“I am, reluctantly, a self-confessed carbon chauvinist. Carbon is abundant in the Cosmos. It makes marvelously complex molecules, good for life. I am also a water chauvinist. Water makes an ideal solvent system for organic chemistry to work in and stays liquid over a wide range of temperatures. But sometimes I wonder. Could my fondness for materials have something to do with the fact that I am made chiefly of them? Are we carbon- and water-based because those materials were abundant on the Earth at the time of the origin of life? Could life elsewhere—on Mars, say—be built of different stuff? I am a collection of water, calcium and organic molecules called Carl Sagan. You are a collection of almost identical molecules with a different collective label. But is that all? Is there nothing in here but molecules? Some people find this idea somehow demeaning to human dignity. For myself, I find it elevating that our universe permits the evolution of molecular machines as intricate and subtle as we. But the essence of life is not so much the atoms and simple molecules that make us up as the way in which they are put together. Every now and then we read that the chemicals which constitute the human body cost ninety-seven cents or ten dollars or some such figure; it is a little depressing to find our bodies valued so little. However, these estimates are for human beings reduced to our simplest possible components. We are made mostly of water, which costs almost nothing; the carbon is costed in the form of coal; the calcium in our bones as chalk; the nitrogen in our proteins as air (cheap also); the iron in our blood as rusty nails. If we did not know better, we might be tempted to take all the atoms that make us up, mix them together in a big container and stir. We can do this as much as we want. But in the end all we have is a tedious mixture of atoms. How could we have expected anything else? Harold Morowitz has calculated what it would cost to put together the correct molecular constituents that make up a human being by buying the molecules from chemical supply houses. The answer turns out to be about ten million dollars, which should make us all feel a little better. But even then we could not mix those chemicals together and have a human being emerge from the jar. That is far beyond our capability and will probably be so for a very long period of time. Fortunately, there are other less expensive but still highly reliable methods of making human beings. I think the lifeforms on many worlds will consist, by and large, of the same atoms we have here, perhaps even many of the same basic molecules, such as proteins and nucleic acids—but put together in unfamiliar ways. Perhaps organisms that float in dense planetary atmospheres will be very much like us in their atomic composition, except they might not have bones and therefore not need much calcium. Perhaps elsewhere some solvent other than water is used. Hydrofluoric acid might serve rather well, although there is not a great deal of fluorine in the Cosmos; hydrofluoric acid would do a great deal of damage to the kind of molecules that make us up, but other organic molecules, paraffin waxes, for example, are perfectly stable in its presence. Liquid ammonia would make an even better solvent system, because ammonia is very abundant in the Cosmos. But it is liquid only on worlds much colder than the Earth or Mars. Ammonia is ordinarily a gas on Earth, as water is on Venus. Or perhaps there are living things that do not have a solvent system at all—solid-state life, where there are electrical signals propagating rather than molecules floating about. But these ideas do not”
―
Carl Sagan,
Cosmos
