Unlike fish, ancient aquatic arthropods apparently fared well out of water. New evidence suggests that they adapted to life on shore by finding new uses for their gills, which eventually evolved into a variety of structures, including insect wings and the organs spiders use to breathe and spin webs.
The ancestors of all arthropod species found their home at sea. Some 400 million to 450 million years ago, different branches of the arthropod family tree--including the insects, myriapods such as centipedes and millipedes, chelicerates like scorpions and spiders, and a handful of crustaceans--began crawling ashore and invading the land. The groups' independent forays onto terra firma forced each to evolve its own methods for coping with drier surroundings.
In spite of separate origins, modern arthropods' solutions to a terrestrial lifestyle share a common history, according to developmental geneticist Michalis Averof at the Institute of Molecular Biology and Biotechnology in Crete. When Averof and his colleagues examined the activity of two genes, pdm/nubbin and apterous, in horseshoe crabs--ancient representatives of the chelicerates--they found that both shift into high gear during the development of the crabs' book gills. In the tropical hunting spider Cupiennius salei, a modern chelicate, the same genes are revved up during development of the animals' recently evolved breathing apparatus--book lungs and tracheae--and their web-spinning spinnerets, the researchers report in the 1 October issue of Current Biology. The findings support earlier hypotheses about the structures' shared origins that were based on their shapes and, coupled with the team's earlier discovery that the duo is also turned on during insect wing development, suggests that arthropods' array of land-based adaptations arose from the gills of their water-bound ancestors.
"It's a nice piece of work," says developmental geneticist Michael Akam of the University of Cambridge in the United Kingdom. "The congruence of the morphological and molecular data is particularly compelling in this case," he adds. That gills, wings, spinnerets, and lungs could all arise from the same organ, adds evolutionary biologist William Shear of Hampden-Sydney College in Virginia, "suggests an extraordinary malleability of the original structure."