Our single-celled ancestors darted around the world’s vast ocean a billion years ago, propelling themselves with tiny flagella tails and feeding on primitive plants, algae, and one another. Around this time, two groups of these ancient creatures branched into what would become two of life’s most successful kingdoms. One group developed into animals. The other became fungi. Animals and fungi both breathe oxygen and replenish their energy by eating food. Their cells are similar. The two closely akin kingdoms have occupied the Earth through most of their histories in an awkward fraternal tussle. When environmental conditions change quickly, fungi turn into opportunistic parricides, attacking and feasting on their enfeebled animal kin. Deadly fungi are thriving today amid environmental tumult, wiping out nests of bumblebees, colonies of bats, and hundreds of species of frogs.
And they are coming for us.
Animals typically gulp down and then digest their food. Most fungal species have a different strategy: They stretch ravenous tentacles called mycelia into their meals, squirt out digestive enzymes, and slurp up the dislodged nutrients. Thanks to this feeding style, fungus acts as the world’s great decomposer. It breaks down dead plants and animals, freeing up and recycling organic compounds.
After BP’s Deepwater Horizon blowout, nematodes and other tiny animals virtually disappeared from oil-coated swaths of sand around the Gulf of Mexico. The tainted habitats are now teeming instead with molds, the same types of fungus that speckle shower curtains. The molds are breaking down the crude oil into carbohydrates that will be more palatable to animals.
As useful as these decomposers are, fungi don’t hesitate to feast on living cells when they get the chance. Fungi’s ability to switch effortlessly between different diets, sometimes eating living bodies and sometimes eating dead ones, boosts their pathogenic pestilence. And their greatest trick is their ability to shape-shift. Fungi can retreat to spore form and survive long periods without food. As spores, they can float vast distances through water or air. When conditions suit them, they can quickly grow long mycelia and burrow into flesh, alive or dead, or in some instances slip into living cells to feast on their prey.
These twin abilities—to subsist for long periods without eating and to change diets as needed—mean that fungal diseases are particularly dangerous. Bacterial and viral diseases burn themselves out when they kill their victims before spreading to new hosts. But fungi can wipe out entire populations of their hosts without jeopardizing their own existence.
During Earth’s greatest mass extinction, triggered 250 million years ago possibly by sudden climate change, soil-dwelling fungi rose from the ground to feed on forests that were weakened by environmental bedlam. “Less healthy plants are more prone to become infected by such fungi,” said Cynthia Looy, a biologist at the University of California, Berkeley who investigates how plants respond to environmental change. “Fungi can accelerate the demise of already stressed, unhealthy plants.”
During the comet-induced mass extinction that doomed the dinosaurs 65 million years ago, fungal spores suddenly saturated the world. Some researchers speculate that fungi dealt final deathblows to weakened dinosaurs, helping to give rise to the age of mammals. Fungi are poorly suited to growing in hot conditions, and mammals’ warm blood provides some protection against them.
During the past century, fungal diseases have felled great forests of elms, chestnuts, pines, and other trees around the world, overturning ecosystems and leaving grassy wastelands in their wake. The chytrid fungus Batrachochytrium dendrobatidis is thought to have wiped out hundreds of species of amphibians and has been fingered as the cause of frog die-offs worldwide. Scientists recently reported that the fungus is also infecting and killing crayfish. White-nose syndrome was discovered affecting a few unfortunate bats in New York in 2006; the fungus responsible for the disease has since killed more than 5 million hibernating bats in 21 states and four Canadian provinces.
Scientists have never before witnessed pathogens tearing such virulent paths of destruction through wildlife. Increasingly, humans are succumbing to fungal diseases, too.
Many people have immune systems that are debilitated by age, diseases such as AIDS, or therapies that keep organ-transplant and cancer patients alive. “This cadre of immunosuppressed patients is at major risk of fungal pathogens,” said Joseph Heitman of Duke University Medical Center’s Center for Microbial Pathogenesis. “And that group will grow.”