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The Mosaic of Human Origins

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New research challenges the story of human evolution, revealing a more complex picture than anyone imagined.

"Memento mori" by Nathaniel Gold

"Memento mori" by Nathaniel Gold

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.

Capuchin Monks Crypt

Capuchin Monks Crypt. Photo: John Mosbaugh / Flickr

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.”

Australopithecus sediba

Australopithecus sediba fossils of an adult female (left) and juvenile male (right). Image courtesy of University of Witwatersrand.

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.

Eric Michael Johnson About the Author: Eric Michael Johnson has a Master's degree in Evolutionary Anthropology focusing on great ape behavioral ecology. He is currently a doctoral student in the history of science at University of British Columbia looking at the interplay between evolutionary biology and politics.

Follow his work on Facebook and Google+. Follow on Twitter @primatediaries.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. greg_t_laden 9:43 am 04/17/2013

    I love the metaphor of monks messing around with bones and mosaic evolution.

    Regarding time, that may not be important. We can assume that all identified fossil hominids are samples that point to, but are not equal to, and are not exactly like, ancestral species. We triangulate on the ancestral species by looking at the traits seen in a collection of known species. A. sediba could be walking around today and still “point to” an ancestral Last Common Ancestor of Australopiths and Homo. Time as a factor only works when you have excellent sampling across time and space and the absence of a particular species in the museum drawer is reasonable (though provisional) evidence of absence of the species in antiquity.

    I remember coming back from a research trip to South Africa several years ago and sitting on the plane next to an American returning to the US after visiting there. He was very excited to have just met this amazing young researcher starting up work in human evolution in SA. “You’re going to be hearing about this guy” he said. He was talking about Lee Berger!

    South Africa has always had more bones of hominid ancestors than any given East African country, I’m pretty sure, and the number of species has been rising steadily as well. The main reason people say that East Africa is “the cradle” is because East Africa has better press and has for a long time. There are good reasons to believe that southern Africa is just as much a key location for the evolution of hominid species as Kenya, Tanzania or Ethiopia.

    Finally, one thing that has been true for many decades in African palaeoanthropology: Every find demonstrates increasing complexity of the story, very few finds simplify things.

    Anyway, great essay.

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  2. 2. EricMJohnson 12:37 pm 04/17/2013

    I completely agree. Each new species allows us to narrow in on what our ancestors were like through triangulation even if the fossil discoveries do not represent a direct line of descent. It is for this same reason that we look for common behavioral features between humans, chimpanzees, and bonobos in order to better understand what the ancestor or all three species may have been like. Each new discovery may complicate the story, but in that complication lay the insights that help us piece together the complete picture.

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  3. 3. kso721 3:05 pm 04/17/2013

    Badass article.

    Link to this
  4. 4. EricMJohnson 3:39 pm 04/17/2013

    Thank you.

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  5. 5. Torbjörn Larsson, OM 11:50 am 04/18/2013

    Really good article! Duly bookmarked. [I'm mostly interested in astrobiology, but hominid evolution is peripheral to that.]

    I didn’t know the stratigraphy for the putative Homo maxilla was contested. On the other hand I thought it was uncertain to place it as belonging to H. habilis, which resolves the timing (“clear the way “).

    I could guess Berger’s reasoning now goes something like this:

    - If evolution is mosaic and/or exhibit homoplasy, a single maxilla isn’t really telling us exactly how close its relationship is to Homo.

    - Specifically, it doesn’t mean habilis was older than Au. sediba.

    So far, so fair, it seems to me as an outsider.

    I’m impressed with that the dentistry of Au. africanus + Au. sediba naturally cluster with Homo, if I understand the press. (Paywalled articles.) If it is homoplasy, it is a neat match.

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  6. 6. HertfordshireChris 5:00 pm 04/18/2013

    How different have two “species” to be before they can be guaranteed not to be able to interbred. In comparatively modern times we have had modern humans, Neanderthals, Denisovans and the ancestor of Albert Perry interbreeding and there is no reason to suspect that our earlier ancestors did not split up long enough to evolve different features and then interbreed to combine the best feature of both “species”. Could this have happened, rather than homoplasty, in at least some cases? After all you might only require one male/female union to produce fertile offspring to combine features from two different species – as long as the combination gave the offspring competitive advantage in the survival stakes.

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  7. 7. aldarwin 12:43 pm 04/21/2013

    What defines a “Species”? Can it be argued that a number of species of Homo are extant or represented on the earth now, each with unique ancient phenotypic features admixed with gene-sharing features from interbreeding but also sharing the adaptations common to all mammals? Did certain Homo species persist in geographic isolation (in locations like Africa?) long enough to preserve some of their features that were later transferred and admixed to other Homo species by interbreeding as contact expanded. Are there skeletal dimensions/proportions/features within groups or amongst individuals that suggest a more direct lineage from one Homo species as opposed to another?

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  8. 8. aldarwin 11:17 am 04/22/2013

    Do genes/traits survive if they are “neutral” or have no competitive disadvantage? If traits do not manifest until after reproductive years are over, do they not survive, like aging?

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  9. 9. aldarwin 11:17 am 04/22/2013

    Do genes/traits survive if they are “neutral” or have no competitive disadvantage? If traits do not manifest until after reproductive years are over, do they not survive, like aging?

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