The Evolution of Complexity

The evolution of the biosphere exhibits a trend of increasing complexity of the most complex organisms. Even though we are uncertain about the proper way to measure complexity, it is hard to deny that the earliest prokaryotic cells are simpler than the eukaryotic cells that arose from them, and these were simpler than the multicellular life forms that evolved from them, and so on. But it is controversial how to interpret and explain this trend, and even how to describe it properly (see, e.g., Gould 1989, McShea 1991, Gould 1996, Sterelny and Griffiths 1999). Some think that increasing maximal organism complexity has for all intents and purposes been explained by Darwin, Miller and Urey, Watson and Crick, or some other past or present biologists. This chapter defends the alternative view that the trend is not yet adequately explained but instead is one of the major remaining challenges in understanding biological phenomena.