Earth's most creative evolutionary transitions were reviewed recently . They seem to occur relatively abruptly, compared to the background rate of evolutionary change. Some of the major transitions noted were: origin of chromosomes, origin of eukaryotes, origin of sex, origin of multi-cellular organisms, origin of social groups.
Of these major transitions, perhaps the most dramatic, and best known, was the rapid origin and diversification of large multi-cellular organisms from microscopic single celled ancestors, in what has come to be known as the Cambrian explosion of diversity. It has understandably been called evolution's ``big bang'', when there was a dramatic inflation of complexity of organisms, and species diversified rapidly into an ecological void [9,21].