by Isaac Asimov
The first chlorophyll-using organisms may have been no more complicated than individual chloroplasts today. There are, in fact, 2,000 species of a group of one-celled photosynthesizing organisms called blue-green algae (they are not all blue-green, but the first ones studied were). These are very simple cells, prokaryotes, rather bacterialike in structure, except that they contain chlorophyll and bacteria do not. Blue-green algae may be the simplest descendants of the original chloroplast, while bacteria may be the descendants of chloroplasts that lost their chlorophyll and took to parasitism or to foraging on dead tissue and its components.
As chloroplasts multiplied in the ancient seas, carbon dioxide was gradually consumed and molecular oxygen took its place. The present Atmosphere III was formed. Plant cells grew steadily more efficient, each one containing numerous chloroplasts. At the same time, elaborate cells without chlorophyll could not exist on the previous basis, for new food did not form in the’ ocean except within plant cells. However, cells without chlorophyll but with elaborate mitochondrial equipment that could handle complex molecules with great efficiency and store the energy of their breakdown, could live by ingesting the plant cells and stripping the molecules the latter had painstakingly built up. Thus originated the animal cell. Eventually, organisms grew complex enough to begin to leave the fossil record (plant and animal) that we have today.
Meanwhile, the earth environment had changed fundamentally, from the standpoint of creation of new life. Life could no longer originate and develop from purely chemical evolution. For one thing, the forms of energy that had brought it into being in the first place—ultraviolet and radioactive energy—were effectively gone or at least seriously diminished. For another, the well-established forms of life would quickly consume any organic molecules that arose spontaneously. For both these reasons, there is virtually no chance of any new and independent breakthrough from nonlife into life (barring some future human intervention, if we learn to turn the trick). Spontaneous generation today is so highly improbable that it can be regarded as essentially impossible.
Life in Other Worlds
If we accept the view that life arose simply from the workings of physical and chemical laws, it follows that in all likelihood life is not confined to the earth. What are the possibilities of life elsewhere in the universe?
When it was first recognized that the planets of the solar system were worlds, it was taken for granted that they were the abode of life, even intelligent life. It was with certain shock that the moon was recognized as lacking air and water and, therefore, probably lacking life as well.
In the modern age of rockets and probes (see chapter 3), scientists are pretty well convinced that there is no life on the moon or on any of the other worlds of the inner solar system, except for Earth itself.
Nor is there much chance for the outer solar system. To be sure, Jupiter has a deep and complex atmosphere with a temperature very low at the visible cloud layer and very high within. Somewhere at moderate depths and moderate temperatures, and with the known presence of water and organic compounds, it is conceivable (as Carl Sagan suggests) that life may exist. If true of Jupiter, it may be true of the three other gas giants as well.
Then, too, the Jovian satellite of Europa has a world-girdling glacier; but beneath it, may be a water ocean warmed by Jupiter’s tidal influence. Titan has an atmosphere of methane and nitrogen and may have liquid nitrogen and solid organic compounds on the surface—and so may Neptune’s satellite Triton as well. On all three satellites, it is conceivable that some form of life may exist.
These are all long shots, however. We can hope, wistfully, but we cannot honestly expect much; and it is only fair to suppose that as far as the solar system is concerned, the earth, and only the earth, seems to be an abode of life. But the solar system is not all there is. What are the possibilities of life elsewhere in the universe?
The total number of stars in the known universe is estimated to be at least 10,000,000,000,000,000,000,000 (10 billion trillion). Our own galaxy contains well in excess of 200,000,000,000 stars. If all the stars developed by the same sort of process as the one that is believed to have created our own solar system (that is, the condensing of a large cloud of dust and gas), then it is likely that no star is solitary, but each is part of a local system containing more than one body. We know that there are many double stars, revolving around a common center, and it is estimated that at least half of the stars belong to a system containing two or more stars.
What we really want, though, is a multiple system in which a number of members are too small to be self-luminous and are planets rather than stars. Though we have no means (so far) of detecting directly any planet beyond our own solar system, even for the nearest stars, we can gather indirect evidence. This has been done at the Sproul Observatory of Swarthmore College under the guidance of the Dutch-American astronomer Peter Van de Kamp.
In 1943, small irregularities of one of the stars of the double-star system 61 Cygni showed that a third component, too small to be self-luminous, must exist. This third component, 61 Cygni C, had to be about eight times the mass of Jupiter and therefore (assuming the same density) about twice the diameter. In 1960, a planet of similar size was located circling about the small star Lalande 21185 (located, at least, in the sense that its existence was the most logical way of accounting for irregularities in the star’s motion). In 1963, a close study of Barnard’s star indicated the presence of a planet there, too—one that was only one and one-half times the mass of Jupiter.
Barnard’s star is second closest to ourselves; Lalande 21185, third closest; and 61 Cygni, twelfth closest. That three planetary systems should exist in close proximity to ourselves would be extremely unlikely unless planetary systems were very common generally. Naturally, at the vast distances of the stars, only the largest planets could be detected and even then with difficulty. Where super-Jovian planets exist, it seems quite reasonable (and even inevitable) to suppose that smaller planets also exist.
Unfortunately the observations that yield the supposition that these extrasolar planets exist are anything but clear-cut and are close to the limits that can be observed. There is considerable doubt among astronomers generally that the existence of such planets has really been demonstrated.
But, then, a new kind of evidence made its appearance. In 1983, an Infrared Astronomy Satellite (IRAS) was orbiting Earth. It was designed to detect and study infrared sources in the sky. In August, the astronomers Harmut H. Aumann and Fred Gillett turned its detecting system toward the star Vega and found, to their surprise, that Vega was much brighter in the infrared than seemed reasonable. A closer study showed the infrared radiation was coming not from Vega itself but from its immediate surroundings.
Apparently Vega was surrounded by a cloud of matter stretching outward twice as far as Pluto’s orbit from our sun. Presumably, the cloud consisted of particles larger than dust grains (or it would long since have been gathered up by Vega). Vega is much younger than our sun, for it is less than a billion years old, and, being 60 times as luminous as the sun has a much stronger stellar wind which can act to keep the particles from coalescing. For both these reasons, Vega might be expected to have a planetary system still in the process of formation. Included among the vast cloud of gravel may already be planetsized objects that are gradually sweeping their orbits clean.
This discovery strongly favors the supposition that planetary systems are common in the universe, perhaps as common as stars are.
But even assuming that all or most stars have planetary systems and that many of the planets are earthlike in size, we must know the criteria such planets must fulfill to be habitable. The American space scientist Stephen H. Dole made a particular study of this problem in his book Habitable Planets for Man (1964) and reached certain conclusions, admittedly speculative, but reasonable.
He pointed out, in the first place, that a star must be of a certain size in order to possess a habitable planet. The larg
er the star, the shorter-lived it is; and, if it is larger than a certain size, it will not live long enough to allow a planet to go through the long stage of chemical evolution prior to the development of complex life forms. A star that is too small cannot warm a planet sufficiently, unless that planet is so close that it will suffer damaging tidal effects. Dole concluded that only stars of spectral classes F2 to Kl are suitable for the nurturing of planets that are comfortably habitable for human beings: planets that we can colonize (if travel between the stars ever becomes practicable) without undue effort. There are, Dole estimated, 17 billion such stars in our galaxy.
Such a star might be capable of possessing a habitable planet and yet might not possess one. Dole estimated the probabilities that a star of suitable size might have a planet of the right mass and at the right distance, with an appropriate period of rotation and an appropriately regular orbit; and by making what seem to him to be reasonable estimates, he concluded that there are likely to be 600 million habitable planets in our galaxy alone, each of them already containing some form of life.
If these habitable planets are spread more or less evenly throughout the galaxy, Dole estimated that there is one habitable planet per 80,000 cubic light-years. Hence, the nearest habitable planet to ourselves may be some twenty-seven light-years away; and within one hundred light-years of ourselves, there may be a total of fifty habitable planets.
Dole listed fourteen stars within twenty-two light-years of ourselves that might possess habitable planets, and weighed the probabilities that this might be true in each case. He concluded that habitable planets are most likely to be found in the stars closest to us—the two sun-like stars of the Alpha Centauri system, Alpha Centauri A and Alpha Centauri B. These two companion stars, taken together, have, Dole estimates, 1 chance in 10 of possessing habitable planets. The total probability for all fourteen neighboring stars is about 2 chances in 5.
If life is the consequence of the chemical reactions described in the previous section, its development should prove inevitable on any earthlike planet. Of course, a planet may possess life and yet not possess intelligent life. We have no way of making even an intelligent guess as to the likelihood of the development of intelligence on a planet; and Dole, for instance, was careful to make none. After all, our own Earth, the only habitable planet we can study, existed for more than 3 billion years with a load of life, but not intelligent life.
It is possible that the porpoises and some of their relatives are intelligent, but, as sea creatures, they lack limbs and could not develop the use of fire; consequently, their intelligence, if it exists, could not be bent in the direction of a developed technology. If land life alone is considered, then it is only for about a million years or so that the earth has been able to boast a living creature with intelligence greater than that of an ape.
Still, this means that the earth has possessed intelligent life for 1/3500 of the time it has possessed life of any kind (at a rough guess). If we can say that of all life-bearing planets, lout of 3,500 bears intelligent life, then out of the 640 million habitable planets Dole wrote of, there may be 180,000 intelligences. We may well be far from alone in the universe.
This view of a universe rich in intelligent life forms, which Dole, Sagan (and I) favor, is not held unanimously by astronomers. Since Venus and Mars have been studied in detail and found to be hostile to life, there is the pessimistic view that the limits within which we may expect life to form and to be maintained for billions of years are very narrow, and that Earth is extraordinarily fortunate to be within those limits. A slight change in this direction or that in any of a number of properties, and life would not have formed or, if formed, would not have remained in existence long. In this view, there may not be more than one or two life-bearing planets per galaxy, and there may only be one or two technological civilizations in the entire universe.
Francis Crick takes the view that there may be a sizable number of planets in each galaxy that are habitable but do not have the much narrower range of properties required for life to originate. It may then be that life originates on one particular planet and, once a civilization arises that can manage interstellar flights, spreads elsewhere. Clearly, Earth has not yet developed interstellar flights, and it may be that some travelers from far off billions of years ago unwittingly (or deliberately) infected Earth with life on a visit here.
Both these views, the optimistic and the pessimistic—a universe full of life and a universe nearly empty of life—are a priori views. Both involve reasoning from certain assumptions, and neither has observational evidence.
Can such evidence be obtained? Is there any way of telling, at a distance, whether life exists somewhere in the vicinity of a distant star? It can be reasoned that any form of life intelligent enough to have developed a high-technological civilization comparable, or superior, to our own will certainly have developed radio astronomy and will certainly be capable of sending out radio signals—or will, as we ourselves do, send them out inadvertently as we go about our radio-filled lives.
American scientists took such a possibility seriously enough to set up an enterprise, under the leadership of Frank Donald Drake, called Project Ozma (deriving its name from one of the Oz books for children) to listen for possible radio signals from other worlds. The idea is to look for some pattern in radio waves coming in from space. If they detect signals in a complexly ordered pattern, as opposed to the random, formless broadcasts from radio stars or excited matter in space, or from the simple periodicity of pulsars it may be assumed that such signals will represent messages from some extraterrestrial intelligence. Of course, even if such messages were received, communication with the distant intelligence would still be a problem. The messages would have been many years on the way, and a reply also would take many years to reach the distant broadcasters, since the nearest potentially habitable planet is 4⅓ light-years away.
The sections of the heavens listened to at one time or another in the course of Project Ozma included the directions in which lie Epsilon Eridani, Tau Ceti, Omicron-2 Eridani, Epsilon Indi, Alpha Centauri, 70 Ophiuchi, and 61 Cygni. After two months of negative results, however, the project was suspended.
Other attempts of this sort were still briefer and less elaborate. Scientists dream of something better, however.
In 1971, a NASA group under Bernard Oliver suggested what has come to be called Project Cyclops. This would be a large array of radio telescopes, each 100 meters (109 yards) in diameter; all arranged in rank and file; all steered in unison by a computerized electronic system. The entire array, working together, would be equivalent to a single radio telescope some 10 kilometers (6.2 miles) across. Such an array would detect radio beams of the kind Earth is inadvertently leaking at a distance of a hundred light-years, and should detect a deliberately aimed radio-wave beacon from another civilization at a distance of a thousand light-years.
To set up such an array might take twenty years and cost 100 billion dollars. (Before exclaiming at the expense, think that the world is spending 500 billion dollars—five times as much—each year on war and preparations for war.)
But why make the attempt? There seems little chance we will succeed, and even if we do, so what? Is there any possible chance we can understand an interstellar message? Yet there are reasons for trying.
First, the mere attempt will advance the art of radio telescopy to the great advantage of humanity in understanding the universe. Second, if we search the sky for messages and find none, we may still find a great deal of interest. But, what if we actually do detect a message and do not understand it? What good will it do us?
Well, there is another argument against intelligent life existing on other planets. It goes as follows: If they exist, and are superior to us, why haven’t they discovered us? Life has existed on Earth for billions of years without being disturbed by outside influences (as far as we can tell), and that is indication enough that there are no outside influences to begin with.
Othe
r arguments can be used to counter this. It may be that the civilizations that exist are so far away that there is no convenient way to reach us; that interstellar travel is never developed by any civilization; and that every one of us is isolated so that we can reach each other only by long-distance messages. It may also be that they have reached us but, realizing that we are a planet that is in the process of developing life and an eventual civilization, are deliberately refraining from interfering with us.
Both are weak arguments. There is another, stronger, and very frightening one. It is possible that intelligence is a self-limiting property. Perhaps as soon as a species develops a sufficiently high technology, it destroys itself-as we, with our mounting stores of nuclear weapons and our penchant for overpopulating and for destroying the environment, seem to be doing. In that case, it is not that there are no civilizations, and nothing more. There may be many civilizations not yet at the point of being able to send or receive messages, and many civilizations that are destroyed, and only one or two that have just reached the point of message sending and are about to destroy themselves but have not yet done so.
In that case, if we receive a message—one message—the one fact it may reveal to us is that somewhere one civilization anyway has reached a high level of technology (beyond ours, in all likelihood) and has not destroyed itself.
And if it has managed to survive, might we not as well?
It is the kind of encouragement that humanity badly needs at this stage in its history and something that I, for one, would gladly welcome.
Chapter 14
* * *
The Microorganisms
Bacteria
Before the seventeenth century, the smallest known living creatures were tiny insects. It was taken for granted, of course, that no smaller organisms existed. Living beings might be made invisible by a supernatural agency (all cultures believed that in one way or another), but no one supposed there to be creatures in nature too small to be seen.
