Some ants are impressive farmers. You might have seen leaf-cutter ants carrying plant bits back to their colony — not for eating, but for farming.
Ants use these plants to cultivate fungi, which produce food for the colony. This is a form of agriculture, and ants have even domesticated several fungi, plants, and even animals for their benefit. Ant agriculture is relatively recent discovery. Still, researchers previously determined that ants have done this for at least 50 million years.
Now, a new study suggests this practice might be even older — around 66 million years old. And it could be linked to the event that wiped out the dinosaurs.
Ants have been at it for millions of years
Hundreds of ant species practice agriculture nowadays.
“The origin of fungus-farming ants was relatively well understood, but a more precise timeline for these microorganisms was lacking. The work provides the smallest margin of error to date for the emergence of these fungal strains, which were previously thought to be more recent,” explains André Rodrigues, professor at the Institute of Biosciences of São Paulo State University (IB-UNESP) in Rio Claro, Brazil, and a study author.
The research builds on decades of fieldwork and genomic data, building a complex family tree. But the researchers didn’t look at the ants; they looked at the fungus the ants grow. In this family tree, the team focused on something called ultraconserved elements (UCEs).
Ultraconserved elements are regions of a genome that are well-retained across different species, often remaining unchanged for millions or hundreds of millions of years. UCEs are thought to be involved in essential regulatory processes, such as gene expression, development, and maintaining genomic integrity. Their precise functions are not fully understood but their extreme conservation across distantly related species highlights their evolutionary importance.
The researchers looked for these elements throughout the genomes of 475 fungal species cultivated by ants and collected from different parts of the Americas. They found that the ancestors of modern ant-grown fungi evolved 66 million years ago, when a massive asteroid hit the Earth, triggering the mass extinction that wiped out the dinosaurs.
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Ants, dinosaurs, and meteorites
The development of agriculture is often considered the start of human civilization, but ants have been doing it for way longer. There’s evidence that ants and fungi proliferated at the end of the Cretaceous, when the dinosaurs went extinct. This new study suggests the idea that fungi had already undergone some pre-adaptations before being grown by the ants.
The theory is that the fungi were growing near ant colonies and the ants would eat them every now and then. The chain of events triggered by the cataclysmic meteorite could have pushed the two groups closer together. The ants might have suffered a nutritional decline from other food sources and turned more and more to the fungi.
“The fungi were not an essential part of the ants’ diet. The pressure exerted by the meteor impact may have turned this relationship into an obligatory mutualism, in which these fungi come to depend on the ants for food and reproduction, while at the same time the ants depend exclusively on the fungi as a food source,” Rodrigues says.
This practice was beneficial for all parties, the researchers say.
“To feed itself, the fungus decomposes the organic matter carried by the ants. In turn, the ant consumes substances produced by the fungus that it couldn’t obtain from any other source. It’s as if the fungus were the insect’s external stomach,” adds the researcher.
Coevolution and agriculture
The researchers also pieced together a part of the ants’ evolutionary trajectory. The first fungus-farming ants lived in humid forests, but some 27 million years ago, they started to diversify, extending into more open and arid areas — ultimately evolving into the leafcutter ants we see today.
Nowadays, researchers know of 247 species of ants that tend to the fungi. Curiously, however, the ants’ agricultural techniques still hold some secrets.
“When we cultivate them in the lab, the fungi take the expected form of hyphae. However, inside the colony, one of these hyphae types becomes swollen and forms structures similar to grape clusters, rich in sugars. We still don’t know how the ants do this,” says Kooij.
Journal Reference: Ted R. Schultz et al, The coevolution of fungus-ant agriculture, Science (2024). DOI: 10.1126/science.adn7179