  Considering that life on earth evolved about 3.7 billion years ago,
vertebrates are young, appearing in the fossil record during the Cambrian
explosion about 542 to 515 million years ago. Results from sequence analyses of
genomes from bacteria, yeast, plants, invertebrates and vertebrates indicate
that receptors for adrenal steroids (aldosterone, cortisol), and sex steroids
(estrogen, progesterone, testosterone) also are young, with receptors for
estrogens and 3-ketosteroids first appearing in basal chordates
(cephalochordates: amphioxus), which are close ancestors of vertebrates. An
ancestral progesterone receptor and an ancestral corticoid receptor, the common
ancestor of the glucocorticoid and mineralocorticoid receptors, evolved in
jawless vertebrates (cyclostomes: lampreys, hagfish). This was followed by
evolution of an androgen receptor and distinct glucocorticoid and
mineralocorticoid receptors in cartilaginous fishes (gnathostomes: sharks).
Adrenal and sex steroid receptors are not found in echinoderms: and
hemichordates, which are ancestors in the lineage of cephalochordates and
vertebrates. The presence of steroid receptors in vertebrates, in which these
steroid receptors act as master switches to regulate differentiation,
development, reproduction, immune responses, electrolyte homeostasis and stress
responses, argues for an important role for steroid receptors in the
evolutionary success of vertebrates, considering that the human genome contains
about 22,000 genes, which is not much larger than genomes of invertebrates,
such as Caenorhabditis elegans (~18,000 genes) and Drosophila (~14,000 genes).
