“The study of a single instance of extraterrestrial life, no matter how humble, will deprovincialize biology. For the first time, the biologists will know what other kinds of life are possible.” When Carl Sagan spoke these words over 30 years ago, he was arguing for caution against our human tendencies to over-emphasize our own significance. He wanted to allow for the possibility of life forms, however different from us to inhabit worlds equally as unimaginable. However, he was not advocating for a “no-holds-barred” free-for-all of ideas. Sagan was very much a rationalist and understood the limits of chemistry and physics. He knew that any instance of life, anywhere, would have to play by the rules. And he knew, very well, what those rules are. He was, as he put it, a carbon chauvinist and a water chauvinist when it came to musings on alien life. (He was also very much a nucleic acid and amino acid chauvinist, although less vocally so.) But how could he know? How could any of us really know what another example of life might be like? We only know of one example of life. Aren’t we hopelessly biased? Isn’t every speculation, by our very circumstance, provincial?
As it turns out, we can know quite a lot from our limited sampling. “Follow the water” is a grade school prerequisite, but it speaks volumes to our present understanding and expectations for life on other worlds. It says that we fully expect life to be chemically based, as it is here. Atoms and molecules need to collide into each other to react, and those collision take place best in water. Water is a good solvent for organic molecules and is liquid over a wide range of temperatures. Plus, water is everywhere. All of our water is extraterrestrial in origin. There is water on the moon and Mars, and in the atmospheres of the gas giants, the moons of Jupiter, and even on our Sun’s closest planet, Mercury. It’s everywhere. Throughout the Universe, the watery stage is easily set for life to make its appearance.
Water is only the beginning. An examination of life reveals a remarkable pattern; one that reverberates throughout the Universe. The living machinery of life, the chemistry, is composed of a relatively small number of simple compounds. The tens of thousands (perhaps hundreds of thousands) of unique proteins and many millions (maybe infinite) versions of DNA, belies the simplicity of their construction. Just a few dozen monomers comprise virtually all of the chemical parts of cells. These building blocks include the 20 or so amino acids used to build all proteins, the 5 nucleotides found in all nucleic acids (DNA and RNA), and some odd and ends in the form of sugars and lipids. Notice the qualifier “all” in the previous sentence. There are no exceptions. The rich diversity of living things, past and present (and future), is the result of the near infinite ways in which these building block can be put together. Individually, these monomers are quite boring, but collectively the polymers they form can make everything from butterfly scales, to pollen grains, to tooth enamel.
Perhaps most amazingly is the fact that these few dozen monomers are not unique to Earth. They, like water, are found throughout the Universe. Life is built from these monomers because (but not solely “because”) these monomers are everywhere we look. From the tails of comets, to the pools of dark between the Stars, the Universe is littered with the stuff of life.
We can interpret the close match between the chemistry of life and the chemistry of the Universe as extreme coincidence. Or we can propose a simpler alternative --as direct cause and effect; as the inevitable outcome of a Universe primed for life. But, to do so requires more evidence.
-End of part 2 (part 3)
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