Read Online Cosmos by Carl Sagan - Free Book Online

recently evolved from more complex organisms rather than from simpler ones. Indeed, it is hard to imagine a still simpler organism that is in any sense alive. Viroids are composed exclusively of nucleic acid, unlike the viruses, which also have a protein coat. They are no more than a single strand of RNA with either a linear or a closed circular geometry. Viroids can be so small and still thrive because they are thoroughgoing, unremitting parasites. Like viruses, they simply take over the molecular machinery of a much larger, well-functioning cell and change it from a factory for making more cells into a factory for making more viroids.
    The smallest known free-living organisms are the PPLO (pleuropneumonia-like organisms) and similar small beasts. They are composed of about fifty million atoms. Such organisms, having to be more self-reliant, are also more complicated than viroids and viruses. But the environment of the Earth today is not extremely favorable for simple forms of life. You have to work hard to make a living. You have to be careful about predators. In the early history of our planet, however, when enormous amounts of organic molecules were being produced by sunlight in a hydrogen-rich atmosphere, very simple, nonparasitic organisms had a fighting chance. The first living things may have been something like free-living viroids only a few hundred nucleotides long. Experimental work on making such creatures from scratch may begin by the end of the century. There is still much to be understood about the origin of life, including the origin of the genetic code. But we have been performing such experiments for only somethirty years. Nature has had a four-billion-year head start. All in all, we have not done badly.
    Nothing in such experiments is unique to the Earth. The initial gases, and the energy sources, are common throughout the Cosmos. Chemical reactions like those in our laboratory vessels may be responsible for the organic matter in interstellar space and the amino acids found in meteorites. Some similar chemistry must have occurred on a billion other worlds in the Milky Way Galaxy. The molecules of life fill the Cosmos.
    But even if life on another planet has the same molecular chemistry as life here, there is no reason to expect it to resemble familiar organisms. Consider the enormous diversity of living things on Earth, all of which share the same planet and an identical molecular biology. Those other beasts and vegetables are probably radically different from any organism we know here. There may be some convergent evolution because there may be only one best solution to a certain environmental problem—something like two eyes, for example, for binocular vision at optical frequencies. But in general the random character of the evolutionary process should create extraterrestrial creatures very different from any that we know.
    I cannot tell you what an extraterrestrial being would look like. I am terribly limited by the fact that I know only one kind of life, life on Earth. Some people—science fiction writers and artists, for instance—have speculated on what other beings might be like. I am skeptical about most of those extraterrestrial visions. They seem to me to rely too much on forms of life we already know. Any given organism is the way it is because of a long series of individually unlikely steps. I do not think life anywhere else would look very much like a reptile, or an insect or a human—even with such minor cosmetic adjustments as green skin, pointy ears and antennae. But if you pressed me, I could try to imagine something rather different:
    On a giant gas planet like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water and ammonia, there is no accessible solid surface, but rather a dense, cloudy atmosphere in which organic molecules may be falling from the skies like manna from heaven, like the products of our laboratory experiments. However, there is a characteristic