American Scientist

Symbiotic Planet: A New Look At Evolution. Lynn Margulis. 147 pages. Basic Books, 1998. $21.

Scientists often delight at pointing out that Darwin devotes very little space to the origin of species in On the Origin of Species. After doing just this, Lynn Margulis tells us in Symbiotic Planet that she will pick up where Darwin left off. Her premise is that symbiosis is the primary cause of speciation events throughout history. I find it interesting, therefore, that she never actually defines what a species is! That fault aside, this book sheds light on the oft-debated, somewhat-abstruse processes that led to the formation of the major divisions of life.

Margulis contrasts what she terms the "branching" and "fusing" views of evolutionary change. She is perhaps the strongest and most vocal proponent of fusing—that is, much evolutionary novelty is brought about by organisms fusing and "becoming one." Here I (and Margulis) use "fusing" in a loose sense, sometimes literally meaning to fuse and become one structure and sometimes figuratively, for two organisms to become intimately associated with one another and have large effects on one another. A classic example of the first type of fusing is that of ancient cyanobacteria being the ancestors of extant chloroplasts, for which there is very good indirect evidence, whereas an example of the second is the association between mycorrhizal fungi and their plant hosts. Many of her arguments on this general point make sense. However, her logic falters at times, mostly because she takes what is often seen as an extremist, but not necessarily wrong, viewpoint. For example, Margulis launches into a diatribe about how making phylogenies based purely on nucleic acid sequences is no good: "Organisms must be classified on the basis of their entire biology." This entire page is some of the most anticladist literature I have ever read; it's not clear whether Margulis is interested in an accurate history of life or a convenient, human-biased way of cataloging diversity. Her general argument, backed up by none other than Ernst Mayr, is the following: Because archaebacteria are very "similar" to eubacteria and both are less obviously similar to all other forms of life (the eukaryotes), classifying them as separate entities obscures the diversity of the plants, animals, fungi and protists. To the contrary, many people (myself included) believe that the grand "diversity" of the eukaryotes is simply a human bias and that we truly do not recognize the extreme diversity of the bacteria. Although we can agree to disagree on this topic, Margulis correctly points out that both views of life's history are entirely consistent with her fusing view of macroevolutionary change. This brings me to comment on a third point about this relatively short book: Margulis gives fair treatment to competing arguments. Then she proceeds to disintegrate them into oblivion.

One outstanding feature is the author's description of natural history: Margulis's depictions of a multitude of organisms and their symbioses makes interesting reading even for nonprofessionals, if they can get past the numerous Latin species designations. Another noteworthy feature is the personal nature with which the writing is done: She peppers her scientific exploits with stories of her personal life—and with colorful language, at one point calling Gaia "a tough bitch."—Mark D. Drapeau, Ecology and Evolutionary Biology, University of California, Irvine