Cosmic Connection by Carl Sagan

planets, that as we penetrate below the visible clouds, the temperatures increase. There is always a region in the atmospheres of Jupiter, Saturn, Uranus, and Neptune that is at quite comfortable temperatures by terrestrial standards.
    But why is it necessary to have temperatures like those on Earth in order for life to proliferate? A human being is seriously inconvenienced if his body temperature is raised or lowered by a mere 20 degrees. Is this because we happen to live by accident on the one planet in the Solar System that has a surface at the right temperature for biology? Or is it that our chemistry is delicately attuned to the temperature of the planet on which we have evolved? The latter is almost surely the case. Other temperatures, other biochemistries.
    Our biological molecules are put together in complex three-dimensional arrangements. The functioning of these molecules, particularly the enzymes, are turned on and off by altering these three-dimensional arrangements. The chemical bonds that do these rearrangements must be weak enough to be broken conveniently at terrestrial temperatures, and at the same time strong enough not to fall to pieces if left alone for short periods of time. A chemical bond known as the hydrogen bond has an energy appropriately intermediate between these unreactive and unstable alternatives. The hydrogen bond is intimately connected with the three-dimensional biochemistry of terrestrial organisms.
    On a much hotter planet like Venus, our biological molecules would fall to pieces. On a much colder planet in the outer Solar System, our biological molecules would be rigid, and our chemical reactions would not proceed at any useful rate. However, it is conceivable that much stronger bonds on Venus and much weaker bonds in the outer Solar System play the same role that hydrogen bonds play on Earth. We may have been much too quick to reject life at temperatures very different from those on our planet. There are not many chemical reactions known that can proceed at useful rates at some very low temperature such as might exist on Pluto, 30 or 40 degrees above absolute zero. But there are also very few chemical laboratories on Earth where experiments are performed at 30 or 40 degrees above absolute zero. With a few exceptions, such experiments have not been performed at all.
    We are thus at the mercy of observational selection. We examine only a small fraction of the possible range of cases because of some unconscious bias, or the fact that scientists wish to work in their shirtsleeves. We then conclude that all conceivable cases must conform to what our preconceptions have forced upon us.
    Another common chauvinism–one which, try as I might, I find I share–is carbon chauvinism. A carbon chauvinist holds that biological systems elsewhere in the universe will be constructed out of carbon compounds, as is life on this planet. There are conceivable alternatives: Atoms like silicon or germanium can enter into some of the same kinds of chemical reactions as carbon does. It is also true that much more attention has been paid to carbon organic chemistry than to silicon or germanium organic chemistry, largely because most biochemists we know are of the carbon, rather than the silicon or germanium, variety. Nevertheless, from what we know of the alternative chemistries, it appears clear that–except in very lowtemperature environments–there is a much wider variety of complex compounds that can be built from carbon than from the alternatives.
    In addition, the cosmic abundance of carbon exceeds that of silicon, germanium, or other alternatives. Everywhere in the universe, and particularly in primitive planetary environments in which the origin of life occurs, there is simply more carbon than alternative atoms available to make complex molecules. We see from laboratory experiments simulating the primitive atmosphere of the Earth or the present environment of Jupiter, as well as in radioastronomical