Eureka! Heureka! An Astonishing New Ant!

Martialis heureka Rabeling & Verhaagh 2008

drawing by the inimitable Barrett Klein for PNAS

Most scientific discoveries these days emerge through carefully planned and controlled research programs.  Every now and again, though, something unexpected just pops up in a distant tropical jungle.  Martialis heureka is a fantastic discovery of that old-fashioned kind.  This little ant simply walked up to myrmecologist Christian Rabeling in the Brazilian Amazon.  It is not only a new species, but an entirely different sort of ant than anything known before.

The remarkable find was announced today in a paper by Rabeling, Manfred Verhaagh (who, a decade ago, collected and subsequently lost the elusive Martialis before he had a chance to properly examine it), and Jeremy Brown in the early edition of PNAS. If the DNA evidence is correct, Martialis is as ancient and as odd as an ant can be and still be called an ant.  The lineage emerged right at the dawn of the family and provides a new line of sight back to the elusive ant ancestors.

What do we know about Martialis?  Not all that much.  We have a single insect that was found walking about in the leaf litter, away from its presumed nesting site.  The details of its biology must be inferred from the morphology of the preserved specimen and the DNA sequence of a few genes.  Until someone locates live colonies,  the situation is a bit like having a well-preserved fossil with a smattering of genetic information.

We can say that Martialis really is an ant and not just another wayward wingless wasp.  The insect bears all the telltale traits marking the ant family: a metapleural gland on the thorax, a constricted waist segment, and an elongate first antennal segment.  In Rabeling et al's analysis, DNA sequence from three nuclear genes (18S, 28S, and EF-1alpha) places Martialis outside the rest of the living ants, but only slightly.  At left I have drawn up a simplified phylogeny, an amalgamation of Rabeling et al's finding and the landmark 2006 studies of Brady et al and Moreau et al.

Martialis is blind and pale, traits normally associated with subterranean species.  It was collected in rainforest leaf litter, at dusk, near Manaus, Brazil.  The elongate mandibles imply a predatory specialization, although on what we do not know.  Rabeling et al suggest "annelids, termites, insect larvae, and other soft-bodied arthropods".  The ant has a stinger, as do all the early lineages.  The presence of a metaplueral gland- thought to be associated with ant social behavior- indicates that Martialis lives in colonies.

Given the antiquity of the lineage, the temptation to view Martialis as an ur-ant of sorts is strong.  E.O. Wilson certainly felt that way when interviewed for a recent NYTimes article:

Dr. Wilson...is trying to contain his excitement: the 14,001st ant species has just been discovered in the soils of a Brazilian forest. He steamrolls any incipient skepticism about the antâs uniqueness â the new species is a living coelacanth of ants, a primitive throwback to the first ant, a wasp that shed its wings and assigned all its descendants to live in earth, not their ancestral air. The new ant is so alien, Dr. Wilson explains, so unlike any known to earthlings, that it will be named as if it came from another planet.

With due respect to Wilson, such a view is a mistake.  Martialis has over 120 million years' separation since the ur-ant, plenty of time to develop along its own trajectory.  The surviving species is not a throwback but a mix of primitive traits retained from the ancestor and unique traits acquired in the elapsed time.  The same is true of most other living ants.  Those impossibly long jaws, for example, are not present in any of the other early lineages nor in any of the fossils, almost certainly arising in the intervening millenia as Martialis developed a predatory specialization.  And even though both Martialis and the next earliest lineage, the Leptanillines, are blind and pale, such traits evolve so readily among other ant groups (see here, here, and here) it is difficult to infer confidently that the ur-ant was also a yellow eyeless wonder.

Rather, Martialis is important because we have a new window backward from which to view the ur-ant.  This perspective, when combined with knowledge of the other early lineages (Poneromorphs and Leptanillines), will provide a stronger triangulation on the nature of the first ants.  We will be able to infer with greater confidence the sequence of evolutionary events early in ant history.  It gives new data where the existing knowledge was fuzzy.

As an example, the most troublesome aspect of current ant phylogenies is uncertainty surrounding the very earliest events in ant evolution. The genetic studies of Moreau (2006) and Brady et al (2006) unexpectedly fingered the subterranean Leptanilline ants as sister to all other species.  Further exploration by Brady et al (2006) indicated that the Leptanilline arrangement might be an artifact of the data, leaving myrmecologists feeling a bit like we were back where we started.  In Rabeling et al's work, Martialis falls in exactly the right place to clear up the confusion: the ancient age of Leptanillines is likely real, not an artifact.  And Martialis is older still.

Where we go from here will depend on whether someone succeeds in finding living Martialis.  The missing link now is not the ant itself but the knowledge about what it does.

Original Paper: Rabeling, C., Brown, J. M., and Verhaagh, M. 2008. Newly discovered sister lineage sheds light on early ant evolution. Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.0806187105.

Other sources:

Brady SG, Fisher BL, Schultz TR, Ward PS (2006) Evaluating alternative hypotheses for the early evolution and diversification of ants. Proc Natl Acad Sci USA 103:18172â18177.

Moreau CS, Bell CD, Vila R, Archibald SB, Pierce NE (2006) Phylogeny of the ants: Diversification in the age of angiosperms. Science 312:101â104.

Specimen images by Rabeling & Verhaagh, used with permission