Mammals arose approximately 225 million years ago in the mid to late Triassic period, as shy, small, secretive creatures springing from a reptilian lineage. At the time mammals evolved, the reptilian order that generated them probably enjoyed tetrachromacy. Most mammalian families became nocturnal (and in many cases arboreal) because of intense dinosaur predation and/or competition. These nocturnal mammalian families lost two visual pigments to become dichromats. It is believed that the now extinct, last common ancestor to the mammalian class probably gave rise to the monotremes (BJO March cover, 2005), the marsupials, and the placentals, although the sequence of descent and cladistic relation remains controversial.

Approximately 65 million years ago when a comet struck the Yucatan peninsula ending the Cretaceous period and snuffing out dinosaurs forever, the evolutionary door swung open for synapsids including all mammalian clades. Mammalian families extant at that time were mostly dichromatic, and only a few of the primates would later recover trichromacy (BJO December cover, 2001).

Evolutionary recapitulation is rare, often impossible, since metabolic solutions once …