Riley Black

Science Correspondent

When an at least six-mile-wide asteroid struck 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 the dinosaurs, only leaving beaked birds behind, but also triggered extinctions and ecological collapses the world over. Survivors of the heat and fire on the first day had to contend with more than three years of impact winter that followed, photosynthesis nearly halted as life struggled to hang on. During this dark time, however, new relationships were beginning to evolve. In a world where plants struggled to grow back, fungus proliferated, and ants began to cultivate this abundant new resource to survive. In the shadow of Earth’s fifth mass extinction, ants evolved agriculture.

Even though farming might seem to be a human endeavor, many species of ants have evolved intertwined relationships with fungus. So far, entomologists have recognized about 250 different ant species in the Americas and Caribbean that farm fungi. In its most basic form, ants collect and spread fungi, defending their food source in a mutually beneficial relationship. When this close relationship began, however, has been unclear. To find out, the Smithsonian’s National Museum of Natural History curator of ants Ted Schultz and a multi-institution team of colleagues looked to a library of genetic samples from ants and fungi to estimate when their connection began.

“I’ve been working on fungus-farming ants for over 35 years, ever since I was a graduate student,” Schultz says. During all that time, researchers were unsure how ants and fungus evolved together. “Because we did not have a good sample of non-ant-cultivated fungi closely related to the ant-cultivated fungi, we did not know from what fungal ancestors the ant fungi arose,” Schultz says. The relationship couldn’t be understood without knowledge of both ants and the fungi they grow. Recent research has filled in the gap, however, allowing scientists to finally map when ants and fungi began their close relationship.

Published Thursday in Science, the research from Schultz and colleagues examined genetic data from 276 species of ants and 475 fungi species to create evolutionary trees of both groups. The resulting trees not only indicate how fungus-farming ants and their cultivars relate to each other, but also can be calibrated with additional evidence to estimate when particular traits evolved. The results suggest that catastrophe sowed the ground for new relationships to emerge among surviving species.

Yeast agriculture
A worker of the fungus-farming ant species Cyphomyrmex cf. rimosus, collected in Mindo, Ecuador, in 2011, on its fungus garden Alex Wild

Agricultural ants evolved about 66.65 million years ago, the researchers found, just about the time that life was struggling to recover from the asteroid’s aftermath. “That really surprised me and struck me as way beyond coincidental,” Schultz says. Paleontologists have previously documented spikes in fungal spores from the time period just after impact, which would have provided ants plenty of fodder to farm. Experts hypothesize that ant agriculture originated only once, and so it seems that the impact aftermath marks the beginning of the fungus farming many ant species busy themselves with today.

“I think the paper makes a solid case,” says University of Copenhagen entomologist Jonathan Shik, who was not involved in the new study. Previous research proposed that farming ants arose around 66 million years ago, and the new research now indicates that fungus evolution shows signs of the shift, as well, as would be expected in such a close evolutionary relationship. In a world where photosynthesis was reduced, and nearly halted, “detritivores ruled,” Shik notes, and ants that were already feeding on fungus had an unexpected advantage that bloomed into a deeper form of interaction.

In those cloudy days of 66 million years ago, ants farmed fungus by inadvertently helping it spread and defending the food source from other organisms that might eat it. Over time, however, some ants evolved more multifaceted relationships with their fungal food. Leafcutter ants today, for example, snip off leaves to take back to their nests and deposit them as food for the fungus that they then eat. Experts refer to this as “higher agriculture,” which the new study indicates began among ants about 27 million years ago.

“Agriculture, whether practiced by humans or by ants, is a complex process,” Schultz says, noting that “crops need to be planted, weeded and nourished, and they need to be transmitted from one farmer generation to the next.” It’s a huge undertaking, but one that might have allowed both ants and fungus to survive during another time of significant shifts to Earth’s habitats.

Around 27 million years ago, Earth was going through a cold period. Dense, humid forests drew back as grasslands spread in the cooler, drier climate conditions. Ants that had been cultivating fungi in the ancient forests began to spread across and into the more open grasslands, places that were not wet enough for the favored fungus to spread on its own. The ants needed the fungus, however, and so sheltered the fungus in their nests and fed it materials like leaves so that their food might grow. Ants created their own gardens during a time when our own ancestors were still scrambling through the trees in search of fruit.

With a timeline in place, Schultz notes, researchers can better investigate how ants evolved their farming skills. Humans can think about what to grow and how to farm, but ants carry out similar tasks without our foresight or planning abilities. Further study will likely turn up new evidence of how ecological shifts led some ants to become careful farmers and how some fungus has been able to survive thanks to their cultivation. As distant as 66 million years ago feels from our present moment, farming ants are emblems of how Earth’s ecosystems bounced back from one of its worst disasters of all time.

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Riley Black

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.