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. 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.
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 moulds, the same types of fungus that speckle shower curtains. The moulds 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. 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.
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. Scientists project 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 immunosupressed patients is at major risk of fungal pathogens," said Joseph Heitman of Duke University Medical Center's Centre for Microbial Pathogenesis. "And that group will grow."
As we tear up forests and turn over soil, we unleash spores from their slumber, including species that humans and other animals have rarely encountered before. World trade is helping strains of fungus spread and hybridiae. And our wanton use of antibacterial medicine, including in farm animals, kills the microbes that could help keep fungus levels in check.
"The environment is changing quite dramatically," Heitman said. "Logging, gardening, forestry and other things that perturb the environment and move around soil or trees contaminated with the fungus are a major contributor."
Fungus doesn't just eat away at our organs and cells; it tucks voraciously into our food. Moldy bread can be a minor bother, but fungal agricultural pandemics have the potential to occur on a staggering scale. Researchers writing in Nature calculated that the known fungal pathogens could wipe out more than one-third of the world's supply of major crops if severe epidemics struck simultaneously. And they point out that diseases such as rice blast and wheat rust are already having a major impact on agricultural productivity: "Our calculations show that even low-level persistent disease leads to losses that, if mitigated, would be sufficient to feed 8.5 per cent of the 7 billion humans alive in 2011."
This year, 39 people have died and 581 others have been sickened after mold spores infiltrated immunosuppressant medicine. The tainted drugs were injected directly into patients' central nervous systems, where the fungus blossomed. Researchers are racing to develop vaccines against some of the most deadly fungal pathogens. Anti-fungal medicines are readily available; they work by damaging fungal membranes and cell walls. But because we're so closely related, potent medicines that damage fungal cells can also harm human organs they were designed to protect.