Charting uncertainty about ant origins

Over a wide range of environments, up to five ant species forage every square meter of ground (1). In Amazonian rainforests, the biomass of ants dwarfs that of vertebrates (2), and in many rainforest trees, ants make up a large fraction of individual insects (3). This ecological dominance and the complexity of their societies makes their phylogeny of great interest as a glimpse into the development of the modern world in terms of the relationships between the various groups of ants, how their characteristics evolved, and when they originated. This year we have seen not one but two blockbuster articles examining ant phylogeny and time of origin of the group, one of which is by Brady et al. (4) in this issue of PNAS. The two articles (4, 5) agree in several important respects but disagree in others.

Early thought on ant phylogeny was bedeviled by the belief that all or most of the genera with armored cuticles and strong stings belonged in a single subfamily, the Ponerinae (6). Brown (7) pointed the way forward by suggesting that various other ant subfamilies arose within the ponerines, which are thus paraphyletic; presciently, he proposed a close relationship between the Ectatomminae (then a ponerine tribe) and the giant subfamily Myrmicinae [>4,500 species (8)]. However, he made no nomenclatural change, and subsequent authors tended to treat the ponerines as a single group. This tendency to agglomerate seriously compromised the ability to make sense of ant phylogeny, and for decades the procession of phylogenetic schemes was notable in its diversity rather than its stability. The crucial breakthrough came from Bolton (24), who erected a host of new subfamilies and subdivided the original subfamily Ponerinae into six; although he still placed all of these together, this recognition of difference liberated phylogeneticists to make …

*E-mail: ross.crozier{at}jcu.edu.au