Carl R. Woese, "On the evolution of cells" (2002)

"Evolving the cell requires evolutionary invention of unprecedented novelty and variety, the likes of which cannot be generated by any familiar evolutionary dynamic....Thus, what needs explaining is not why the major cell designs are so similar, but why they are so different.  This apparent contradiction can be resolved by assuming that the highly diverse cell designs that exist today are the result of a common evolution in which each of them began under (significantly) different starting conditions."
[...]
"Extant life on Earth is descended not from one, but from three distinctly different cell types."
 
Carl R. Woese, "On the evolution of cells," PNAS 99 (2002):8742-8747; pp. 8745-6.