How Vertebrates Left the Water / Как позвоночные покинули воду

Life appeared in the oceans in a past so distant that it is diffi cult to imagine. The exact age of life on Earth is debated because the structures once considered to represent the oldest fossils (remains of ancient organisms, or traces which they left) have been reinterpreted as mineral crystallization in microscopic fractures by some paleontologists (this reinterpretation is itself debated). The fi rst life forms were very simple and resembled extant bacteria, some of which formed stromatolites, the oldest of which are about 3 Ga (billion years) old (Fig. p.1). Stromatolites are still being formed today in some coastal regions. For at least 1.5 Ga, life remained in its native aquatic environment. Thus, for the greatest part of the history of the biosphere, life remained in water, diversifi ed, and radiated into several ecological niches. The oceans and seas teemed with life well before the fi rst animal ventured out of the water. <...>