ANATOMY

Detailed explanation-1: -Tetrapods evolved from a clade of primitive semiaquatic animals known as the Tetrapodomorpha which, in turn, evolved from ancient lobe-finned fish (sarcopterygians) around 390 million years ago in the Middle Devonian period; their forms were transitional between lobe-finned fishes and true four-limbed tetrapods.

Detailed explanation-2: -All tetrapods descended from a common ancestor that just happened to have limbs with five digits. And over the eons of evolution following that, natural selection worked with variations on pentadactyly rather than starting over again to produce tetrapods with another number of digits, be it two, seven, or 17.

Detailed explanation-3: -The first crown-tetrapods (last common ancestors of extant tetrapods capable of terrestrial locomotion) appeared by the very early Carboniferous, 350 million years ago.

Detailed explanation-4: -The first fossils of tetrapods, Acanthostega and Ichthyostega, were dated to 365 mya. So, Shubin and his colleagues hypothesized that the fish in the process of becoming more tetrapod-like lived between 385 and 365 mya. This 20-million-year-span is within a geological epoch called the Upper Devonian.

Detailed explanation-5: -This 350 million year old animal, the first tetrapod, had limbs with one long bone (the humerus) attached to two other long bones (the radius and ulna). Its descendants, including whales, lizards, humans, and birds, as well as many others, inherited the tetrapod limb from this ancestor.