The superclass Tetrapoda (Ancient Greek τετραπόδηs tetrapodēs, "four-footed"), or the tetrapods , comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, mammals, birds, and some extinct fish. Tetrapods evolved from the lobe-finned fishes around 390 million years ago in the middle Devonian Period, with modern tetrapod groups having appeared by the late Devonian, 367.5 million years ago. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.