Introduction

Considered as a whole, the two most striking aspects of the evolution of life on earth are the staggering diversity of living forms that have come into existence, and the fact that older forms have given way to new and improved forms that seem (for the most part) to be admirably adapted for their respective ways of life. Darwin captured both aspects of evolution in the closing words of the Origin, where he remarked that “from so simple a beginning, endless forms most beautiful and most wonderful have been, and are being, evolved” (Darwin 1859, p. 490). Although there are fascinating problems associated with the evolution of diversity (for example, why are there so many different kinds of living things? How do new species come into existence? What are “species,” anyway?), it is the second aspect of the evolutionary process that is at issue here. Life has not only diversified from its initial humble beginnings, it has also advanced. Multicellular organisms (“metazoa”) arose from unicellular organisms; mammals arose from earlier, nonmammalian ancestors; and in general larger and more complex organisms arose from smaller and simpler ones.