Underground! From rabbit warrens to nuclear fallout shelters, it’s a great place to hide out. If you’re a terrestrial vertebrate, the safest place you could conceivably be is underground, where you’re protected by a temperature-stable bunker, an ocean of dirt, rocks, and roots. But it’s one thing to dig a burrow, and another to spend all your time underground, swimming through the soil. If you’re a full-time tunneler, there are really just two body types you can evolve to fit: the “mole” model, and the “earthworm” model.
Consider the difficulties of underground travel. You don’t want to be too large, or digging would be exhausting. You don’t want large eyes, which would be useless and become full of grit. You’ll need a keen sense of smell and touch, as you’re likely to be finding food by chemical and tactile signals rather than visual ones. This star-nosed mole from North America is a great example of the mole archetype: small, compact, wedge-shaped, with sealed-off eyes and ears, powerful front claws, and 22 fleshy appendages that are among the most sensitive touch receptors in the animal kingdom. Star-nosed moles are true swimmers; they breaststroke through soil, but are also quite adept at catching prey in the water. Moles are insectivores, related to that most ancient of mammals, the shrew. But thanks to the awesome power of convergent evolution, you don’t have to be related to the moles to become a mole.