to the weight of empirical evidence. But from this point onwards the equation moves into the realm of conjecture. Estimates regarding the size of f l have no empirical support and rest on a widely disputed theory of how life developed. At present biologists have not reached a consensus regarding explanations as to how terrestrial life originated, which renders Earth an unsatisfactory model for the rest of the universe. The dominant theories refer to the evolution of organic compounds out of the oceans, when the Earth’s atmosphere consisted primarily of hydrogen, nitrogen, carbon dioxide, methane and water vapour. It is argued that the action of the Sun, together with volcanic activity on Earth, was primarily causative of the emergence of organic compounds (such as the amino acids) out of inorganic molecules. If correct, this explanation provides the necessary, but not the sufficient, conditions for life. All we have is a model of the possible emergence of life; there is no guarantee that life must emerge. The theory does not show how matter came to life; this problem has yet to be resolved. As yet life has not been produced in a laboratory. Until we know much more about the origins of terrestrial life the Earth cannot be regarded as a reliable model for the emergence of life on other planets.
f i: the number of planets with intelligent life
Conjectures regarding the number of life-bearing planets with intelligent life are predicated on widely disputed concepts of intelligence. Philosophers, psycholo- gists, anthropologists and sociologists have failed to produce an authoritative definition of intelligence which can be applied to life-forms on Earth. There is no satisfactory definition of intelligence for either humans or other species. There are disputes whether intelligence can be attributed to non-humans, such as chimpanzees and dolphins, and whether machines can exhibit forms of intelligent behaviour. We know very little about the origin of human intelligence, of the processes and mechanisms by which it is formed. Measuring the size of brains is far from satisfactory as a means of detecting or examining intelligence, as many terrestrial species with large brains do not appear to exhibit intelligence but use their large brains to control the basic functioning of their bodies. In the case of humans, intelligence appeared to occur with developments in manual dexterity.
It should, however, be admitted that the nature of the search for ETI will itself determine the parameters of intelligent life. It is of little use speculating on the likely existence of forms of intelligence that cannot be detected by current instruments. For this reason Jill Tarter points out that the search for intelligent life is a search for technology that is detectable by our technology. She offers a more pragmatic definition of intelligence: ‘A species’ ability to technologically modify its local environment in ways that can be detected over interstellar distances’ (Tarter, 1995: 9). This, she considers to be preferable to the ‘overly complex and convoluted definition of “intelligence” offered by researchers in other fields’ (ibid.).
53
In what might be described as a ‘leap of faith’, some SETI exobiologists have maintained that life, as it evolves, is likely to evolve greater forms of intelligence; that intelligence must necessarily follow life. The belief is that intelligence will enhance survival. This is not always the case, as many non-intelligent species, bacteria, and plants, have an excellent survival record. A study of terrestrial natural history is unlikely to support the belief that intelligence enhances survival, and even if an evolution towards greater intelligence does occur, we are still left with the problem of determining the nature of intelligence. SETI is not helped with speculations concerning intelligence other than a criterion for intelligence which is bound up with the emergence of a technological civilization capable of radio communication.
It may be easier to detect life than intelligent life. NASA already has a project called TOPS (Towards Other Planetary Systems) which is based on techniques for carrying out spectroscopic examinations of distant planetary atmospheres, seeking evidence that a planet’s chemistry has been altered by metabolism. Evidence of a large amount of oxygen may indicate the presence of biochemical life.
Recognition that life may be more prevalent in the universe has followed a number of claims that evidence of simple organic life has been detected in Martian meteorites. Sceptics who would generally oppose SETI’s belief in the prevalence of life might, however, concede that simple organic structures are more widespread than previously held, but nevertheless resist any move from f l to f i. Biochemists would now concede that single cellular organisms may be quite prevalent in conditions throughout the universe which resemble those on Earth some 4 billion years ago. But, having made this concession, the sceptic might insist that later stages, f i and f c are improbable. The argument is that while bacteria appeared fairly rapidly on Earth, it nevertheless took over 3 billion years for multicellular life to develop. This appears to support the suggestion that the transition from simple life to the more ‘advanced’ multicellular organisms, and then to technological civilizations, is extremely rare.
f c: the number of planetswhere intelligent life has the capacity to communicate with other planets
Conjectures regarding the fraction f c frequently rest on the belief that a scientific civilization like ours is an inevitable consequence of intelligent life. This presupposes that other evolving species are likely to pass through similar stages from the Stone Age to the Nuclear Age. But if we consider the contingencies in the development of human natural and social history such a repetition appears very unlikely. Mammals might not have survived long enough to develop a human species. Natural disasters of the kind that wiped out the dinosaurs could have arrested human development, allowing some entirely different dominant species to evolve. Humans might not have developed sophisticated coordination between hands and eyes, which is necessary for tool-making
54
Pages: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104