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For many years, these three competing hypotheses--climate change, overhunting, hyperdisease--seemed to be little more than intriguing ideas. But at long last scientists are beginning to buttress them with hard data. Within the past year, for example, a team led by MacPhee and his colleague, molecular biologist Alex Greenwood, has managed to isolate viral material from frozen mammoth tissues and bone marrow. So far they haven't identified a microbial culprit for the extinctions, but candidates under consideration include the species-crossing viruses that cause rabies and distemper.
For his part, vertebrate paleontologist Russ Graham of the Denver Museum of Nature and Science is collecting evidence of climate-driven habitat changes that occurred during the critical time frame. Among other things, he notes, the forests and savannas in which mastodons and mammoths foraged not only became less diverse as glacial conditions wound down but also dwindled in size. As the preferred habitat shrank, so did the numbers of these huge animals, rendering the animals vulnerable to all sorts of bad influences, including disease and overhunting.
Proponents of the overkill hypothesis have also been busy. In another article published in Science last week, Tim Flannery and his colleagues describe how they used a sophisticated new dating technique to pin down the timing of a megafauna extinction that occurred in Australia. A total of 55 species of vertebrates, including giant kangaroos and an ostrich-size bird known as Genyornis newtoni, all disappeared about 46,400 years ago, they report--within 5,000 to 10,000 years of the presumed arrival of humans. The same close correlation between the appearance of humans and the disappearance of large-bodied animals has been documented in New Zealand and Madagascar.
Many find the progress being made on such a number of fronts encouraging. It means that scientists may soon be able to rule out some scenarios and bolster others. From growth rings on fossil tusks, for example, University of Michigan paleobiologist Dan Fisher can already tentatively say that climate change alone does not seem to be an adequate explanation for the demise of the mammoths. In the tusks examined to date, he notes, there is no discernible pattern of growth retardation or delayed maturity, both of which would be expected if climate change disrupted the plant communities that sustained these large herbivores.
On one thing, at least, all the scientists agree, and that is the importance of solving this long-standing mystery. Globally, Flannery observes, the disappearance of the megafauna amounts to "the biggest extinction event since the age of the dinosaurs," and in some respects, it is even more troubling. The dinosaurs, after all, were wiped out after a comet or asteroid smashed into Earth. The extinction of the megafauna, directly or indirectly, may have been our doing.
--With reporting by Andrea Dorfman/New York