A diversity of organisms are endowed with the ability to emit light, and to display and control it in a variety of ways. Most of the luciferins (substrates) of the various phylogenetically distant systems fall into unrelated chemical classes, and, based on still limited data, the luciferases (enzymes) and reaction mechanisms are distinctly different. Based on its diversity and phylogenetic distribution, it is estimated that bioluminescence may have arisen independently as many as 30 times in the course of evolution. However, there are several examples of cross-phyletic similarities among the substrates; some of these may be accounted for nutritionally, but in other cases they may have evolved independently.