Until it was moved to make room for a Tyrannosaurus skull replica a few weeks ago, a stilt-legged mammal skeleton stood behind me during my weekly sessions cleaning a fossil crocodile at the New Jersey State Museum. Almost every visitor who stopped to comment on the skeleton had a different idea about what it might be. Some were convinced that it was a moose – what other animal had such broad antlers? – while others suspected that the animal was an elk or deer. A few were completely baffled by the creature’s osteological architecture. Missing the enveloping layers of flesh and fur, the skeleton was an enigma.
I would have been shocked if a visitor walked up to the ancient skeleton and identified it as Cervalces scotti. The Pleistocene herbivore is known only to those who make prehistory their business. An entirely extinct genus of deer, Cervalces looked like a mash-up between an elk (Cervus) and a moose (Alces), and the animal’s name – coined by Princeton paleontologist William Berryman Scott in 1885 after studying the skeleton now on display in Trenton – is a tribute to the familiar, yet strange nature of the “elk moose.”
(Scott did not propose the species name for himself. That would have been a serious paleontological faux pas. Scott coined Cervalces, and the species epithet of C. scotti was added later to honor his contributions to the study of fossil mammals.)
Though not the most famous or charismatic of North America’s recently-lost megafauna, Cervalces epitomizes the strange nature of the Late Pleistocene world. The anatomy of the elk moose resembles that of its namesakes, yet it was neither an elk nor a moose, just as woolly mammoths were variations on the elephant archetype and saber-toothed cats were a specialized subset of the cat family. Strange, yet familiar, the awesome assemblages of Pleistocene megamammals disappeared tantalizingly close to us in time – most between 15,000 and 8,000 years ago – and, as Sharon Levy writes in her new book Once & Future Giants, the proximity of this lost world to our own has fueled an ongoing debate about what happened to the titans of the Ice Age.
Levy begins her well-written account with the story of how extinction was discovered in the first place. The death of species was once thought to be impossible, and the bones of Pleistocene megamammals played a key role in demonstrating the fact of extinction at the end of the 18th century. In 1799 the young French anatomist Georges Cuvier convincingly argued that the bones of the American mastodon and Siberia’s mammoths represented two species that were distinct from existing varieties of elephants, and these fossils were soon followed by the remains of other animals that did not exist in the modern world.
Cuvier’s simple discovery had major implications. Nature was not held in a static balance, but was constantly in flux – existing species perished and new species took their place. Identifying the exact causes of extinction, however, has always been a difficult and controversial enterprise. In his monograph on Cervalces, for example, Scott proposed that the impressively-antlered herbivore may have lost out in ecological competition with the “more perfectly adapted moose”, but this was only a vague just-so story. We still don’t know why the elk moose or so many of its giant Pleistocene contemporaries disappeared.
Catastrophic local floods, competition with surviving mammals, superdiseases, and even swarms of comets have all figured into scenarios of Pleistocene extinction, but Levy only mentions the two main contenders – climate change and hunting by humans. Of these two, only a bloody blitzkrieg by our prehistoric relatives receives a full hearing, with death by climate change treated as the standard, baseline hypothesis that has been forcefully challenged.
Both extinction triggers have the potential to explain the global pattern of the end-Pleistocene extinctions. In general terms, the transformation of the dry, cold Ice Age climate into a warmer, wetter one has traditionally been thought to be a major driver of the extinction, but during the past several decades some paleontologists have pointed out there is a “deadly syncopation” between the movements of Homo sapiens and the disappearance of large animals. Neither hypothesis is airtight – and there is no reason to believe that only one of these causes accounts for the all the species disappearances worldwide – but Levy more thoroughly considers the overkill hypothesis.
Curiously, the way Levy composed her account may reveal why many Pleistocene specialists have come to loggerheads over the causes of the extinctions. Though the pattern of extinction was global, much of the debate has focused on the activities of humans in North America and Australia. The less severe extinctions in Africa are treated as an enigma, the loss of species in Eurasia has been more often attributed to climate change, and what happened in South America is largely a mystery, leaving North America and Australia as the battlegrounds for the ongoing arguments about what humans did or didn’t do when they arrived. Additionally, the focus is on the loss of large species – the extinction of small species at the end of the Pleistocene and the changes in ranges of still-living species hardly figure into the debate at all.
Much of the debate about the Pleistocene extinctions – at least in North America – has been narrowed down even further into what happened to the woolly and Columbian mammoths. These symbols of the Ice Age have also been co-opted into symbols of the overkill hypothesis. A few skeletons damaged by stone tools have been found, and the detailed work of paleontologist Dan Fisher has provided indirect evidence that mammoths were under the sort of population pressure experienced by animals being hunted, and so it would appear that the rest of the Pleistocene fauna suffered dearly when humans developed a taste for mammoth meat. What happened to the giant ground sloths, sabercats, glyptodonts, camels, horses, deep-snouted bears, American lions, and other creatures is barely considered. We know so little about their natural history that it is difficult to reconstruct their last days, but mammoths are plentiful, charismatic, and well-studied enough that they have often been taken as indicators of the most powerful extinction pressures.
Levy’s lack of detail regarding the many other species that disappeared during the Pleistocene left me disappointed, but I must admit that her focus on the overkill hypothesis and the continents of North America and Australia makes rhetorical sense. After all, the book’s title is Once & Future Giants, and after Levy summarizes the ongoing debate over Pleistocene extinctions she connects the long-lost beasts to our future by invoking the concept of Pleistocene rewilding.
Though the proposal could be applied almost anywhere megamammals have been lost, Pleistocene rewilding has most often been discussed in reference to North America. The basic idea is to reinvigorated Pleistocene ecologies – the rudiments of which still exist – by introducing animal proxies to take the place of extinct species. Mammoths and mastodons may be gone, but African and Asian elephants might be able to take their place, and some species, like lions, would simply have their ranges expanded back into areas they occurred in long ago. The proposal does not suggest dumping African mammals into the middle of Kansas and stepping back to watch what happens, but it does advocate a fenced-in park or reserve where an artificial assemblage of native and alien animals could be mixed together in an attempt to recreate Pleistocene interactions. (How Pleistocene mammals without proxies – such as giant sloths, glyptodonts, and sabercats – might be substituted, no one has yet determined.)
Pleistocene rewilding is closely intertwined with the overkill hypothesis. We have no impetus to recreate Pleistocene ecologies if climate change did in the megamammals – especially since the environments they inhabited will have undoubtedly changed in the past few thousand years – but if our own species destroyed so many fantastic creatures, then Pleistocene rewilding can act as a penance. Other benefits to this brand of “resurrection ecology” have been named – mainly providing endangered animals safe places to shore up their numbers – but the overkill hypothesis is the most powerful rhetorical flourish used to argue for it.
Herds of elephants walking across the western plains and cheetah chasing after pronghorn makes for some powerful imagery, but those not already enamored by the proposal of Pleistocene rewilding will probably not be convinced to change their minds by Once & Future Giants. Levy pulls together a variety of sources – both from the fossil record and modern ecosystems – but the basic arguments in favor of transplanting lions and elephants to enclosures North America are the same that have been heard in books like Will Stoltzenburg’s Where the Wild Things Were and Caroline Fraser’s Rewilding the World. Levy beautifully documents how apex predators and immense herbivores play critical roles in modern ecosystems, but why we should strive to use these findings to create of facsimile of the Late Pleistocene remains poorly defined. Given the persistent resistance to the reintroduction of wolves to the Greater Yellowstone Ecosystem, Levy points out, the main battle advocates of Pleistocene rewilding face is political rather than scientific.
Contrary to what you might believe by this point in my review, however, I truly enjoyed Levy’s book. Other books on this topic have been written before – notably Twilight of the Mammoths by Paul S. Martin, a chief advocate of the overkill hypothesis and Pleistocene rewilding who recently passed away – but Levy’s ability as a storyteller is unmatched among the cadre of authors who have tackled this topic. Levy deftly shifts between interviews with scientists, the findings of scientific studies, and ongoing debates about conservation, making the book an informative and pleasurable read. Even if I would have preferred more detail about the state of the debate surrounding the end-Pleistocene extinction, Levy’s new book provides an excellent way to quickly get up-to-speed about the general state of the argument and the lines of evidence involved.
Once & Future Giants beautifully connects the world as we know it to one that, on a geological scale, disappeared only yesterday. We still understand so little about the natural history of the animals that went extinct, and a nuanced view in which both humans and climate change played significant roles in the decline of many species is beginning to emerge, but, for the moment, Levy’s book effectively states the case for those who want to heal ecological wounds thought to have been opened by prehistoric humans. In the modern era – when pristine, truly wild environments untouched by human influence no longer exist – we must take a more active role in ensuring the survival of imperiled species, and Levy’s book offers the outline of one way we might choose to create a different, wilder world.
Top Image: Cervalces scotti on display at the New Jersey State Museum. Photo by author.