The American Naturalist, Vol. 154, No. S4, Evolutionary Relationships Among Eukaryotes A Symposium Organized by Laura A. Katz (Guest Editor) and Mitchell L. Sogin (October 1999), pp. 96-124 (29 pages) ...

"We tend to think of evolution as a one-way march toward greater ... blocks for the nervous system were already in place in our protozoan ancestors—before the nervous system even existed." ...