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wiped out the large vertebrates through directpredation pressure; although he acknowledgesthe potential role of less direct human influ-ences, such as pathogen propagation and arti-ficial fires, he focuses primarily on hunting.Second, he posits that the extinctions occurredextremely quickly; for example, he thinks thewhole of North America could have lost itsmegafauna in a few tens or hundreds of years.
It is convenient that Martin takes such anextreme position because it renders his modeltestable. If it can be shown that humans co-existed with megafauna for a thousand yearsor more, his blitzkrieg version of the overkillmodel would be in trouble. In most cases, suchas in Australia and South America, thepalaeontological record is not known in suffi-cient detail to determine the precise dates ofhuman arrival and megafaunal extinction. Butthe situation is better in North America and onthe large oceanic islands, where the timing ofthe two phenomena correlate very closely.However, if humans were responsible for suchlarge-scale devastation, where are the archaeo-logical kill sites? In North America, only adozen or so late Quaternary sites have beenfound with clear evidence of butchery (andalmost all of these are mammoth), and thereare no known sites with artistic depictions ofhumans interacting with any members of theextinct megafauna.
To most observers, the paucity of kill sitesconstitutes strong evidence against the overkillmodel. Martin, however, argues that thisabsence of evidence actually supports his posi-tion: if the extinction event was extremely rapid,the devastation would fail to register in thearchaeological record. Overkill seems to havebecome an ide fixe for Martin, and his logicbecomes circular at times. For example, hewrites: I have grave doubts about the existence
of a widespread and biologically effectivehuman population in the Americas before13,000 years ago precisely because large, slow-moving, eminently huntable animals such as ground sloths continued to occupy theirfavorite dung caves in North and South Americaas late as they did. In other words, he takes thepresence of megafauna to signal the absence ofhumans, because the two could not possiblyhave coexisted. Martin is certainly more effec-tive when he is working to test his model thanwhen he is assuming it.
As if his overkill hypothesis were notcontroversial enough, Martin finishes the bookwith a call to restore lost megafauna throughthe introduction of closely related species, orecological proxies, into the blighted areas. In North America, he declares, we ought torestore not only bison and brown bears, butalso proboscideans, camels and lions. If giantground sloths are not available, we could userhinos as surrogates. Martin recognizes that hissolution is not perfect (and that in some casesthe political hurdles may be insurmountable),but he argues that this attempt at restoration isinfinitely better than continual attrition.
Whatever scientists may think of Martinscall for resurrection ecology, it must be agreedthat his perspective fundamentally challengesour conception of what is natural. Many ecol-ogists seek to study systems without a contem-porary human presence so they can betterassess the pure dynamics of nature. Martinswork impresses on us the fact that Earths ter-restrial ecosystems were profoundly restruc-tured long before European colonialism andindustrialization. That, at least, no longer seemscontroversial. Alan B. Shabel is in the Department of IntegrativeBiology, University of California, Berkeley,Berkeley, California 94720-3140, USA.
A mammoth murder mysteryTwilight of the Mammoths: Ice AgeExtinctions and the Rewilding of Americaby Paul S. Martin University of California Press: 2005. 269 pp.$29.95, 18.95
Alan B. ShabelIn 1877, Richard Owen suggested that thegreat Pleistocene fossil mammals of Australiahad been driven to extinction by the hostileagency of man. Seven years later, C. S. Wilkin-son replied that a reduction in rainfall, leadingto the impoverishment of a once rich flora, was a more likely trigger for the loss of thesebeasts. And so it has always been in the studyof late Quaternary extinctions: arguments foranthropogenic causes are countered by argu-ments in favour of climate change.
It is widely accepted that in the past 50,000years, the world has lost a diverse array of rep-tiles, birds and mammals. A striking feature ofthis mass extinction event is that it dispropor-tionately affected large-bodied vertebrates(more than 45 kg). With few exceptions, smallvertebrates, plants and insects did not sufferdramatic losses. Another striking feature is theextinction events geographic asynchrony: theextinctions occurred first in Australia (about46,000 years ago) and much later in the Ameri-cas (13,000 years ago), the West Indies (5,000years ago), Madagascar (2,300 years ago) andNew Zealand (500 years ago). On each landmass, the extinctions occurred soon after theimmigration of Homo sapiens, and the timingof the extinctions does not correlate withanomalous changes in climate.
In the 1960s, Paul Martin sparked the mod-ern study of the late Quaternary extinctionswith his overkill hypothesis, which arguesforcefully for the role of human hunting in theextinction event. In his new book, Twilight ofthe Mammoths, Martin synthesizes fifty yearsof his research on the subject in a palaeonto-logical memoir. He takes an extreme position,in two respects. First, he argues that humans
BOOKS & ARTS NATURE|Vol 441|25 May 2006
Was climate change or hunting by humans to blame for the demise of large mammals in the Pleistocene?
ELECTRIFYING BOOKWINS AVENTIS PRIZEDavid Bodanis has won this year's Aventis Prizefor science books. His book, Electric Universe: How Electricity Switched on the Modern Worldpublished by Little, Brown, was described by thejudges as a very good read that opens up auniverse of facts that would scarcely be crediblein an imaginary tale.The junior prize went to Kate Petty, JennieMaizels and Corina Fletcher for their book TheGlobal Garden, inspired by the Eden Project andpublished by Eden Books (Transworld). The winners received their awards at a ceremonyon 16 May 2006 at the Royal Society in London.
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