The Link: Uncovering Our Earliest Ancestor
by Colin Tudge and Josh Young
Reviewed by Chris Beard
Recent events cause me to wonder whether we are in the midst of an arms race being waged by various scientists and their marketing gurus over how best to communicate results to the lay public. A case in point is the commotion over the Eocene primate skeleton known as Ida. At roughly 47 million years old, Ida is a remarkably complete specimen of a juvenile female primate from the Messel Pit, a UNESCO World Heritage site in Germany, near Frankfurt. Variously hailed as "the Holy Grail of paleontology," "the eighth wonder of the world" and a "Rosetta stone" for reconstructing our distant ancestry, the fossil made its public debut at a gala event at the American Museum of Natural History in New York on May 19, 2009. A press conference at the museum coincided precisely with the online publication of a technical paper describing the fossil in the journal PLoS One. By the time the press conference had ended, a glitzy Web site promoting the fossil had gone live, a television documentary was being advertised on the History Channel, and thousands of copies of The Link, a book describing the discovery, had been distributed to retail outlets worldwide.
The fossil Ida is paradoxical on several levels, and so is this book. Perhaps the most perplexing question is why any paleontologist fortunate enough to acquire such a fossil -- which some have likened to the Mona Lisa and the Lost Ark -- would refrain from writing his own book about it, opting instead to farm the project out to professional writers Colin Tudge and Josh Young. Back in 1981 Donald Johanson coauthored what still stands as the definitive book in this genre, Lucy: The Beginnings of Humankind. During that bygone era before the Internet abbreviated our collective attention span, Johanson and his colleagues could afford to let precious years elapse between the publication of their initial technical analyses and the popular book. Apparently time was of the essence in writing and publishing The Link.
In many ways, this book stands as further testimony to the old adage that haste makes waste. It is riddled with errors. These range from the relatively trivial but geographically challenged claim that the monkey-eating eagle Pithecophaga jefferyi inhabits South America (these majestic birds are actually confined to the Philippines) to the more fundamental but no less mistaken contention that "Ida herself is the only nearly complete fossil primate ever found." The latter assertion would no doubt surprise the legendary comparative anatomist William King Gregory, who described exquisitely complete skeletons of the closely related and equally ancient primate Notharctus in 1920. Much closer to us in terms of time and evolutionary position is the late Miocene ape Oreopithecus bambolii, which is also documented by a relatively complete skeleton, even if it -- like Ida -- is nearly two-dimensional.
The Link begins by painting a highly anthropomorphized portrait of Ida as a "petite being" no more than "two feet tall" bearing "opposable thumbs." This depiction is intended to heighten the drama of poor Ida's untimely demise (she was less than a year old). I doubt that many of us would describe our house cats as petite beings that stand two feet tall. Yet in terms of Ida's general posture and brain size, she was clearly more catlike than humanlike. The exact circumstances surrounding Ida's death approximately 47 million years ago remain unknown. The vivid speculative account provided in The Link agrees with what we know about the geologic history of the ancient lake that would later become the Messel oil shale pit: Magma from beneath the lake probably leaked carbon dioxide into the water, and Ida may have been killed by a cloud of carbon dioxide gas rising from the lake.
Ida's fossilized remains came into the possession of Jorn Hurum, a paleontologist based at the Natural History Museum at the University of Oslo in Norway, through a circuitous path. The specimen was allegedly unearthed sometime in 1982 by an unidentified private collector. Whoever found Ida did so by splitting apart chunks of the Messel oil shale. This process typically divides fossils into a "part" and a "counterpart," each of which contains a fraction of the original skeleton. The less photogenic slab was spruced up to make it appear more complete than it really was; it was then sold to a private museum in Wyoming. Its complement, containing most of Ida's skeleton, was kept in a private collection until Hurum learned about it at a "fossil fair" in Hamburg, Germany, in 2006 and arranged for the University of Oslo to purchase it.
These commercial transactions expose the seedy underbelly of paleontology. Many academic paleontologists, myself included, disapprove of treating fossils like commodities. Paying large sums of money for fossils inevitably encourages nonscientific collecting. All too often, this results in the permanent loss of critical data, including information on the fossil's exact age, its depositional context, associated fauna and flora, and so forth. Equally important, when academic paleontologists have to compete with collectors whose sole motivation is a financial incentive, the academics often lose access to important fossils or entire fossil sites. Among the most disheartening effects of the media campaign surrounding Ida is the widespread knowledge that extraordinary fossil primates can command a substantial premium on the open market.
The bulk of The Link is devoted to fleshing out Ida's fossilized remains. To do so, Hurum assembled what he has referred to as a "dream team" of paleontologists. Their analyses suggest that Ida was a vegetarian (her stomach contents indicate that her last meal consisted of fruit and leaves), that she moved about her arboreal domain by making acrobatic leaps from tree to tree, and that she was already injured when she died. They interpret Ida as belonging to a new genus and species of adapiform primate, Darwinius masillae (awkwardly, this name never actually appears in the book, which was written and printed prior to the online technical publication in which the taxon was formally proposed).
The extinct adapiforms are a group of lemurlike primates that were abundant and widespread during the Eocene. This aspect of Ida's taxonomic placement is uncontroversial. The book, however -- particularly in its title -- goes further, hinting that Hurum and his dream team consider Ida to be a transitional fossil between primitive primates (a loose concept that often gets conflated in The Link with the term prosimians, which itself is an oversimplification of primate evolutionary relationships that is meant to include lemurs and tarsiers), and anthropoids, or simians (the group that includes living monkeys, apes and humans). The possibility that Ida might be our distant ancestor is what set in motion the high-octane public relations machine of which The Link is but one cog.
The idea that adapiforms might somehow be ancestral to anthropoids was taken seriously as recently as a couple of decades ago. However, I think it is fair to say that this hypothesis now lies well outside the scientific mainstream, and the discovery and description of Ida have done little to rehabilitate it.
Hurum and his colleagues point to several of Ida's anatomical features as support for their claim that this fossil is transitional between primitive primates and anthropoids, and these assertions are faithfully repeated in The Link. Like all adapiforms and other primitive primates, Ida lacked a lemurlike toothcomb. Based on the shape of her terminal phalanges, Ida also appears to have lacked a lemurlike grooming claw on her second toe, although this inference is hindered by the orientation of the bone in question. The absence of a toothcomb and the probable absence of a grooming claw readily distinguish Ida from lemurs, but primitive traits such as these provide no more support for placing Ida closer to anthropoids than does the fact that, like them, she has 10 fingers and 10 toes. Details of Ida's ankle anatomy (specifically, the shape of the joint between the fibula and the astragalus) are also cited as indicating a special relationship between Ida and anthropoids, but the excellent photographs provided in The Link fail to substantiate this claim. Ida's two-dimensional and fully articulated skeleton precludes any meaningful assessment of details like these.
Among the numerous problems that confront those who would like to see Ida near the base of the anthropoid family tree, three stand out as particularly serious obstacles. The first of these, which The Link never really attempts to address, is the inherent conflict that would exist between fossils and the biology of living primates if Ida really were as close to modern anthropoids as Hurum's team would have us believe. Those tiny, bug-eyed, nocturnal denizens of Southeast Asian rainforests known as tarsiers are the elephants in the room in this regard. Tarsiers are perhaps the world's strangest primates, even when we throw humans into the mix. Tarsier biology, despite its general oddness, shares various features in common with anthropoid biology. Examples include an inability to synthesize vitamin C (which tarsiers and anthropoids must therefore ingest), the loss of the reflective layer on the back of the eyeballs that makes the eyes of deer and most other nocturnal mammals "glow in the dark" when illuminated by headlights, and some rare genetic rearrangements known as SINE insertions. Given these evolutionarily recent commonalities, most scientists acknowledge that tarsiers lie closer to anthropoids on the primate family tree than do lemurs. Ida and her adapiform kin share many features in common with lemurs, but none that are specifically tarsierlike. It is therefore difficult to envision Ida being a transitional fossil between lemurs and anthropoids, when tarsiers play such a critical role at this juncture. One might just as well posit a transitional fossil linking fish and reptiles, forgetting all the while that amphibians ever existed.
A second problem for Ida stems from what The Link posits as her chief attribute -- her incredible preservation and anatomical completeness. Simply put, Ida's anatomy is so well characterized that it leaves little to the imagination. Twenty years ago, as advocates of the adapiform theory of anthropoid origins championed their cause, they tried to make their case with one fragmentary fossil after another. Unfortunately for them, as soon as we found reasonably complete specimens of Hoanghonius stehlini and Catopithecus browni, we learned that the former were vanilla-flavored adapiforms and the latter were full-blown anthropoids. None of the supposedly transitional fossils predicted by the adapiform theory of anthropoid origins held up when subjected to detailed scrutiny. Here, Ida's remarkable completeness preempts any similar proposal. She differs in minor details from other adapiforms, but not in the ways that would make her a more convincing candidate for anthropoid ancestry.
The biggest problem for Ida and The Link is that the fossil record has already yielded far more compelling contenders for the role of transitional fossils between primitive primates and modern anthropoids. These animals -- notably including Eosimias from China -- do not conflict with the privileged role of tarsiers in reconstructing anthropoid origins. They have the right anatomy, and they lived in the right place and time, to be close relatives of modern anthropoids. Because they remain only partly documented anatomically, Eosimias and its Asian relatives have outstanding potential to become even more convincing members of our extended family tree as we learn more about them.
Hurum and his colleagues attempt to dismiss the hypothesis that Eosimias and its Asian relatives are close relatives of anthropoids, primarily on the grounds that we still don't have complete skeletons of them. This strikes me as an odd critique. There is no rationale for positing that anatomical completeness should influence where any fossil, whether it be Ida or Eosimias, is placed on the primate family tree. The reconstruction of evolutionary relationships depends on a fossil's anatomy alone, and in this respect Eosimias is infinitely better qualified as a transitional fossil between modern anthropoids and more primitive primates than Ida ever will be. Many of us will continue to search for better fossils of Eosimias and animals like it, but our inability to find complete skeletons of them hardly justifies pretending that early Asian anthropoids simply don't exist.
Reading The Link is a lot like watching the pitch for the latest must-have gadget being sold late at night on cable television. The book is unabashedly biased, it makes remarkable claims, and its conclusions probably won't hold up to further scrutiny. But isn't this the point of the whole enterprise? How else can you sell that amazing potato peeler that doubles as a night-light? Yet, toward the end of The Link, a quote from Hurum explains the rationale for the marketing campaign behind Ida as follows: "I knew that a fossil can be metaphorically destroyed... if it is explained wrong or ridiculously in the media." Perhaps this is Ida's biggest paradox of all.
Chris Beard is the Mary R. Dawson Chair of Vertebrate Paleontology at the Carnegie Museum of Natural History and the author of The Hunt for the Dawn Monkey: Unearthing the Origins of Monkeys, Apes, and Humans (University of California Press, 2004).