Quest to figure out why mammals are warm-blooded led to new theory
By Carolyn Y. Johnson, Globe Correspondent | February 22, 2005
In the still unsolved mystery of how the dinosaurs died, there's a new suspect -- fungus.
After a meteor slammed into the Earth 65 million years ago, "the great dying" began, decimating life in the oceans and killing off the dinosaurs -- with mysteriously little effect on mammals. Conjecture over what did in the reptiles has long fascinated everyone from school children to paleontologists, but a new theory suggests that a less earth-shaking possibility could have played a role.
"The forests went out. The fungi proliferated, and the Earth became a giant compost pile. An enormous number of spores were released," said Dr. Arturo Casadevall, an infectious disease researcher who proposed last month that air thick with fungal spores after the meteor hit could have overwhelmed animals' immune systems, causing sickness and death. If he's right, the large numbers of warm-blooded mammals and birds that survived the mass extinction might have had a natural advantage -- body temperatures too hot for fungal infections to take hold.
"It's just a beautifully creative suggestion," said Nicholas Money, a mycologist, or mold expert, from Miami University of Ohio and author of "Carpet Monsters and Killer Spores: A Natural History of Toxic Mold."
Casadevall, of Albert Einstein College of New York, laid out his suggestion in this month's issue of Fungal Genetics and Biology when considering a much larger question: "I ask you, why are we so hot?"
He has long been troubled by the lives of warm-blooded animals, who must live a virtual food-finding mission because they burn so many calories each day just heating their bodies. Cold-blooded animals, on the other hand, need only eat once every few days. Where, he wondered, is the advantage in a life of constant scurrying, foraging, and saving up food for the winter?
That question coincided with another puzzling trend: Fungal infections rarely give mammals more than a mildly irritating case of athlete's foot or a yeast infection but are often deadly to plants, fish, and insects.
At a crucial time in natural history, the world's 1.5 million species of molds, yeasts, rusts, and mushrooms, also might have been a vehicle for natural selection.
In the aftermath of the meteor that carved out the Chicxulub crater on the Yucatan Peninsula, the Earth probably was a cool, shady place. Researchers last year discovered fossil evidence of a post-collision "fungal spike," and in a world dense with potentially pathogenic fungi, warm-blooded animals might have had a unique advantage.
In such a situation, "every warm-blooded generation has a little advantage, and when the dust settles and the sun comes out again . . . the warm-blooded find themselves in a world with a lot more space," Casadevall said.
Other evidence shows that the mass die-off didn't occur immediately after the collision, but about 300,000 years afterward -- raising the possibility that an intermediary factor, like fungi, could have played a part.
The trouble with the theory, experts said, is that no one is sure whether the dinosaurs were warm- or cold-blooded. Smaller cold-blooded animals like turtles, lizards, snakes, and frogs were able to weather the mass extinction, indicating that size, not body temperature, may have been a deciding factor.
And, while there is wide agreement that a massive meteor struck the Earth 65 million years ago, other theories suggest that increased volcanic activity could have played a role in the extinction.
Stephen McLoughlin, a geologist from Queensland University of Technology in Australia who discovered evidence of the long-ago fungal explosion, said the spores that his group studied, which were preserved in a layer of coal in New Zealand, probably did not harm animals.
He stated in an e-mail that he finds Casadevall's idea "intriguing" but, "while this may have been the case, it is virtually impossible to test."
Nonetheless, the main idea behind Casadevall's research -- that deadly fungi could have helped establish the age of the mammals -- is timely.
Fungal infections are now emerging as an important force in nature again: Fungal diseases also may be contributing to the worldwide decline of the coral reefs, and appear to play a poorly understood role in the steady decline of amphibians.
A study last year reported that a third of all amphibian species worldwide are facing extinction -- and while climate change, pollution, and habitat loss are all thought to play a role, many of the extinct and endangered frog species have been infected with the chtyrid fungus, which may interfere with their delicate, breathable skin, produce a toxin, or something else.
"Like everything in life, it wasn't just one thing" that killed the dinosaurs, Casadevall said. In the case of the amphibians, "you can imagine [the culprit] could be a weakening of their immunity caused by a fungus."
Carolyn Y. Johnson can be reached at firstname.lastname@example.org.