Four Billion Years of Evolution from Our Microbial Ancestors.
Lynn Margulis and Dorion Sagan.
Summit Books, New York, 1986. 301 pp., illus.
For the past 20 years, Lynn Margulis has been an important intellectual force in the fields of evolutionary biology and Earth history. She has authored provocative hypotheses on such disparate topics as the origins of life, the eukaryotic cell, sex and multicellularity and has championed novel ideas concerning bacterial evolution and the biological regulation of Earth surface environments.
As this new book, written with Dorion Sagan, makes clear, her many ideas are really of a piece. Collectively, they constitute a coherent world view that stands in sharp contrast to conventional wisdom about our planet and its biota. Although Microcosmos can be read with enjoyment as a slightly idiosyncratic chronicle of evolution, it is really a sustained argument in support of an alternative biology.
In the world according to Margulis, bacteria are the protagonists. Not only were they the only organisms on Earth for most of its history; they are the organisms in which virtually all major metabolic pathways evolved and they remain the crucial intermediates in biogeochemical cycles. Eukaryotic cells are viewed as tightly integrated bacterial assemblages, plants and animals as assemblages of assemblages. Cooperation, not competition, is the wellspring of evolutionary change, giving rise to a biota so highly integrated that the concept of the biosphere as a "global superorganism" has a significance beyond metaphor.
Microcosmos was written for a general audience, and it succeeds admirably in describing four billion years of evolution in accessible, of-ten elegant prose. I've spent most of the past decade studying Pre-cambrian rocks and microfossils, yet Margulis and Sagan's evocative account of the early Earth is so fresh and beautifully constructed that in reading it I felt the excitement of entering a new world. It is hard to argue with the enthusiasm of the authors, who write that "life on earth is such a good story you can't afford to miss the beginning." Their brief exposition of Phanerozoic evolution is also enjoyable, approached as it is from their singular perspective of animals, plants and fungi as highly organized microbial collectives.
On the whole, however, this section is less critically informed than that on early evolution, and theory occasionally receives support from debatable facts. For example, Margulis and Sagan note that "it may not be a coincidence that the first mammals . . . date almost exactly to the period of the first flowers, evolving some 125 million years ago." This observation fits neatly into their conception of evolution by symbiosis, but the first mammals in the fossil record actually precede the initial radiation of flowering plants by nearly 100 mil lion years.
Homo sapiens fares poorly in Microcosmos. We are but an evolutionary pup, whose capacity even for pollution is dwarfed by that of Archean microorganisms. Fortunately, the biosphere is robust, so that life on Earth will continue de spite our predilection for self-destruction. "A human Armageddon might" even, the authors assert, "prepare the biosphere for less self-centered forms of life." This is a deliberately provocative perspective, but is it a useful one for those of us whose principal concerns are for our grandchildren rather than for the invulnerability of the biosphere?
All of the most provocative ideas in Microcosmos compete in the intellectual marketplace with markedly different hypotheses, which are not always acknowledged in the book as being more than straw men. For example, the authors espouse the theory of Sonea and Panisset that genetic exchange among bacteria is so pervasive and complete that all prokaryotes constitute "a unique, complex type of clone, composed of highly differentiated (specialized) cells"-in other words, a superorganism. In contrast, many, perhaps most, microbial geneticists believe that strict rules govern the amounts and types of genetic material that can be passed laterally among populations. Perhaps the microtubules of eukaryotic cells originated as proteinaceous structures in symbiotic spirochetes, but many protistologists view perceived similarities between the two as convergent. Per-haps organisms regulate the biosphere for some ill-defined biotic good, but many biogeochemists believe that physical processes constrain Earth surface conditions within relatively narrow limits, despite biological activity. These alternative hypotheses can be tested, and it is only fair to point out that the conventional wisdom in all cases enjoys empirical support equal to or greater than that marshaled by Margulis and Sagan in defense of their preferred theories.
In short, Microcosmos provides one way of looking at the world, but not the only one, and perhaps not one that most biologists will find congenial. Therefore, read this book with a critical mind as a source of questions rather than of answers. But do read it. Microcosmos is an entertaining and enlightening, if sometimes exasperating, introduction to the ideas of one of our generation's most innovative and challenging thinkers.
Andrew H. Knoll is professor of biology, Botanical Museum, Harvard University, Cambridge, MA 02138.