Did modern ants evolve up from subterranean ancestors? Or did they diversify from above-ground species?
This straightforward question about ant history does not have a straightforward answer. If we look at early ant fossils, most sport the long limbs and large eyes typical of surface-dwelling species. Consider the Cretaceous ant Haidomyrmex:
If the fossils tell a tale of large-eyed, surface-dwelling ancestors spawning the modern ant fauna, genetic data from modern species give an apparently conflicting story. A recent paper by Andrea Lucky and others in PLOS ONE took the known habits of modern ants and triangulated back over an evolutionary tree to infer the ancestral state. The odds of a subterranean ancestor were more than 90%:
Which story do we believe?
Possibly, both. The existence of a conflict depends on how the extinct, large-eyed species are related to modern ants. Many of them are in the subfamily Sphecomyrminae, and their phylogenetic position is not known with certainty. If sphecomyrmines are merely another ant lineage contained within the other known ants, as beautifully illustrated in the below diagram that took at least 2 minutes to sketch, then we indeed have a conflict.
In this latter scenario, one particular lineage went underground (where they were less likely to be preserved as amber fossils) and from there radiated into the ants we know and love today. Meanwhile, their sphecomyrmine sisters persisted, large-eyed and above ground, until extinction. It is entirely possible that the diversity and success of modern ants traces to modifications forced by this subterranean existence.
Finally, the underground result of Lucky et al depends on particular assumptions of how traits evolve. Specifically, that above- and below-ground species go extinct at similar rates. If subterranean ants tend, on average, to go extinct less often than their above-ground relatives, then many of the surviving members of older lineages will be subterranean and we might infer a subterranean ancestor as an artifact.
source: Lucky A, Trautwein MD, Guénard BS, Weiser MD, Dunn RR (2013) Tracing the Rise of Ants – Out of the Ground. PLoS ONE 8(12): e84012. doi:10.1371/journal.pone.0084012