This diagram from Convergent Adaptation of True Crabs (Decapoda: Brachyura) to a Gradient of Terrestrial Environments is going to reinforce the idea that everything converges on crabs…which is kind of cool anyway.

Summary of phylogeny and divergence time estimates for Brachyura (88 brachyuran families, 263 genera, 333 species, 338 individuals plus 6 outgroups). Posterior ages were estimated in BEAST2 using a fixed topology resulting from the concatenated ML analysis in IQ-TREE, 36 vetted node calibrations, a birth–death tree prior, and relaxed lognormal clock model. Shaded circles at nodes represent ultrafast bootstraps. Pie slices are colored by superfamily, with the outermost ring colored by taxonomic section. Line drawings, one representative per superfamily (numbers corresponding to taxa in Supplementary Table S7), by Javier Luque and Harrison Mancke.

Please note, this diagram illustrates the evolution of a single, large, successful clade, the Brachyura. It does not imply that humans and salamanders and spiders are going to converge on a crablike form, OK? The interesting thing is that all the descendants of this Triassic lineage, despite going through multiple independent transitions from marine to terrestrial and back again, have assumed these very similar (superficially, at least) forms, and that tells us something interesting. The Brachyura have some internal constraints that shape their evolution, and studying them help us understand the balance between inherited patterns and external forces. That’s the conclusion of the paper, that there are constraints on evolution.

Herein, we inferred a large molecular phylogeny of true crabs, estimated divergence times that were older than previously thought, and estimated the number of transitions from marine to non-marine lifestyles. We found up to 17 convergent transitions through direct and indirect pathways, with at least 3 climbing to higher degrees of terrestrial adaptation. The most highly terrestrial clades were some of the oldest non-marine inferences in our data, with their common ancestors having diverged over 66 Ma. At least 9 more recent events throughout the Cenozoic led to crabs living in intertidal and marginal marine environments, a shift that is estimated to be much easier based on lower threshold liability and likely fewer traits required. As instances of convergent evolution provide emerging models in the form of “natural experiments,” the framework we have developed to compare the gradient of adaptations will enable future research that aims to “predict” the constraints leading to repeated trait evolution and better understand the drivers of biodiversity across related groups.