Despite widespread variations, evolutionary developments require that the parts contribute to the function of the whole; hence non-functional unecon-omical parts are either discarded or never develop. Puccetti also appeals to a principle of convergent evolution in order to show limits on the form of possible extraterrestrials who, he argues, if they exist, would be not that dissimilar to terrestrials. For example, most predatory animals with complex nervous systems have an anterior mouth, with its sensing organs and brain relatively near, and a posterior anus. With the transfer to terrestrial surfaces there is a likelihood of greater integration of nerve centres, and legs (wheels do not evolve in living organisms) would be required for mobility, especially in a creature with a large brain which would require a heavy structure. In fact two pairs of legs are most promising for terrestrial creatures with a large brain, and this would facilitate the development of arms in the tool-manipulation phase. Convergence, argues Puccetti, is not accidental; it represents a series of common solutions ‘available in the physico-chemical environment common to all planetary surfaces on which life-systems can evolve’ (ibid.: 96). Thus, for example, a land predator will develop skin, not feathers or scales, and a tool-making animal will evolve fingers, rather than claws.
Although the evolutionary process is based upon random mutation and is consequently unpredictable, there are nevertheless certain convergent solutions which reveal an element of predictability. Most animals with muscles and nerves equipped for mobility are likely to develop a visual system, as the ability to see confers selective advantage. Sight has independently evolved at least three times: in vertebrates, in insects and in molluscs (squids and octopuses). Some evolutionary developments may be one-off accidents, but mechanisms such as sight are more inevitable. On these terms, extraterrestrials – if they exist – would not be greatly unlike terrestrial creatures, sharing a similar environment, similar structures and similar sense organs.
The contingency of life as we know it
SETI optimists could very well underestimate the precarious nature of continuous life. Knowledge about the Earth’s history suggests that the continuance of living phenomena is precarious. The dinosaurs lasted about 150 million years, while humans have been around for less than 150,000 years. In 1980 Luis and Walter Alvarez, at the University of California at Berkeley, proposed a theory that the dinosaurs were destroyed as the result of a collision between the Earth and a large meteorite at the end of the Cretaceous period, about 66 million years ago. The original scenario implied the existence of a dust cloud, caused by the impact, which blocked photosynthesis, leaving plants and other members of the food chain to die. In a period between one and ten years most species on Earth died out. Others have contested this extinction rate, claiming that they disappeared over a period of 500,000 years, one species dying out after another. Alvarez’s thesis was supported by the discovery of large amounts of a
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rare element, iridium, in rocks in Denmark, Italy and New Zealand. Although rare on Earth, iridium is common in meteors. However, some palaeontologists cite fossil evidence of dinosaurs in rocks below those containing iridium, which suggests that dinosaurs were not destroyed by meteor impact; others have appealed to analogies with volcanic eruptions, like Krakatoa in Indonesia, which did not wipe out all plant life in the vicinity, as evidence in favour of the tenacity of living phenomena.
In 1992 scientists at the University of Chicago added a new dimension to the catastrophe theory of extinction. Dating asteroid collisions from impact craters and checking them against known mass extinctions, they argued that during the 600-million-year history of cellular life, some 60 per cent of species extinctions may have been caused by disasters due to the impact of asteroids, comets or other extraterrestrial bodies. Nevertheless, it has been pointed out that not all extinctions have been due to impacts. It has been suggested that extinctions toward the end of the Permian period, about 250 million years ago, were the result of massive volcanic activity in Siberia.
It can also be argued that impact disasters with large comets have been minimized on Earth due to the unique positioning and masses of the planets in the solar system, thus giving life on Earth an opportunity not shared by other stellar systems. For example, George Wetherill of the Carnegie Institution of Washington, suggests that intelligent life on Earth might not have had sufficient time to evolve between impact disasters were it not for the presence of the two giant planets, Jupiter and Saturn, whose masses are respectively 318 times and 95 times that of the Earth and whose strong gravity deflects comets out of the solar system. Were it not for Jupiter’s ‘house-cleaning’ role in the solar system, life on Earth might not have developed. Wetherill’s suggestion is admittedly speculative, but is nevertheless based on a series of computer simulations of a solar system wherein these gas giants have a considerably reduced mass. Whereas catastrophic impacts take place every 100 million years on Earth, Wetherill’s computer simulation which reduced Jupiter and Saturn’s respective masses to a mere 15 times that of the Earth, revealed a likelihood of collision every 100,000 years, thus greatly reducing the time for intelligent life to develop (Crosswell, 1992a). If life were to be given a chance to develop in other stellar systems, the protection provided by similar gas giants to the planet Jupiter might be crucial. On the other hand, it can be argued that while gas giants such as Jupiter can deflect comets and other large masses, it can also attract them. There is growing evidence that asteroids wandering inwards from the main belt beyond Mars have been disturbed by instabilities in Jupiter.
While many recent discoveries of ex-solar planets indicate that gas giants are fairly widespread, their closeness to their parent star, which in some cases is closer than Mercury is to the Sun, calls into question the standard theory that gas giants exist in the outer regions of their stellar system, growing larger as their lengthy orbits sweep up material. If, according to some recent theories, gas giants tend to migrate inwards towards their stars, then the majority of planetary
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