In the isolated islands of the Pacific Ocean, a lack of large land mammals created openings for some truly unusual animals to dominate the ecosystem. Land crabs once ruled Hawaii...at least until humans and their pets showed up.
Most crabs are tied completely to the sea because they need to refresh themselves with saltwater. But we've found fossil evidence of crabs on Hawaii that made it several miles inland, apparently taking up residence in caves at shockingly high altitudes, as much as 3000 feet above sea level. These crabs, which went extinct when Polynesian settlers arrived about 1000 years ago, appear to have evolved the unique capacity to survive far away from saltwater, including perhaps being able to reproduce without returning to the ocean.
Naturalist Gustav Paulay explains how these crabs once ruled Hawaii:
"When you look at the islands of the Pacific, things don't get there easily, so land animals are scarce - and the biggest things on some of them are crabs. They control the ecosystems on atolls to an incredible extent. This particular species would have been important as a predator and as an omnivore - it would probably have had a big impact on the insect population, on land snails, and also maybe on nesting birds."
These crabs weren't huge - they were only about the size of a human fist - but that was big enough in the unique ecosystems of the pre-human Pacific for the species to become dominant. It appears that it wasn't humans who did them in - rather, it was the Polynesian rat, which was a frequent (and not necessarily welcome) traveling companion of these early island settlers.