The evolution of terrestrial creatures from aquatic fish with fins may have begun with the need for a breath of fresh air. Animals with limbs, feet and toes—a group known as the tetrapods (literally, “four-footed”)—arose between 380 million and 375 million years ago. Scientists long believed that limbs evolved as an adaptation to life on terra firma. But recent discoveries have revealed that some of the key changes involved in the fin-to-limb transition occurred while the ancestors of tetrapods were still living in the water.