of 2.3 million years could potentially be hundreds of thousands of years off. The origin of our genus might not have been as early as previously claimed considering that the next best candidate doesn’t appear in the fossil record for about 400,000 years. Without AL 666 it would essentially clear the way for Au. sediba’s parentage of Homo, at least in terms of the timeline. But other researchers see more than mere temporal concerns when contemplating the bones in our past.
“The conflict is not time,” said John Hawks, an associate professor of anthropology at the University of Wisconsin-Madison who was not involved in the current study. “The conflict is anatomy. The anatomies of the early fossils that are candidates for Homo are not like Au. sediba.”
What remains clear is that our most recently discovered hominin forebear refuses easy categorization. Based on the results published Friday, Au. sediba’s dental traits are close to the South African Au. africanus as would be expected if the first was a descendant of the second, and they remain distinct from the East African australopiths. However, the mandibular remains are just the opposite and share similarities with these other australopiths while differing from Au. africanus. To complicate matters even further, Au. sediba shares more ape-like upper limbs and rib cage with all of the australopith species—suggesting an adaptation for climbing and suspension—while the lower rib cage is more human-like, narrowing to the lower thorax and revealing a chest anatomy not seen in any of these other species. Au. sediba had the same number of lumbar vertebrae as modern humans but possessed a longer and more flexible lower back, a morphology that is distinct from Au. africanus. Finally, the forelimb anatomy was quite unusual for an australopith. While this species would have been capable of bipedalism they likely walked with a hyperpronating gait, meaning an excessive rotation of the foot, and would have hobbled along with some difficulty.
“The forelimbs may be the most problematic aspect of these results,” said Hawks. “We also know from the earlier papers that Au. sediba had a very human-like hand. But what are they doing with a human-like hand when they have a very ape-like arm? This is odd and is an area that demands resolution.”
Ultimately, Australopithecus sediba displays a mosaic of features and researchers will need to reconcile this complexity in order to make the case that they have discovered the ancestor of Homo. One of the most important factors to consider is known among evolutionary scholars as homoplasy, or convergent evolution amongst unrelated lineages. If our australopith forebears had each independently adapted to similar evolutionary pressures, the common features in their anatomy would not have been related by descent but by the environmental constraints imposed that resulted in a commonality of form. For this South African clade to be the origin of Homo there would need to be a range of features in common extending from Au. africanus through Au. sediba and ultimately appearing in our Homo ancestors. However, while this mosaic of features may not answer the question of lineage, they do address a more profound mystery.
“The reason why these researchers are emphasizing the mosaic nature of Au. sediba so much,” said Hawks, “is that it is evident in these skeletons that you have features that shouldn’t go together. If you looked at only one part you would come to a very different conclusion than looking at the whole.”
It is this aspect of Au. sediba that offers what is potentially the most provocative and far-reaching insight into our hominin origins. Given that the majority of fossil discoveries by paleoanthropologists are mere fragments—particularly those interpreted as showing the transition to Homo—it reveals how highly variable an individual feature can be when interpreted in isolation rather than as part of the larger morphological pattern. It is akin to the African proverb of blind men each touching a different part of the same elephant only to end up identifying the animal in dramatically different ways.
These results suggest that the priority of interpretation should go to the more complete fossil skeletons and highlights the problematic nature of basing conclusions on fragmentary evidence. Reflecting on this mosaic of bones ultimately challenges us to reconsider the pride with which we have told the story of human origins. In this way, Australopithecus sediba reveals what the Capuchin monks spent more than three centuries creating their own mosaic to express. The whole truly is greater than the sum of its parts.