Science fiction has envisaged the possibility of everything from kind, wise, and even cute extraterrestrials, like ET, to utterly malicious, scheming monsters, like Giger's Alien. On balance, ever since H. G. Wells unleashed his marauding Martians (see War of the Worlds, novel), the fictional creatures from "out there" have tended to be of the usurping, death-ray variety - not surprisingly, since this makes for a more compelling plot. But if we do encounter other intelligences among the stars, will they in reality prove to be friendly or hostile?
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A poll conducted for The Planetary Society by the Marist Institute in 1998 suggested that 86% of Americans who think there is life on other planets believe it will be friendly (see opinion polls, about extraterrestrials). Similar optimism has been expressed by many scientists who have figured prominently in the search for extraterrestrial life, including Frank Drake, Philip Morrison, Carl Sagan, and Ronald Bracewell. An argument in favor of alien beneficence is that any race which has managed to survive the kind of global crises currently facing humanity (and which presumably confront all technological species at some stage in their development) is likely to have resolved the sources of conflict we still have on Earth (see extraterrestrial civilizations, hazards to). Morrison, for instance, doubted that advanced societies "crush out any competitive form of intelligence, especially when there is clearly no danger." Similarly, Arthur C. Clarke has stated that:
As our own species is in the process of proving, one cannot have superior science and inferior morals. The combination is unstable and self-destroying. However, there can be no assurance on this point. After all, human beings appear to have made very little progress, over the past two millennia or so, toward eliminating or controlling their aggressive tendencies. And there is no reason to suppose we shall change much in this respect over the next few centuries, during which time we may well develop the means of reaching the stars in a realistic timescale. Those who are pessimistic about the general nature of extraterrestrials argue that Darwinism, and its fundamental tenet "survival of the fittest", virtually guarantees that any advanced species will be potentially dangerous.Michael Archer, professor of biology at the University of New South Wales, Australia, has put it this way:1
Any creature we contact will also have had to claw its way up the evolutionary ladder and will be every bit as nasty as we are. It will likely be an extremely adaptable, extremely aggressive super-predator.
In a similar vein, medical anthropologist Melvin Korner has written that:
Evolution predicts the existence of selfishness, arrogance and violence on other planets even more surely than it predicts intelligence. If they could get to Earth, extraterrestrials would do to us what we have done to lesser animals for centuries.
In 1964, Freeman Dyson considered the two polar extremes to which intelligence might evolve:2
Intelligence may indeed be a benign influence, creating isolated groups of philosopher-kings far apart in the heavens and enabling them to share at leisure their accumulated wisdom ... [On the other hand] intelligence may be a cancer of purposeless technological exploitation, sweeping across a galaxy as irresistibly as it has swept across our own planet ... the technological cancer could spread over a whole galaxy in a few million years, a time very short compared with the life of a planet... Our business as scientists is to search the universe and find out what is there. What is there may conform to our moral sense or it may not... It is just as unscientific to impute to remote intelligences wisdom and serenity as it is to impute to them irrational and murderous impulses. We must be prepared for either possibility and conduct our searches accordingly.
Perhaps the most reasonable assumption, in the absence of any data, is that, just as in our own case, the potential for good and evil will exist in every intelligent extraterrestrial race. Civilization is unthinkable without some measure of compassion, and yet how could a species that had emerged successfully after several billion years of live-and-let-die biological competition not also possess a ruthless streak? The question is surely not whether any advanced race we may meet among the stars is capable of aggression – it certainly will be unless it has genetically or otherwise altered itself to be purely pacific – but whether it has learned to override its more basic instincts. A further point to bear in mind is the variation in character which can exist between individuals within a species. Will the first representative of an alien race that we encounter be a Hitler or a Gandhi? Fears about what consequences might follow from making contact with a superior species, malevolent or benign, have led to calls from some leading scientists to avoid attempts at CETI (see CETI, opposition to).3
Regardless, two schools of thought exist on the question of what extraterrestrial life will be like on other worlds. These fall under the headings of "divergionism" and "convergionism", or to use Harold Blum's terminology, "opportunism" and "determinism". In reality, the truth may well lie somewhere between these extremes. Divergionism stresses the unpredictable and non-repeatable aspects of evolution which generate novelty and therefore might tend to lead to a spectacularly wide range of organisms across the Universe. Convergionism, on the other hand, argues that, while life will inevitably exhibit some differences from one world to another, natural selection will tend to come up with the same optimum designs, reflected in similar biological structures and processes, wherever living things develop. Both these points of view can cite evidence based on observations of life-forms on our own planet.
In favor of divergionism is the astonishing biodiversity found on Earth. Bearing in mind the remarkable differences between, say, hummingbirds and oak trees, or whales and bacteria, all of which have a common heritage, it is difficult to set limits on what richness of forms might occur among extraterrestrial species. Even a replay of terrestrial evolution would almost certainly lead to substantially different end-products, a claim first put forcibly by Wallace and echoed many times since. Conceivably, for instance, an evolutionary rerun would fail to give rise to the land vertebrates at all. And even if it did, the probability of a repeat of the cataclysm which ended the dinosaur dynasty and, as a consequence, allowed higher forms of mammalian life, including ourselves, to fill the vacated niches, would be low indeed at that same pivotal point in Earth's history. This incident alone is enough to suggest that random cosmic events, especially asteroidal or cometary collisions, can have profound and totally unpredictable effects on the course of a planet's biological development (see cosmic collisions, biological effects).
Given the serendipitous nature of mutations and major external influences, divergionists would argue that even if life is constrained to having essentially just one biochemical basis, evolution, in its blind exploration of all biological options, will generate across the vastness of space and time every conceivable, viable type of organism. Still greater diversity could be anticipated if life can arise from a variety of different molecular building blocks. An important factor in this is where the first few chemical steps leading to life tend to take place. If prebiotic synthesis happens almost exclusively on the surface of suitable worlds then it is more likely that wide, homegrown variations in biochemical development occur, than if the organic starter kit arrives from the communal resource of space. That at least some prebiotic chemicals are seeded on infant worlds by asteroid and comet impacts now seems very likely, suggesting that certain fundamental sub-units of life such as amino acids, which have been found in interstellar clouds, may be common biological currency. Yet at what stage up the ladder of prebiotic synthesis major variations between worlds become apparent is not clear. Some biologists believe that wherever life occurs it will use at least some of the same molecules, like sugars, and therefore have some similar biosynthetic pathways, such as the citric acid cycle used to burn sugar and yield energy. Certainly, on an extrasolar planet, the chance of anything like the same gene combinations emerging that underpin contemporary terrestrial life must be vanishingly small. But whether genes themselves, DNA, and proteins lie routinely at the heart of life throughout the Galaxy has yet to be established. The greater the possible biochemical diversity of organisms, the more scope for divergionism to envisage novel forms of cosmic life, including perhaps creatures so different from anything we know that we might not recognize them as being alive. Our very definitions of life might need to be reexamined (see life, nature of).
Convergionism, on the other hand, while acknowledging that chance and circumstances play a part in shaping how life evolves, places greater emphasis on the fact that natural selection is subject to universal laws. Therefore the same motifs will tend to recur, in subtly different guises, as evolution keeps discovering the same best solutions to survival in different environments on different worlds. At the level of gross anatomy, for example, it would be surprising if certain features, such as legs, wings, fins, eyes, and ears, or their near-equivalents, did not evolve independently in many different locations in space. The phenomenon of convergent evolution on Earth suggests that this is true. On the other hand, there is the danger of anthropomorphism and we need to consider carefully the bold attempts by both scientists and science fiction writers, over the past century, to explore the outer limits of what life elsewhere might be like. H. G. Wells offered an early vision of something disturbingly alien with his Martians and selenites, while his Victorian contemporary, Arthur Conan Doyle raised the prospect of a single planet-wide organism in "The Day the Earth Screamed"-a concept given some scientific credibility by the Gaia Hypothesis. Wildly unanthropomorphic aliens go back at least as far as the Lensmen novels of E. E. "Doc" Smith in the 1930s and Olaf Stapledon's Starmaker of 1934, and earlier if one includes the now-neglected late 19th century writings of J. H. Rosny. Biologists have mused on whether life could have an entirely different chemical basis - silicon, perhaps, instead of carbon (see silicon-based life), or ammonia in place of water (see ammonia-based life). Some scientists have speculated further. Fred Hoyle described an intelligent nebula in The Black Cloud, while Frank Drake, an early pioneer of SETI, wondered if life might evolve on a neutron star (see neutron star, life on), an idea taken further and cast into fictional form by the aerospace engineer, Robert Forward.
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