Ants grew mushrooms after the dinosaurs became extinct 66 million years ago

When an asteroid at least six miles wide hit Earth 66 million years ago, life on our planet suffered one of the worst mass extinctions of all time. The cataclysm not only wiped out almost all dinosaurs, leaving only beaked birds, but also caused extinctions and ecological collapse around the world. Those who survived the heat and fire on the first day had to contend with the more than three-year winter that followed. Photosynthesis almost stopped and life could barely sustain itself. However, during this dark time, new relationships began to evolve. In a world where plants struggled to grow back, fungi proliferated and ants began cultivating this new, abundant resource to survive. In the shadow of Earth’s fifth mass extinction, ants developed agriculture.

Although farming may seem like a human activity, many ant species have evolved in association with fungi. So far, entomologists have identified about 250 different species of ants that grow mushrooms in the Americas and the Caribbean. In their most basic form, ants collect and spread fungi, defending their food source in a mutually beneficial relationship. However, it is unclear when this close relationship began. To find out, Ted Schultz, curator of ants at the Smithsonian National Museum of Natural History, and a multi-institutional team of colleagues looked through a library of genetic samples of ants and fungi to estimate when their association began.

“I have been raising mushroom ants for over 35 years, since I was a student,” Schultz says. All this time, researchers weren’t sure how ants and fungi evolved together. “Because we didn’t have a good sample of non-ant-cultivated fungi that were closely related to ant-cultivated mushrooms, we didn’t know what fungal ancestors these ants descended from,” says Schultz. This relationship cannot be understood without knowledge about ants and the fungi they breed. But recent research has filled this gap, allowing scientists to finally determine when ants and fungi began their close relationship.

Published on Thursday in ScienceSchultz and colleagues’ study examined genetic data from 276 species of ants and 475 species of fungi to create evolutionary trees for both groups. The resulting trees not only show how mushroom-growing ants and their strains are related to each other, but can also be calibrated with additional evidence to estimate when particular traits evolved. The results suggest that the disaster sowed the groundwork for new relationships among surviving species.

Scientists found that farm ants evolved about 66.65 million years ago, around the time when life was struggling to recover after the asteroid disaster. “It really surprised me and seemed more than accidental,” Schultz says. Paleontologists have previously documented spikes in fungal spore numbers in the period just after an impact, which would provide the ants with plenty of fodder for farms. Experts hypothesize that ant farming originated only once, so the consequences of this impact appear to mark the beginning of fungus farming, which is practiced by many ant species today.

“I think the paper is solid evidence,” says University of Copenhagen entomologist Jonathan Shik, who was not involved in the new study. Previous research suggested that farm ants emerged about 66 million years ago, and new research now shows that fungal evolution also shows signs of this change, which would be expected for such a close evolutionary relationship. In a world where photosynthesis was reduced or almost stopped, “detritivores” ruled, Shik notes, and ants that were already feeding on the fungi gained an unexpected advantage that developed into a deeper form of interaction.

On those cloudy days 66 million years ago, ants were growing fungi, inadvertently helping them spread and protecting the food source from other organisms that might eat them. However, over time, some ants have evolved more complex relationships with their fungal food. For example, today’s leafcutter ants cut off leaves to take them back to their nests and lay them down as food for fungi, which they then eat. Experts call this “higher agriculture,” which a new study shows began among ants about 27 million years ago.

“Agriculture, whether practiced by humans or ants, is a complex process,” says Schultz, noting that “crops must be planted, weeded and nourished, and passed down from generation to generation of farmers.” It’s a huge undertaking, but it could perhaps enable both ants and fungi to survive during the next period of significant change in Earth’s habitats.

About 27 million years ago, Earth went through a cold period. Dense, moist forests retreated as grasslands spread in colder, drier climates. Ants that had been growing mushrooms in the ancient forests began to spread across and into more open grasslands, places that were not moist enough for their favorite mushroom to spread on its own. However, the ants needed the fungus, so they sheltered it in their nests and fed it materials such as leaves so that their food could grow. Ants created their own gardens back when our ancestors still crawled through trees in search of fruit.

Schultz notes that by having a timeline, researchers can better examine how ants evolved their farming skills. Humans can think about what to grow and how to farm, but ants perform similar tasks without our ability to predict or plan. Further research will likely provide new evidence of how ecological changes prompted some ants to become careful farmers and how some fungi were able to survive by cultivating them. Even though it seems like 66 million years ago from our present, farmer ants are symbols of how Earth’s ecosystems have bounced back from one of the worst disasters of all time.