It is commonly assumed that the first terrestrial vertebrates reproduced in a way similar to that of "typical" extant amphibians, in which relatively small, externally fertilized eggs are laid in fresh water. According to this view, early tetrapods were tied to standing water by the physiological requirements of their eggs. Full liberation from water required extensive modification of this ancestral reproductive mode, and culminated in the evolution of the amniote egg, which is regarded as an adaptation to the physical rigors that eggs allegedly encounter on land.

Several independent lines of evidence suggest that this traditional account is false. There appear to be no theoretical reasons to assume that the evolution of terrestrial egg-laying was difficult, or required a structure as elaborate as the amniote egg. Oviparous vertebrates that reproduce on land typically, and probably primitively, lay eggs in underground nests.

The physical conditions that eggs encounter in these nests are generally quite mild-not only milder than conditions above ground, but also milder than conditions in standing water. In addition, analysis of the distribution of key reproductive character states among vertebrates provides no evidence that the "typical amphibian" reproductive mode is primitive for tetrapods. When mapped on any plausible phylogeny of the vertebrates, these character states show so much homoplasy that they cannot be polarized. Amniotes are as likely as frogs or salamanders to retain primitive reproductive character states. We do not know how the first terrestrial vertebrates or their ancestors reproduced, or even among which group terrestrial reproduction began. Evolutionary scenarios that assume such knowledge cannot be tested.

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