Ants are the world's most diverse and ecologically dominant eusocial organisms. Resolving the phylogeny and timescale for major ant lineages is vital to understanding how they achieved this success. Morphological, molecular, and paleontological studies, however, have presented conflicting views on early ant evolution. To address these issues, we generated the largest ant molecular phylogenetic data set published to date, containing Ϸ6 kb of DNA sequence from 162 species representing all 20 ant subfamilies and 10 aculeate outgroup families. When these data were analyzed with and without outgroups, which are all distantly related to ants and hence long-branched, we obtained conflicting ingroup topologies for some early ant lineages. This result casts strong doubt on the existence of a poneroid clade as currently defined. We compare alternate attachments of the outgroups to the ingroup tree by using likelihood tests, and find that several alternative rootings cannot be rejected by the data. These alternatives imply fundamentally different scenarios for the early evolution of ant morphology and behavior. Our data strongly support several notable relationships within the more derived formicoid ants, including placement of the enigmatic subfamily Aenictogitoninae as sister to Dorylus army ants. We use the molecular data to estimate divergence times, employing a strategy distinct from previous work by incorporating the extensive fossil record of other aculeate Hymenoptera as well as that of ants. Our age estimates for the most recent common ancestor of extant ants range from Ϸ115 to 135 million years ago, indicating that a Jurassic origin is highly unlikely.divergence dating ͉ Formicidae ͉ long-branch attraction ͉ phylogeny A nts (Hymenoptera:Formicidae) are the world's most successful group of eusocial insects. They constitute 15-20% of the animal biomass in tropical rainforests (1, 2) and occupy keystone positions in many terrestrial environments (3). Ants are among the leading predators of invertebrates in most ecosystems and are also prominent herbivores in many neotropical communities. Various ant species participate in symbiotic relationships with Ͼ465 plant species in Ͼ52 families (4), with thousands of arthropod species (5, 6), and with as-yet-unknown numbers of fungi and microorganisms (7). Some ant lineages have evolved astonishing adaptive specializations [agriculture of fungi, seed harvesting, herding and milking of other insects, communal nest weaving, cooperative hunting in packs, social parasitism, and slave-making (6)] that have fueled the curiosities of scientists as well as the general public.Understanding the sequence of events contributing to the rise of ants to ecological dominance requires a robust phylogeny of their early evolution and a reliable timescale for their diversification. However, both the age of ants and the relationships among their earliest evolving lineages remain controversial. Ant fossils from the Cretaceous are relatively scarce (8, 9), although their abundance and diversity inc...

Agriculture is a specialized form of symbiosis that is known to have evolved in only four animal groups: humans, bark beetles, termites, and ants. Here, we reconstruct the major evolutionary transitions that produced the five distinct agricultural systems of the fungus-growing ants, the most well studied of the nonhuman agriculturalists. We do so with reference to the first fossilcalibrated, multiple-gene, molecular phylogeny that incorporates the full range of taxonomic diversity within the fungus-growing ant tribe Attini. Our analyses indicate that the original form of ant agriculture, the cultivation of a diverse subset of fungal species in the tribe Leucocoprineae, evolved Ϸ50 million years ago in the Neotropics, coincident with the early Eocene climatic optimum. During the past 30 million years, three known ant agricultural systems, each involving a phylogenetically distinct set of derived fungal cultivars, have separately arisen from the original agricultural system. One of these derived systems subsequently gave rise to the fifth known system of agriculture, in which a single fungal species is cultivated by leaf-cutter ants. Leaf-cutter ants evolved remarkably recently (Ϸ8 -12 million years ago) to become the dominant herbivores of the New World tropics. Our analyses identify relict, extant attine ant species that occupy phylogenetic positions that are transitional between the agricultural systems. Intensive study of those species holds particular promise for clarifying the sequential accretion of ecological and behavioral characters that produced each of the major ant agricultural systems.Attini ͉ divergence dating ͉ Formicidae ͉ phylogeny ͉ symbiosis A ttine ants (subfamily Myrmicinae, tribe Attini) comprise a monophyletic group of Ͼ230 described species, exclusively New World and primarily Neotropical in distribution (1-4). All attine ants obligately depend on the cultivation of fungus gardens for food. So complete is this dependence that, upon leaving the maternal nest, a daughter queen must carry within her mouth a nucleus of fungus that serves as the starting culture for her new garden (5-7). Attine agriculture achieves its evolutionary apex in the leaf-cutting ants of the genera Acromyrmex and Atta, the dominant herbivores of the New World tropics (8, 9). Unlike more primitive attine ants that forage for and cultivate their fungus gardens on organic detritus, leaf-cutting ants have acquired the ability to cut and process fresh vegetation (leaves, flowers, and grasses) to serve as the nutritional substrate for their fungal cultivars. This key evolutionary innovation renders a mature Atta colony the ecological equivalent of a large mammalian herbivore in terms of collective biomass, lifespan, and quantity of plant material consumed (9).Attine ant agriculture is the product of an ancient, quadripartite, symbiotic relationship between three mutualists and one parasite. The mutualists include the attine ants, their fungal cultivars (Leucocoprineae and Pterulaceae), and filamentous bacteria in the genus ...