February 7, 2005 (BRONX, NY) We know that
dinosaurs died out about 65 million years ago. And we know that whatever did
them in paved the way for the rise of mammals. Scientists have proposed all sorts of causes for the dinosaurs demise,
ranging from giant meteors to catastrophic volcanoes. But maybe we should try thinking smaller as in
fungi suggests Dr.
Arturo Casadevall, professor of microbiology and immunology at the Albert
Einstein College of Medicine of Yeshiva University. He discusses his theory in a
commentary published in January in the ScienceDirect publication Fungal Genetics and Biology.
Fungi are among the earliest forms of life: The fungal lineages present today emerged about one billion years ago and then proceeded to infect later-evolving organisms, some of which are more susceptible to fungal infection than others.
Fungi grow best at temperatures between 77 and 95 degrees F. and so have trouble surviving at the higher temperatures inside warm-blooded mammals. By contrast, fungi are often associated with disease in cold-blooded organisms such as insects, fish, reptiles and amphibians. Since most of the large dinosaurs are also thought to have been cold-blooded, Dr. Casadevall theorizes that fungi may have contributed to their extinction.
"Whatever happened 65 million years ago killed off many terrestrial species including dinosaurs but spared mammals," says Dr. Casadevall. "Researchers have recently found evidence for massive proliferation of fungi around that time, probably due to decreased sunlight and cooler global temperatures. This fungal outburst would have caused a huge increase in airborne fungal spores that animals would have inhaled in tremendous amounts."
Dr. Casadevall notes that microbial dose is crucial for determining the outcome of an infection and that inhaling microbes in massive numbers can overwhelm even animals with healthy immune systems. "With their higher body temperatures, mammals would have been better poised than the cold-blooded dinosaurs to survive in the fungi-rich world that existed back then," he says.
For further evidence supporting this theory, look no further than the sky. "Birds are the only non-extinct descendants of the dinosaurs," says Dr. Casadevall. They survived the great dinosaur extinction along with mammals and, like the mammals, birds are warm-blooded.