ABSTRACT
When life first appeared more than three billion years ago, it did so in the presence of water. Only in the last half billion years or so have living things become well adapted to living on dry land. For creatures making the difficult transition out of the ocean or fresh water, one of the greatest challenges lies in obtaining and retaining adequate water in their tissues. Among the vertebrates, the earliest forms which spent significant time on land were the ancestors of today’s amphibians, most of which continue to live in wet or damp habitats. The first vertebrates that were truly successful in dry environments were the reptiles, and they managed this by developing a range of improved water-conservation structures, especially waterproof skin and eggs, internal lungs, and water-conserving kidneys. When birds and mammals later arose from reptiles, those water-conserving structures were retained, with mammals eventually dispensing even with waterproof eggs by keeping the babies inside the mother until they hatched.
