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Jokes are a way of dealing with anxiety. There is a class of jokes dealing with extraterrestrial life. In one, the extraterrestrial visitor lands on Earth, walks up to a gasoline pump or a gumball machine - the accounts differ - and asks, "What's a nice girl like you doing in a place like this?"

Elsewhere, beings are doubtless very different from us. But the joke assumes that extraterrestrial organisms will be, if not like human beings, then like gasoline pumps or gumball machines. The most likely circumstance is that extraterrestrial beings will look nothing like any organisms or machines familiar to us. Extraterrestrials will be the product of billions of years of independent biological evolution, by small steps, each involving a series of tiny mutational accidents, on planets with very different environments from those that characterize Earth.

But such jokes underscore a general problem and a general virtue in thinking about life elsewhere. The problem is that we have only one kind of life to study, the co-related biology of the planet Earth, all organisms of which have descended from a single instance of the origin of life. It is difficult for the biologist, as well as the layman, to determine what properties of life on our planet are accidents of the evolutionary process and what properties are characteristic of life everywhere. The assumption that life elsewhere has to be, in some major sense, like life here is a conceit I will call chauvinism.

While such chauvinism has been common throughout human history, clearer views have occasionally surfaced, for example, by the great French astronomer Pierre Simon, the Marquis de Leplace. In his classic work La Mecanique Celeste he wrote: "[The Sun's] influence gives birth to the animals and plants which cover the surface of the Earth, and analogy induces us to believe that it produces similar effects on the planets; for it is not natural to pose that matter, of which we see the fecundity develop itself in such various ways, should be sterile upon a planet so large as Jupiter, which, like the Earth, has its days, its nights, and its years, and on which observation discovers changes that indicate very active forces. Man, formed for the temperature which he enjoys upon the Earth, could not, according to all appearance, live upon the other planets; but ought there not to be a diversity of organization suited to the various temperatures of the globes of this universe? If the difference of elements and climates alone causes such variety in the production of the Earth, how infinitely diversified must be the production of the planets and their satellites?" Laplace wrote these words near the end of the eighteenth century.

The virtue of thinking about life elsewhere is that it forces us to stretch our imaginations. Can we think of alternative solutions to biological problems already solved in one particular way on Earth? For example, the wheel is a comparatively recent invention on the planet Earth. It seems to have been invented in the ancient Near East less than ten thousand years ago. In fact, the high civilizations of Meso-America, the Aztecs and the Mayas, never employed the wheel, except for children's toys. Biology - the evolutionary process - has never invented the wheel, in spite of the fact that its selective advantages are manifest. Why are there no wheeled spiders or goats or elephants rolling along the highways? The answer is clearly that, until recently, there were no highways. Wheels are of use only when there are surfaces to roll on. Since the planet Earth is a heterogeneous, bumpy place with few long, smooth areas, there was no advantage to evolving the wheel. We can very well imagine another planet with enormous long stretches of smooth lava fields in which wheeled organisms are abundant. The late Dutch artist M. C. Escher designed a salamander-like organism that would do very well in such an environment.

The evolution of life on Earth is a product of random events, chance mutations, and individually unlikely steps; small differences early in the evolution of life have a profound significance later in the evolution of life. Were we to start the Earth over again and let only random factors operate, I believe that we would wind up with nothing at all resembling human beings. This being the case, how much less likely it is that organisms evolving over five billion or more years, independently in a quite different environment of another planet of a far-off star, would closely resemble human beings.

Thus, the hoary science-fiction standby of the sexual love between a human being and an inhabitant of another planet ignores, in the most fundamental sense, the biological realities. John Carter could love Dejah Thoris, but, despite what Edgar Rice Burroughs believed, their love could not be consummated. And if it could, a viable offspring would not be possible. Likewise, the category of contact story, now quite fashionable in some UFO enthusiast circles, of sexual contact between human and saucerian - most recently described in a weekly newspaper headline with the modest title "We Sexed a Blonde from a Flying Saucer!" - must be relegated to the realm of improbable fantasy. Such crossings are about as reasonable as the mating of a man and a petunia.

A popular phrase - often encountered in popular books on the planets - is "life as we know it." We read that "life as we know it" is impossible on this planet or that. But what is life as we know it? It depends entirely on who the "we" is. A person who is unsophisticated in biology, who lacks a keen appreciation of the multitudinous adaptations and varieties of terrestrial organisms, will have a meager idea of the range of possible biological habitats. There are discussions, even by famous scientists, that give the impression that an environment that is uncomfortable for my grandmother is impossible for life.

At one time it was thought that oxides of nitrogen had been detected in the atmosphere of Mars. A scientific paper was published on this apparent finding. The authors of the paper argued that life on Mars was, therefore, impossible, because oxides of nitrogen are poisonous gases. There are at least two objections to this argument. First, oxides of nitrogen are poisonous gases only to some organisms on Earth. Second, what quantity of oxides of nitrogen were thought to be discovered on Mars? When I calculated the amount, it turned out to be less than the average abundance above Los Angeles. The oxides of nitrogen are an important constituent of smog. Life in Los Angeles may be difficult, but it is not yet impossible. The same conclusion applies to Mars. The final problem with these particular observations is that they are very likely mistaken; later studies -for example, observations Tobias Owen and I made with the Orbiting Astronomical Observatory - have shown no oxides of nitrogen in the atmosphere of Mars.

Oxygen chauvinism is common. If a planet has no oxygen, it is alleged to be uninhabitable. This view ignores the fact that life arose on Earth in the absence of oxygen. In fact, oxygen chauvinism, if accepted, logically demonstrates that life anywhere is impossible. Fundamentally, oxygen is a poisonous gas. It chemically combines with and destroys the organic molecules of which terrestrial life is composed. There are many organisms on Earth that do without oxygen and many organisms that are poisoned by it.

All of the earliest organisms on Earth did not use molecular oxygen, O2. In a brilliant set of evolutionary adaptations, organisms like insects and frogs and fish and people learned not only to survive in the presence of this poisonous gas but actually to use it to increase the efficiency with which we metabolize foodstuffs. But that should not blind us to the fundamentally poisonous character of this gas. The absence of oxygen on a place such as Jupiter is, therefore, hardly an argument against life on such planets.

There are ultraviolet light chauvinists. Because of the oxygen in the Earth's atmosphere, a variety of oxygen molecule called ozone (O3) is produced high in the atmosphere, about twenty-five miles above the surface. This ozone layer absorbs the middle-wavelength ultraviolet rays from the Sun, preventing them from reaching the surface of our planet. These rays are germicidal. They are emitted by ultraviolet lamps commonly used to sterilize surgical instruments. Strong ultraviolet rays from the Sun are an extremely serious hazard to most forms of life on Earth. But this is because most forms of life on Earth evolved in the absence of a high ultraviolet flux.

It is easy to imagine adaptations to protect organisms against ultraviolet light. In fact, sunburn and high melanin pigmentation in the skin are adaptations in this direction. They have not been carried very far in most terrestrial organisms because the present ultraviolet flux is not very high. In a place like Mars, where there is little ozone, the ultraviolet light at the surface is extremely intense. But the Martian surface material is a strong absorber of ultraviolet light - as most soil and rocks are - and we can easily imagine organisms walking around with small ultraviolet-opaque shields on their backs: Martian turtles. Or perhaps Martian organisms carry about ultraviolet parasols. Many organic molecules also could be used in the exterior layers of extraterrestrial organisms to protect them against ultraviolet light.

There are temperature chauvinists. It is said that the freezing temperatures on planets like Jupiter or Saturn, in the outer Solar System, make all life there impossible. But these low temperatures do not apply to all portions of the planet. They refer only to the outermost cloud layers - the layers that are accessible to infrared telescopes that can measure temperatures. Indeed, if we had such a telescope in the vicinity of Jupiter and pointed it at Earth, we would deduce very low temperatures on Earth: We would be measuring the temperatures in the upper clouds and not on the much warmer surface of Earth.

It is now quite firmly established, both from theory and from radio observations of these 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 low-temperature 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 studies of the interstellar medium, a profusion of simple and complex organic molecules readily produced by a wide variety of energy sources. For example, in one of our experiments, the passage of a single high-pressure shock wave through a mixture of methane (CH4), ethane (C2H6), ammonia (NH3), and water (H2O) converted 38 percent of the ammonia into amino acids, the building blocks of proteins. There were not enormous quantities of other sorts of organic molecules.

Thus, both the atoms and the simple molecules of which we are made are probably common to organisms elsewhere in the universe. But the specific way in which these molecules are put together and the specific forms and physiologies of the extraterrestrial organisms may be, because of their different evolutionary histories, extremely different from what is common on our planet.

In considering which stars to examine for possible radio signals directed at us, much attention is usually given to stars like our Sun. It has been reasonably argued that searches should begin with the one type of star we know has life on at least one of its planets, namely stars like our own Sun. In Project Ozma, the first attempt to search for such radio signals, the two stars examined, Tau Ceti and Epsilon Eridani, were both stars with mass, radius, age, and composition very similar to our Sun, which astronomers call a G-o dwarf. They were, in fact, the nearest two Sun-like stars.

But should we restrict our attention to stars like the Sun? I think not. Stars of slightly smaller mass and of slightly lower luminosity than our Sun are longer lived. These stars, called K and M dwarfs, can be many billions of years older than the Sun. If we imagine that the longer the lifetime of a planet, the more likely it is that intelligent organisms have evolved on it, we should then bias our searches toward K and M stars and avoid G-star chauvinism. It may be objected that planets of K and M stars are much colder than Earth, and that life on them may be less likely.

The premise of this objection does not appear to be true; such planets seem to be closer to their stars than the corresponding planets in our Solar System; and we have already discussed the fallacies of temperature chauvinism. Too, there are many more K and M stars than G stars.

Is there planetary chauvinism? Must life arise and reside on planets, or might there be organisms that inhabit the depths of interstellar space, the surfaces or interiors of stars, or other even more exotic cosmic objects?

In our present state of ignorance, these are very difficult questions to answer. The density of matter in interstellar space is so low that an organism there simply cannot acquire enough material to make a copy of itself in any reasonable period of time. This is not true in dense interstellar clouds, but such clouds live for very short periods of time, condensing to form stars and planets. In the process, they become so hot that any organic compounds contained within them are probably destroyed.

We might imagine organisms evolving on planets with atmospheres slowly leaking away to space, permitting the organisms gradually to adapt to the increasingly severe conditions, and finally acclimatizing to what in effect is an interstellar environment. Organisms leaving such planets - perhaps by electromagnetic radiation pressure, or by solar wind from the local sun - might populate interstellar space, but they would still be faced with insurmountable problems of malnutrition.

A quite different sort of interstellar organism may be much more likely: Intelligent beings who arise on planets as we have, but who have moved their arena of activities to the much vaster volume of interstellar space. Beings in our far technological future should have capabilities at which we cannot today even dimly guess. It is not out of the question that such societies could tap the matter and energy of stars and galaxies for their own uses. Just as we are organisms completely at home only on the land, although we evolved from the sea, the universe may be populated with societies that arose on planets but that are comfortable only in the depths of interstellar space.

Pierre Simon Leplace

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