Fungus helped create us, could destroy us

By John Upton

In a story about plagues of fungus diseases that I wrote for Slate’s pandemics series, I touched on fungi’s role in the rise of the mammals.

The last great extinctions occurred 65 million years ago, when land-dwelling dinosaurs disappeared and mammals began an ascent that eventually led to our own evolution. The dinosaurs were doomed when the Earth clouded over with smoke. The darkened world grew cold, reptiles were unable to bask effectively and plants struggled to photosynthesize. While other kingdoms of life flailed, fossil records indicate that fungi flourished.

Illustrated by Perry Shirley.
Illustrated by Perry Shirley.

Fungi are the world’s great decomposers, and during periods of environmental upheaval they can become savagely pathogenic, feasting on the living flesh of the weak. But fungus does not grow well in hot conditions. In 2005, Arturo Casadevall suggested that the rise of fungus during the Cretaceous–Paleogene extinction event selectively killed off cold-blooded dinosaurs and gave warm-blooded mammals the opportunity to prosper. In August, the Albert Einstein College of Medicine professor published a followup paper in PLOS Pathogens that expanded and built upon his theory.

“Mammals are highly resistant to systemic fungal diseases,” Casadevall wrote in the paper.

Not a real dinosaur / Kamala Kelkar
Not a real dinosaur / Kamala Kelkar

“Primitive mammals like the platypus, with core temperatures near 32°C, are susceptible to Mucor amphibiorum, a fungus with a maximal thermal tolerance of 36°C that would make it avirulent for higher mammals. The resistance of mammals to fungal diseases is in sharp contrast to the vulnerability of other vertebrates, such as amphibians, a group that is currently under severe pressure from a chrytrid. Like mammals, amphibians have adaptive immunity, but unlike mammals, they are ectotherms and lack a thermal environment that is exclusionary to fungi.”

Casadevall tells me he’s more confident now in his theory than he was when he first described it seven years ago. That’s partly because of a study that he co-authored in 2010 that indicated that the human body temperature is almost ideally optimized for warding off fungal diseases while maintaining metabolic needs. He said the spread of white nose syndrome, a soil fungus that in North America has killed millions of hibernating bats, whose temperatures plummet during winter, provides additional support for his theory.

“People have been intrigued with the fungal-mammalian hypothesis,” Casadevall told me. “There has been no significant pushback.”

In his new essay, Casadevall says global warming could help fungi adapt to warmer temperatures, potentially reaching a point where pathogenic species could develop newfound abilities to infect warm-blooded mammals. Such a development could be disastrous for humanity, as I explain in Slate.

[To join a LinkedIn group devoted to the discussion of fungus diseases, click here.]