Your arms and legs began as the fins of ancient fish
The limbs of every four-legged animal, including humans, match the paired pectoral and pelvic fins of jawed fish. They descend from freshwater lobe-finned fish whose fleshy, bone-supported fins became the first legs in the Devonian. Today only eight lobe-finned species survive, including two coelacanths and the lungfish.
Fins push against water to produce thrust and lift. Unpaired fins along the midline drive a fish forward and keep it steady, while paired fins on the sides handle braking, turning, rising, diving and rolling. Some species use them for other jobs entirely: flying fish glide on their pectoral fins, and mudskippers and frogfish crawl on theirs.
Construction varies by group. Ray-finned fish, over 30,000 species and more than half of all living vertebrates, spread thin skin over bony rays like a folding fan. Spiny rays are stiff and always sit in front of soft ones, which are segmented and flexible; catfish can lock their spines outward for defence, and triggerfish wedge themselves into crevices with theirs. Lobe-finned fish instead carry each fin on a scaly, fleshy stalk with jointed bones resembling a tetrapod limb.
Coelacanths, thought to have reached roughly their present form about 408 million years ago, mostly drift on rising and falling currents, steadying themselves with paired fins. They have been seen doing headstands and swimming upside down. Lungfish, living in Africa, Australia and South America, are the closest living relatives of land vertebrates, their swim bladders having become primitive lungs.
Sharks build fins from unsegmented, elastic filaments called ceratotrichia, similar to the keratin of hair. Most have eight fins and a tail whose upper lobe is larger, because the spine runs into it and offers more room for muscle. Tail shape reflects hunting style: the porbeagle's big lower lobe helps it chase mackerel, while the thresher shark stuns prey with a long upper lobe. Rays swim by flapping enlarged pectoral fins.
Source: Fish fin
