From Water to Land in Ontogeny and Phylogeny

A chorus of frogs beside a pond on a spring evening heralds one of nature's dramatic events. Mating frogs produce masses of eggs, which will hatch into limbless, gill-breathing, fishlike tadpole larvae that feed and grow. Then a remarkable transformation occurs. Hindlegs appear and gradually lengthen. The tail shortens. Larval teeth and gills are lost. Eyelids develop. Forelimbs emerge. In a few weeks the aquatic tadpole has completed its metamorphosis to an adult frog.

The evolutionary transition from water to land occurred not in weeks but over millions of years. A lengthy series of alterations cumulatively fitted the vertebrate body plan for life on land. The origin of land vertebrates is no less a remarkable feat for this fact— a feat that would have a poor chance of succeeding today because well-established terrestrial competitors would exclude poorly adapted transitional forms.

Amphibians are the only living vertebrates that have a transition from water to land in both their ontogeny and phylogeny. Even after 350 million years of evolution, few amphibians are completely land adapted; most are quasiterrestrial, hovering between aquatic and land environments. This double life is expressed in their name. Even the amphibians best adapted for a terrestrial existence cannot stray far from moist conditions. Many, however, have developed ways to keep their eggs out of open water where their larvae would be exposed to enemies.