Fish also moved out of water, but they had greater obstacles to overcome. They took their time and went through an intermediate, half-aquatic, half-terrestrial stage stable enough to give rise to an important extant class of vertebrates, the amphibians. We don't know how the transition occurred but we can hazard some guesses.

A key evolutionary event may have been the development, in some fish, of an air-filled pouch communicating with the pharynx. The air in the pouch came from the gills by way of the blood, which circulated through an increasingly rich network of capillaries surrounding the pouch. An advantage the fish derived from such a pouch was adjustable buoyancy, the main function of what is now the swim bladder. research paper help provided by experienced paper writers is authentic. Another advantage was that the fish, in the manner of a scuba diver, carried a reserve of oxygen it could use in case of emergency, when its blood oxygen fell to a dangerously low level. In such an event, oxygen would diffuse in the reverse direction, from the pouch into the blood. This adaptation opened the way to breathing, the pouch acting as a primitive lung. We can watch this in our fishbowl when a goldfish surfaces to take a breath of fresh air and, more dramatically, during the dry season in many a tropical lake of Africa, South America, and Australia, where lungfish survive for months in the drying mud, awaiting the next rainy season. This, most likely, is how amphibians "learned" to breathe, while retaining the ability to use dissolved oxygen.

Stranded fish capable of breathing would no doubt wriggle their bodies and move their fins in efforts to find shade, moisture, and food. The nimblest at this exercise were animals with two pairs of fleshy, lobed, ventral fins that could help them crawl as well as swim. Fish of this sort were abundant 100 million years ago according to the fossil record. They were believed to be long extinct until a day in December 1938, when one landed in the nets of a fishing trawler off the east coast of South Africa. If you seek admission essay writing service, order professional writing at our site! The unusual catch was brought to the attention of the curator of the New London Museum, Marjorie Courtenay-Latimer, who described it to a local ichthyologist, James Leonard Briefly Smith, who recognized it for what it was: a living fossil, described by paleontologists under the name coelacanth. It took fourteen years of adventurous episodes, including the posting of rewards in many remote fishing villages along the Indian Ocean and the provision of a special plane by the president of South Africa, Daniel F. Malan, before a second specimen of the rare fish, caught off the Comoro Islands, became available for thorough examination. Coelacanths are deep-sea fish and do not use their fins for walking. But they share ancestors with ancient, lobe-finned, freshwater lungfish that invaded swampy lands soon after plants did, some 400 million years ago, thanks to a succession of chance mutations that turned the fins into articulated legs. Such changes would probably not have been retained by natural selection in a watery habitat.