A breakthrough innovation in animal evolution.

A mong the many activities of animals, few seem more outlandish than the leaf-cutting and fungus-growing habits of attine ants of the New World tropics. In a typical rainforest, endless processions of workers transport leaf forage on cleared highways across the forest f loor, each worker balancing a leaf fragment like an oversized green umbrella (Fig. 1). The ants do not eat the leaves, but they use them as garden compost to grow a nutritious fungus, the ants’ main diet. So efficient is the conversion of leaves into fungal food that the ant– fungus symbiosis has been called ‘‘one of the breakthroughs in animal evolution’’ (1), on par with such major evolutionary innovations as the ungulate rumen or the powered flight of birds. Leaf-cutter ants consume more vegetation than any other comparable herbivore (2), making them major agricultural pests. Early Brazilian farmers were so frustrated in their battles against the saúva (leafcutter ants) that they concluded ‘‘Brazil must kill the saúva or the saúva will kill Brazil.’’ Leaf-cutter ants are the ecologically conspicuous representatives of a larger group of more than 200 fungus-growing (attine) ant species, most of which do not cut leaves and instead use leaf-litter debris for fungal cultivation (3, 4). The common feature of all fungus-growing ants is their astonishing proficiency in planting, manuring, weeding, and sheltering fungal gardens, but the specific fungicultural habits are remarkably diverse among ant species, suggesting a long and complex coevolutionary history between attine ants and their fungal crops. Now, Schultz and Brady (5) report in this issue of PNAS a comprehensive phylogenetic analysis of the attine ants, providing detailed insights into the evolutionary transitions leading from simple to complex fungiculture and, ultimately, to the breakthrough of leafcutter fungiculture.