The Chicxulub asteroid struck 66 million years ago and ended the nonavian dinosaurs. But a new PNAS study published May 12 is pointing the spotlight at something that may have been happening long before the “final blow”: a world already “beginning to rot,” with fungal blooms showing up tens of thousands of years prior to the impact.

In the study, researchers led by Dr. Arturo Casadevall, chair of the Molecular Microbiology and Immunology Department at Johns Hopkins University, identify an earlier fungal bloom dating roughly 30,000 to 100,000 years before the asteroid hit. They connect it to a cooling period tied to intense volcanic activity in the Deccan Traps in what is now India, and they also report a fungal spike immediately after the Chicxulub impact, lining up with the moment when devastated ecosystems would have been littered with dead plants and animals.

If you are tracking what drives mammals winning the post-cataclysm land race, this matters. The findings revive the fungal infection-mammalian selection, or FIMS, hypothesis. Proposed two decades ago by Casadevall and updated in 2018 and 2026 papers, FIMS does not claim fungi wiped out dinosaurs. Instead, it argues the abundance of fungus before the asteroid impact may have helped mammals gain an evolutionary advantage over dinosaurs that survived the event. The key mechanism is not “dinosaurs died of fungus” but that mammals’ live births and immune systems may have helped them withstand a disrupted environment full of rot, cold, starvation, and polluted air.

That is the thesis. Here is what the new evidence actually did. To find the pre-asteroid fungal bloom, Casadevall and Rosanna Baker, who researches fungus at Johns Hopkins, focused on palynomorphs, tiny fossilized organic remains. These include fungal spores, fungal hyphae, and plant pollen preserved in ancient sediment layers from the Denver Basin in Colorado dating to the K-Pg extinction. Baker counted between 100 and 500 microfossils per sample across sediment layers spanning about 60,000 years before the K-Pg mass extinction event to 30,000 years after it. Most samples were dominated by plant material, but some layers contained 50% or more fungal spores. The researchers interpreted that high proportion as a sign of ecological disturbance: more dead material and more fungi feeding on it, especially after the dinosaur-killing asteroid.

The study also finds an earlier fungal bloom, roughly 10,000 to 30,000 years before the asteroid impact. And the really attention-grabbing part, according to Casadevall, is the timing. “What surprised us was the proliferation in association with Deccan volcanism, which implies ecological disruption prior to [the] meteor impact,” he told Live Science via email. Put plainly: the ecosystems may have already been under stress, and fungus may have been part of that story, before the asteroid delivered the final blow.

This is where FIMS becomes both powerful and controversial. The hypothesis leans on biological reality that mammals tend to be unusually resistant to invasive fungal diseases when their immune systems are healthy. Casadevall’s argument starts with immune defenses: first-line, relatively nonspecific defenses like neutrophils that recognize fungal cell walls, engulf spores, and attack invasive filaments, plus an adaptive immune system that builds customized antibodies and immune memory. He also previously argued that mammals’ generally stable body temperatures, typically around 97 to 104 degrees Fahrenheit (36 to 40 degrees Celsius), can make it harder for many environmental fungi to establish infection.

Then there is the “impact winter” angle. After Chicxulub, the environment likely included increased volcanism and worsening air quality as ash and dust filled the sky. When the asteroid struck, forests burned, plants died, darkness fell, and dead matter piled up, creating a feast for fungi. If spores filled the air, surviving animals could have inhaled enormous doses. FIMS proposes mammals had several adaptations that could blunt that risk. Mammals that could maintain stable body temperatures could forage during the cold impact winter when ash and dust blocked sunlight. Dinosaurs, often described as cold-blooded, would have struggled to stay active and find food, which could have weakened them and made disease exposure harder to survive. Gestation could also matter. Many mammals protect developing embryos inside the mother, where warmth and immunity shield embryos. Dinosaurs, by contrast, laid eggs. Those eggs developed in nests often in contact with soil and decaying plant matter, the kind of environments where fungi thrive.

To support the “fungi were a problem” idea, the source also points to earlier evidence from dinosaur fossils. In 2022, a team led by Cary Woodruff, curator of vertebrate paleontology at the Phillip and Patricia Frost Museum of Science in Miami, described a Late Jurassic sauropod fossil from Montana with unusual lesions in its neck vertebrae, a dinosaur known as MOR 7029 that lived roughly 150 million years ago, long before the asteroid impact. Sauropods had air-filled neck bones connected to their respiratory system. Woodruff told Live Science via email that in fossils of healthy sauropods, the places where air-sac tissue met bone are smooth, sometimes glass-like, while in MOR 7029 the normally smooth sockets were rough and irregular. That line of evidence is meant to show that fungal infection was not just a post-impact phenomenon.

Still, not all experts agree the new data settles the mammal advantage question. Mary O’Connell, Chair of the Zoology department at the University of Manchester, told Live Science via email that the hypothesis does not fit all evidence and “it emerges from a chain of plausibility arguments rather than direct evidence, and the links in the chain are weaker than the framing may suggest.” That is a familiar pattern in science and, frankly, in strategy: plausible mechanisms can be compelling, but they do not automatically equal proof.

For executives and board-level thinkers, the second-order takeaway is not “fungi caused mammal dominance.” It is that narratives about major disruptions are now being rebuilt around micro-level biological stress signals, with time-stamped evidence that can reach tens of thousands of years before a planetary event. If you run research-heavy organizations, publish, invest, or set long-range strategy, this is a reminder that the most consequential breakthroughs often look like updates to timelines. And timelines are where bets get made, budgets get allocated, and credibility gets tested.