The cultivation of fungus by ants is thought to have begun around 50 million years ago, when ancestors of the modern Attini ant tribe dropped the hunter-gatherer lifestyle in favor of becoming farmers. Today, every one of the 200+ species of Attini cultivates some species of fungus, but the most impressive and large-scale farming operations are carried out by the Atta and Acromyrmex genera – the leafcutter ants. These particular ants transitioned from small subsistence farms to large-scale agricultural enterprises about 20 million years ago, thanks to the process of coevolution.
Coevolution is the term used to describe two (or more) species reciprocally affecting one another’s evolutionary strategy, and tends to occur when these species interact closely over long periods of time. The most well-known example of coevolution is driven by predator-prey interaction: a plant develops spikes to deter an herbivore, which in turn grows a pointier snout to get around those spikes, which then grow closer together. This kind of “arms race” is frequently cited to support the view of evolution as a perpetual competition – a “survival of the fittest” power struggle. However, competition is just one piece of the evolutionary puzzle. No complex system can function without the benefits of mutualistic coevolution between species, and leafcutter ants as we know them would not exist without it.
For these ants, the mutualistic leap that changed their agricultural game (and eventually their very biology) occurred 30 million years into the small-scale farming experiment. At this point, one particular lineage of cultivated fungi (Leucoagaricus gongylophorus) evolved gongylidia: nutrient-filled growths occurring on the tips of cultivated fungus. These gongylidia served as specialized food packages for the ants, allowing the colony to sustain a much larger population. They do not exist in any uncultivated fungal species, and would be functionally useless outside an ant colony. Within an ant colony, however, they provide a terrific boost to the fungi’s caretakers, leading to a larger fungal crop and wider distribution.