The timescale of nematode evolution reveals late and convergent radiations of parasitism

Nematodes are among the most diverse animal groups, exerting profound impacts as parasites with far-reaching medical and economic consequences. Despite parasitic species constituting most known extant nematodes, the temporal framework underpinning the evolution of parasitism has remained elusive. Here we integrate expansive genomic datasets (184 representatives, including 156 nematode species) with fossil constraints to recalibrate the timescale of nematode evolution and resolve the evolution of their parasitic and free-living lifestyles. Our analyses indicate that the last common ancestor of nematodes probably emerged as a free-living organism between the late Cambrian and Middle Ordovician. Moreover, we estimate that the earliest shifts toward parasitism occurred between the Late Devonian and end Carboniferous, with subsequent, recurrent transitions marking the evolution of parasitism. For most of their evolutionary history, parasitic nematodes did not exhibit enhanced diversification relative to their free-living counterparts, until approximately 90 million years ago-a temporal juncture that aligns with the rapid radiation of three obligatory zooparasitic lineages (Ascarididae, Onchocercidae and Strongyloidea), coincident with the diversification of terrestrial mammals and modern birds. These findings refine our understanding of nematode evolution and highlight the complex interplay between lifestyle transitions and diversification dynamics in this ecologically pivotal phylum.