Origin of the Tetrapod Limb

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Science  23 Mar 1956:
Vol. 123, Issue 3195, pp. 495-496
DOI: 10.1126/science.123.3195.495


The paired fins of fishes were first used as props and supports for resting on the bottom; these were later used in a clumsy, walking manner, and this behavior perforce began first in the water, because the weak props could not support the animals without the water bouyancy; increased perfection of the mechanics of walking took place in the shallows; which was a refuge from the chief predators; the land was also attractive as a haven and as a source of food; the first vertebrate invaders of land probably had fins, and these became legs by enlargement of the fin base and loss of fin rays; these original limbs and girdles were weak and probably underwent a considerable period of evolution in swampy country; later they were perfected by further selection when it became necessary for early amphibians to move across dry land because of a failing local water supply.

This syllogism conforms to the known behavior and capabilities of fishes and amphibians and to the general facts of zoology and paleontology. It suggests that common, continuous activities and stresses—escape from enemies and food getting—led to the origin of the tetrapod limb. This obviates the necessity for explaining how discontinuous and somewhat catastrophic events, such as the drying up of water bodies, could have led to the origin of limbs, which at the very outset had to be fairly strong.

The general theory stated here is fairly clearly implied by Berry (8), who said, "Those fishy pioneers with air-bladders —and paired fins—which, after ages of using their fins for pushing and paddling themselves over mud flats, gradually ventured onto drier and drier ground—where they avoided the competition for food—and the dangers of swarming hordes of ganoid pirates of the waters, were the ancestors of the amphibians."