Prominent evolutionary biologists have for the most part been hostile to the Search for Extraterrestrial Intelligence (SETI). Their arguments center on the purported "incredible improbability of genuine intelligence emerging" (Mayr 1985; see Simpson's now classic statement, 1964). Tipler (1980), the great opponent of SETI, puts it as follows:
The present-day theory of evolution, the Modern Synthesis, stresses the great contingency of all branches of the evolutionary tree: those possible branches which could terminate in intelligent life are extremely small in number when compared with the total number of branches.
Some evolutionary biologists are not so sure. Gould (1985) says:
But does intelligence lie within the class of phenomena too complex and historically conditioned for repetition? I do not think its uniqueness on earth specifies such a conclusion. Perhaps, in another form on another world, intelligence would be as easy to evolve as flight on ours.
Bieri (1964), in a rejoinder to Simpson, went even further in contending that humanoids would be a likely outcome of parallel evolutionary convergence on other presumably terrestrial planets. Bieri cited the examples of convergence well known in evolution such as between dolphins and sharks, as well as the advantageous arrangements found in hominoids (mouth at one end, anus at the other).
A deep question raised by this controversy is the repeatability of patterns of evolution. If evolution of life on Earth was "run again," how similar would the history of life be to the one we know, assuming the same initial conditions and historical abiotic constraints? We can all agree on the great improbability ofthe exact repetition ofterrestrial history on other Earth-like planets in the galaxy. Yet, to what extent is the general pattern of terrestrial evolution (e.g., procaryotes, eucaryotes, Metazoa, intelligence) "forced" by strong tendencies for life to self-organize in preferred directions? (See discussion by Schwartzman and Rickard 1988.) For example, Russell (1981) has argued that progressive encephalization occurred in a fairly regular pattern over the past 600 million years. This pattern may simply be a result of random evolution from a small brain relative to body weight, to a large brain, given the increasing number of extreme values likely with time as the branches of evolution diverge (see Gould 1996, on Cope's Rule, regarding an analogous trend of larger animal size with evolution).
However, the emergence of oxygenic photosynthesis by cyanobacteria, the endosymbiogenesis of bacteria to form the eucaryotic cell, and the critical inventions necessary for the emergence ofMetazoa (e.g., biochemistry of embryonic development, structural proteins) may be another case altogether, despite the canonical views of such evolutionary biologists as Mayr, as stated in 1961—"probably nothing in biology is less predictable than the future course of evolution"—and 1985—the emergence of eucaryotes by symbiosis was a "most improbable event" If future research confirms that these inventions occurred soon after, on a geological time scale, all the necessary environmental conditions were set, then a certain inevitability is supported. Temperature has been identified here as the last limiting factor holding up the emergence of major organismal groups. This would explain the puzzling lag times in evolution, evidence Gould (1989) has used to support the canonical view of evolutionary biology that historical contingency is central. In the case of oxygenic photosynthesis, is it not plausible that the invention of this metabolism is forced by the very presence of liquid water, carbon diox ide, and the solar flux? Similarly, the emergence of eucaryotes, as previously argued, may have been the highly likely consequence of the coupling of the complementary metabolisms of two or more procaryotes.
Recently, it has been suggested that spontaneous self-organization has played a critical role in evolutionary change (Waldrop 1990; Kauffman 1993, 1995). Thus, a theory of evolving self-organizing systems may yet emerge that predicts finite patterns of biospheric and biotic evolution on terrestrial planets within the habitable zone (HZ). Vernadsky (1945) put it as follows: "the biosphere . . . is being revealed as a planetary phenomenon of cosmic character" (He went on to say that Venus and Mars had life "beyond doubt.")
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