In Bones of Mammoths, Scientist Seeks Clues to Extinctions
Ross MacPhee opened a clear plastic vial that held what looked like finely grated cheese.
Take a whiff, he said.
Two young visitors to his office at the American Museum of Natural History sniffed, wrinkled their faces and turned away from the rancid stench.
No doubt about it. Bone marrow from woolly mammoths really stinks.
But for MacPhee, who’d just returned from Siberia with three dozen vials of ancient bone samples, this scent may lead to a sweet reward: hard evidence for his theory of why mammoths and many other species of big mammals in North and South America disappeared about 13,000 years ago.
The list of animals that once roamed the region but perished in that extinction is startling: Besides mammoths, the North American losses include mastodons, camels, lions, cheetahs, saber-toothed cats, horses and giant sloths as big as buffalo. In South America, the losses include glyptodonts, which resembled armadillos but grew 20 times bigger.
MacPhee, curator of mammalogy at the museum, counts 135 species that disappeared from the Americas. That includes more than two-thirds of North America’s big mammals--those weighing more than 100 pounds. And some scientists believe that it all happened within maybe 400 years, a mere blink in paleontology.
What caused this? Debate has gone on for decades, centering mostly on two proposed explanations: climate change and voracious hunting by newly arrived humans.
“I think it’s sort of reached an impasse at this point,” said Don Prothero, an associate professor of geology at Occidental College in Los Angeles. “The two camps have pretty much said their say.”
MacPhee didn’t buy either idea.
Mammals had survived much bigger climate changes before, and how could a general phenomenon like climate kill animals from Alaska to Tierra del Fuego within a few centuries without bothering others in the West Indies at the same time?
The over-hunting idea, sometimes called the blitzkrieg hypothesis, made no sense to him either. The early Americans were too sparse and ill-equipped to drive so many mammals to extinction, he reckoned. And why would they? They couldn’t use all that meat and fur.
The over-hunting idea did have one thing going for it, he conceded. The fossil record did seem to show that in location after location around the world, extinctions often happened about the time people showed up.
MacPhee got into the debate after reading a magazine article about Ebola virus, the highly lethal germ that infects people. He began to wonder: Could viruses have arrived in the Americas with early humans or accompanying animals like wolf-dogs and scavenging rodents and birds? And could they have then jumped into a wide variety of mammals that had no natural defenses, causing a bunch of diseases that wiped out whole species?
Could this explain a number of catastrophic extinctions over the last 40,000 years?
MacPhee and virus expert Preston Marx developed this “hyperdisease” idea and published it in 1997.
“I don’t give it much chance,” said Paul S. Martin, an emeritus professor in geosciences at the University of Arizona. But the proposal, like any proposed explanation for the mysterious extinctions, is worth considering, he said.
“I like Ross’ risk-taking,” said Martin, the leading exponent of the over-hunting theory. “He’s going out on thin ice and hears the whistle of grapeshot going over his head for coming out with this damned fool idea.”
MacPhee cheerfully admits that he doesn’t have a whit of direct evidence yet for his idea. Nor is it clear what diseases are vicious enough to have done the job.
And there’s another thing. Not everybody thinks that the first humans showed up in the Americas just before all those big mammals died out.
The traditional view puts the human arrival about 13,000 years ago, around the time of the extinctions. But many specialists have recently started to consider the possibility that people reached the Americas as long as 30,000 years ago, said Rob Bonnichsen, director of Oregon State University’s Center for the Study of the First Americans.
Still, the mammal extinctions do roughly coincide with a time of great movement of human populations, unlike anything before or since, Bonnichsen said. So it makes sense that an arriving population might have brought in new germs, he said.
The search for direct evidence to support the hyperdisease explanation began earlier this year, when MacPhee’s colleague, Alex Greenwood, started looking for the smoking gun: traces of viruses in ancient mammal bones.
It would be a longshot to find anything, but not as long as it might appear. Greenwood recently announced that he’d been able to find traces of a special class of viruses in mammoth tooth, rib and marrow samples. Their age ranged from about 4,500 years to more than 23,000 years.
These viruses couldn’t have killed off the mammoths because they were natural residents of the creatures. Long ago, ancestors of these viruses had infected the ancestors of the mammoths Greenwood studied. DNA from those ancestral viruses mingled with the creatures’ own DNA in the sperm and egg cells. So the viral DNA was passed along to future generations, along with the animals’ own genes.
The result was so-called “endogenous retroviruses” that sprang from normal mammoth DNA. In contrast, ordinary viruses like the ones that cause a cold or flu invade from outside the body, causing new infections. That’s the kind of bug MacPhee thinks could have wiped out whole species. And that’s what Greenwood is trying to find now.
That will take some luck. For one thing, he doesn’t know what kind of virus to look for. One goal is to seek DNA typical of adenoviruses, the kind of germ that causes colds. There’s no reason to believe that an adenovirus would have caused extinctions, Greenwood said. But if he could find DNA from one, it would show that it’s feasible to look for traces of a lethal virus.
Another problem is how to recognize viruses that could kill mammoths, mastodons, saber-toothed cats and other mammals that aren’t around anymore for a direct test. The researchers can only hope to gather enough samples to show that a particular kind of virus shows up in the last known populations of various mammals, but not in earlier populations.
In August, the quest for more samples sent MacPhee to Siberia, where mammoths appear to have died out about the same time as in North America.
He joined other scientists and a group of local fishermen in strolling the tundra near Lake Taimyr, between the Kara and Laptev seas. They looked along riverbanks for exposed bones of mammoths and other big mammals.
MacPhee used a cordless drill to extract tissue samples the size of wine corks from these bones, including specimens like a 3 1/2-foot hipbone from a mammoth that stood 8 feet tall at the shoulder.
As he drilled, he hoped to smell the same stench that impressed the young visitors to his office. That would indicate well-preserved marrow and, in turn, a chance for well-preserved DNA.
It’ll take a long time to uncover and recognize any trace of a killer virus, if it’s there to be found at all. In the meantime, MacPhee says he’s not staking his career on the hyperdisease idea.
“If something better comes along tomorrow, it’s dead meat,” he said. But “right now, I don’t know what’s much better.”
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On the Net:
Symposium on extinctions: www.amnh.org/science/biodiversity/extinction
Theories about mammal extinctions:
www.discoveringarchaeology.com/0599toc/5cover1-mammoth.shtml
