boron is substituted for carbon. There is ten times more silicon than carbon in the Earth’s crust, which would suggest that if it were a viable alternative it should have emerged. However, silicon is ten times less abundant than carbon throughout the solar system. This would suggest that if silicon is unsatisfactory where it is most abundant, it is less likely to be prevalent elsewhere. Moreover, there are problems in reproducing the complex storage of information necessary for the genetic code with silicon molecules. Silicon cannot form molecules complex enough to sustain life, whereas carbon can form multiple bonds with other atoms, thus allowing a vast variety of different forms of molecules. So far there has been no success in laboratory attempts to develop an ‘organic chemistry’ based on silicon.
A further problem with silicon is that it cannot form double bonds in the way that carbon can, which allows the latter to combine with oxygen atoms to form CO2. Thus carbon can exist as a gas and can dissolve in water, which enables it to move through the atmosphere and the oceans. Nevertheless, in crystal form silicon can be used to make complex structures. Silicate crystals could store information and possibly reproduce their lattice structure, and even generate mutations such that new crystals could flourish. Jean Schneider, of Meudon Observatory, has proposed a crystalline form of life, but its capacity for reproduction seems limited. More complex developments seem highly improbable. If they breathed, they would breathe out silicon dioxide (sand). But could we call this breathing?
Boron, another candidate, is quite rare; it is about ten times less abundant than carbon on Earth and about one million times less than carbon in the solar system. There is also a problem with regard to the detection of either silicon- or boron-based forms of life. Thus if a radically different biochemistry emerged it would be unlikely to be detected (McDonald, 1998a: 16–17).
It would seem that the most favourable basis for life would consist of carbon compounds forming in water. Carbon is common throughout the universe, which gives it a good chance of replication. But it can only emerge in a living system on surface temperatures similar to Earth. However, it might be noted that even with this limitation there are millions of possible variations. As in the case of Earth, it would only require slightly different environmental conditions to favour very different life-forms. Does this provide credibility for the creatures of science fiction, or are there limits to biological liberalism on other worlds?
In much of science fiction there are accounts of gigantic insects and various exotic life-forms, but an appreciation of the problem of scaling reveals a limit to biological liberalism. Contrary to the beliefs of many science fiction readers, large-scale replicas cannot function if reproduced in exact proportion to small-scale originals. For example, it is often said that given an ant’s ability to lift many times its own weight, were the ant scaled up to the size of an elephant its physical strength would be beyond anything imaginable. Hence a planet inhabited by giant ants could exhibit feats of engineering far beyond anything attempted on Earth. However, an understanding of scaling will soon eliminate such
science fiction scenarios – along with optimistic beliefs in genetically pro-grammed chickens the size of cattle. Since Galileo, physicists, designers and architects have been aware of the problem of scaling. Galileo noted, with regard to structures having the same physical characteristics, such as shape, density or chemical composition, that weight W increases linearly with volume V, whereas strength only increaseslike a cross-sectional area A. Given similar structures V a L3 and A a L2, where L is a characteristic length or height, it can be concluded as shown in Table 3.1.
Table 3.1 Galileo’s definition of weight and volume
Strength A 1 1
—— a —— a —— a ——
Weight V L W1/3
Although smaller animals may appear stronger than larger ones, and ants appear – relatively speaking – stronger than elephants, their advantage would not survive scaling. If an ant were scaled up to the size of an elephant its weight would increase faster than its strength, ultimately causing it to collapse under its own weight. Thus the fantasies of science fiction writers of giant ants, spiders and the like, can never be realized. The giant worms of Dune are physically as well as biologically impossible. For similar reasons, humans cannot fly under their own muscle power, while small animals can leap proportionally greater distances than larger ones. These principles are recognized and applied by architects, designers, ship-builders and biologists and must play a profound regulatory role when considering exotic forms of extraterrestrial life.
According to a thesis developed by Roland Puccetti (1968) there are very strict limits to biological variation, and even stricter limits governing the development of intelligent extraterrestrials. As an example Puccetti cites mythical creatures such as centaurs and griffins, which could not exist anywhere because they make no evolutionary sense. The centaur has the upper half of a human body upon the quadrupedal body of a horse. With its human head we can suppose a human brain, which is hard to imagine evolving in the body of a foraging field animal. Puccetti considers the arms: early humans developed arms by crawling, climbing trees and swinging from branches, which would be impossible with a horse’s body. This evolutionary trend is doomed at the outset: either the upper or lower parts would disappear or, more probably, they would never develop. The evolution of a griffin is equally implausible. The mythical griffin had the head and wings of an eagle and the body and hindquarters of a lion. Flight was clearly impossible, as it would require a massive wingspan to carry the lion’s body with ‘self-spined heavy hindquarters hanging down and contributing nothing to the flight’ (ibid.: 89). And if it did not fly, then what use would wings be to a lion?
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