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Space Topics: Search for Extraterrestrial IntelligenceIs Earth-Life Relevant? A Rebuttalby Carl Sagan The gist of Professor Mayr's argument is essentially to run through the various factors in the Drake equation (see Shklovskii and Sagan, 1966) and attach qualitative values to each. He and I agree that the probabilities concerning the abundance of planets and the origins of life are likely to be high. (I stress again that the latest results [Doyle, 1995] suggest one or even two Earth-like planets with abundant surface liquid water in each planetary system. The conclusion is of course highly tentative, but it encourages optimism.) Where Mayr and I disagree is in the later factors in the Drake equation, especially those concerning the likelihood of the evolution of intelligence and technical civilizations. Mayr argues that prokaryotes and protista have not "evolved smartness." Despite the great respect in which I hold Professor Mayr, I must demur: Prokaryotes and protista are our ancestors. They have evolved smartness, along with most of the rest of the gorgeous diversity of life on Earth. On the one hand, when he notes the small fraction of species that have technological intelligence, Mayr argues for the relevance of life on Earth to the problem of extraterrestrial intelligence. But on the other hand, he neglects the example of life on Earth when he ignores the fact that intelligence has arisen here when our planet has another five billion years more evolution ahead of it. If it were legitimate to extrapolate from the one example of planetary life we have before us, it would follow that:
Alternatively, we could argue that it is improper to extrapolate from a single example. But then Mayr's one-in-50 billion argument collapses. It seems to me he cannot have it both ways. On the evolution of technology, I note that chimpanzees and bonobos have culture and technology. They not only use tools but also purposely manufacture them for future use (see Sagan and Druyan, 1992). In fact, the bonobo Kanzi has discovered how to manufacture stone tools. It is true, as Mayr notes, that of the major human civilizations, only one has developed radio technology. But this says almost nothing about the probability of a human civilization developing such technology. That civilization with radio telescopes has also been at the forefront of weapons technology. If, for example, western European civilization had not utterly destroyed Aztec civilization, would the Aztecs eventually--in centuries or millennia--have developed radio telescopes? They already had a superior astronomical calendar to that of the conquistadores. Slightly more capable species and civilizations may be able to eliminate the competition. But this does not mean that the competition would not eventually have developed comparable capabilities if they had been left alone. Mayr asserts that plants do not receive "electronic" signals. By this I assume he means "electromagnetic" signals. But plants do. Their fundamental existence depends on receiving electromagnetic radiation from the Sun. Photosynthesis and phototropism can be found not only in the simplest plants but also in protista. All stars emit visible light, and Sun-like stars emit most of their electromagnetic radiation in the visible part of the spectrum. Sensing light is a much more effective way of understanding the environment at some distance; certainly much more powerful than olfactory cues. It's hard to imagine a competent technical civilization that does not devote major attention to its primary means of probing the outside world. Even if they were mainly to use visible, ultraviolet or infrared light, the physics is exactly the same for radio waves; the difference is merely a matter of wavelength. I do not insist that the above arguments are compelling, but neither are the contrary ones. We have not witnessed the evolution of biospheres on a wide range of planets. We have not observed many cases of what is possible and what is not. Until we have had such an experience--or detected extraterrestrial intelligence--we will of course be enveloped in uncertainty. The notion that we can, by a priori arguments, exclude the possibility of intelligent life on the possible planets of the 400 billion stars in the Milky Way has to my ears an odd ring. It reminds me of the long series of human conceits that held us to be at the center of the universe, or different not just in degree but in kind from the rest of life on Earth, or even contended that the universe was made for our benefit (Sagan, 1994). Beginning with Copernicus, every one of these conceits has been shown to be without merit. In the case of extraterrestrial intelligence, let us admit our ignorance, put aside a priori arguments, and use the technology we are fortunate enough to have developed to try and actually find out the answer. That is, I think, what Charles Darwin--who was converted from orthodox religion to evolutionary biology by the weight of observational evidence--would have advocated. Carl Sagan |
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