Extraterrestrial Civilizations (25 page)

We can’t do that, but there
is
cool matter in outer space that we can indeed detect; matter in the form of thin gas and dust that fills interstellar space.

The interstellar material was first detected about the turn of the century because certain wavelengths of light from distant stars were absorbed by the occasional atoms that drift about in the vastness of space. By the 1930s, it was recognized that the interstellar medium contained a wide variety of atoms, probably some of every type of atom cooked in the interiors of stars and broadcast into space during supernova explosions.

The density of the interstellar matter is so low that it seemed natural to suppose that it consisted almost entirely of single atoms and nothing else. After all, in order for two atoms to combine to form a molecule, they must first collide, and the various atoms are so widely spread apart in interstellar space that random motions will bring about collisions only after excessively long periods.

And yet, in 1937, stars shining through dark clouds of gas and dust were found to have particular wavelengths missing that pointed to absorption by a carbon-hydrogen combination (CH) or a carbon-nitrogen combination (CN). For the first time, interstellar molecules were found to exist.

To be sure, CH and CN are the kind of combination that can be formed and maintained only in very low-density material. Such atom combinations are very active and would combine with other atoms at once, if other atoms were easily available. It is because such other atoms are available in quantity on Earth that CH and CN do not exist naturally as such, on the planet.

No other combinations were noted in the interstellar dustclouds through dark lines in the visible spectrum.

After World War II, however, radio astronomy became increasingly important. Interstellar atoms can emit or absorb radio waves of characteristic lengths—something that requires far less energy
than emission or absorption of visible light, and therefore takes place more readily. The emission or absorption of radio waves can be detected easily, given the radio telescopes required for the purpose, and the compounds responsible can be identified.

In 1951, for instance, the characteristic radio-wave emission by hydrogen atoms was detected, and the presence of interstellar hydrogen was thus observed directly for the first time and not merely deduced.

It was understood that next to hydrogen, helium and oxygen were the most common atoms in the Universe. Helium atoms don’t cling to any other atoms, but oxygen atoms do. Should there not be oxygen-hydrogen combinations (OH) in space? This should emit radio waves in four particular wavelengths, and two of these were detected for the first time in 1963.

Even as late as the beginning of 1968, only three different atom combinations had been detected in outer space: CH, CN, and OH. Each of these were 2-atom combinations that seemed to have arisen from the chance occasional collisions of individual atoms.

No one expected that the far less probable combination of three atoms would build up to detectable level, but in 1968 the characteristic radio-wave emissions of water and ammonia were detected in interstellar clouds. Water has a 3-atom molecule, two of hydrogen and one of oxygen (H
₂
O) and ammonia has a 4-atom molecule, one of nitrogen and three of hydrogen (NH
₃
).

This was utterly astonishing, and 1968 witnessed the birth of what we now call astrochemistry.

In fact, once compounds of more than two atoms were detected, the list grew rapidly longer. In 1969, a 4-atom combination involving the carbon atom was discovered. This was formaldehyde (HCHO). In 1970, the first 5-atom combination was discovered, cyanoacetylene (HCCCN). That same year came the first 6-atom combination, methyl alcohol (CH
₃
OH). In 1971, the first 7-atom combination was discovered, methylacetylene (CH
₃
CCH).

So it went. Over two dozen different kinds of molecules have now been detected in interstellar space. The exact mechanism by which these atom combinations are formed is not as yet clear, but they are there.

And even in outer space, the direction of formation would seem
to be in the direction of life.
^*
In fact, both in meteorites and in interstellar clouds it is interesting that the carbon chains are forming and that there is no sign of complex molecules that do not involve carbon. This is evidence in favor of our assumption that life (as we know it) always involves carbon compounds.

All of this evidence—in the laboratory, in meteorites, in interstellar clouds—makes it look as though the Haldane-Oparin suggestions are correct. Life
did
start spontaneously on the primordial Earth, and all indications would seem to be that it must have started readily, that the reactions in that direction were inevitable.

It follows that life would therefore start, sooner or later, on
any
habitable planet.

But how much sooner, or later, is “sooner or later”? When did life start on the Earth?

Our knowledge of ancient life forms upon the Earth comes almost entirely from our study of fossils—remnants of shells, bones, teeth, wood, scales, even fecal matter—that have withstood at least some of the ravages of time and have done so sufficiently to tell us something about the structure, appearance, even behavior of the organisms of which they were once part.

Fossils can be dated in various ways, and the oldest ones that we can deal with easily are from the Cambrian period (so called because the rocks from that period were first studied in Wales, which in Roman times was called Cambria).

The oldest Cambrian fossils are 600 million years old, and it was tempting to assume that that was when life on Earth began, more or less. However, since we know Earth is 4,600,000,000 years old, that would mean it lay for 4 billion years without life. Why so long? And if
lifelessness continued for that long, why did life suddenly appear? Why is Earth not still lifeless?

Then, too, at the time the fossil record begins in the Cambrian, life is already plentiful, complex, and varied. To be sure, all the life of which we have a record from that period is marine; there is no freshwater life or land life. Then, too, it is all invertebrate. The earliest chordates (the group to which we belong) did not appear for another 100 million years.

Nevertheless, what does exist seems quite advanced. Thousands of species of trilobites are found in the Cambrian period; these are complex arthropods very much like the horseshoe crabs of today. It is impossible to suppose that they sprang out of nothing and split up into many species. Before the Cambrian time, there must stretch long ages of simpler life. In that case, why is there no record of it?

The most likely answer is that the simpler life was not particularly prone to fossilization. It lacked the kind of parts—shells, bones—that survive easily. And yet despite that, traces of earlier life
have
been found.

The American botanist Elso Sterreberg Barghoorn (1915–), who in the 1960s was working with very ancient rocks, came across faint traces of carbon that, as he could demonstrate, were the remains of microscopic life.

The dim evidence of such microscopic life has now been traced back as far as 3,200,000,000 years, and it probably extended back a few hundred million years before that.

We might conclude, then, that recognizable life forms existed by the time the Earth was one billion years old.

This sounds reasonable intuitively. We can well imagine that during the first half-billion years of Earth’s history the planet may have been in a pretty unsettled state. The crust must have been active and volcanic; the ocean and atmosphere in the process of formation as the planet cooled off from the heat of its initial condensation and its components separated. The second half-billion years may well have been devoted to a slow chemical evolution—the formation of more and more complicated compounds under the lash of the Sun’s ultraviolet light. Finally, a billion years after the Earth’s formation, very simple little bits of life exist here and there.

The Sun’s stay upon the main sequence will be some 12 billion
years, and we might consider this average for Sunlike stars. That means that the Earth (and, on the average, habitable planets generally) will last 12 billion years as the abode of life. If, then, life appears on the Earth after one billion years, it does so after only 8 percent of its lifetime has elapsed.

We can assume that (by the principle of mediocrity) habitable planets in general gain life after some 8 percent of their lifetimes as habitable planets has passed.

Suppose we assume that stars have been forming at a steady rate here in the outskirts of the Galaxy, once the first flurry of star formation in the infancy of the Galaxy had passed.

This is not entirely an assumption. There is evidence that stars have been born in recent times, at least. The giant stars of spectral classes O and B must have been formed a billion years ago or less, or they would not still be on the main sequence. And if stars could form in the last billion years, they must have been forming all along and still be forming now. They must at least be doing so in those galactic regions where clouds of dust and gas (the raw material of stars) are plentiful, and those regions are precisely in the outskirts of galaxies, which, we have already decided, are the only places life can exist.

Moreover, we need not depend entirely on reason to tell us that stars are still being formed today. It is possible we are actually witnessing the process. In the 1940s, the Dutch-American astronomer Bart Jan Bok (1906–) drew attention to certain dust clouds that were opaque, compact, isolated, and more or less spherical in shape. He suggested that these clouds (now called Bok globules) are in the process of condensing into stars and planetary systems. The evidence since then tends to show he is right. Sagan estimates that in our Galaxy, ten stars are born each year on the average.

Assuming, then, a steady rate of star formation, we can say that x percent of the habitable planets have not yet expended x percent of their lifetime. In other words, 50 percent of the habitable planets have not yet expended 50 percent of their lifetime; 10 percent of the habitable planets have not yet expended 10 percent of their lifetime; and so on.

This means that 8 percent of the habitable planets have not yet expended the 8 percent of their lifetime that they should need to form life; that is, they are less than a billion years old.

The converse is that 92 percent of the habitable planets
are
old
enough to have had life develop upon them. That gives us our ninth figure:

Though life may have come to exist on Earth early in its history, its advance was very slow for a long time.

For the first 2 billion years during which life existed on Earth, the dominant forms may have been bacteria and blue-green algae. These were small cells, considerably smaller than the cells that make up our bodies and those of the plants and animals familiar to us. Furthermore, the bacterial cells and the blue-green algae did not have distinct nuclei within which the deoxyribonucleic acid (DNA) molecules that controlled the chemistry and reproduction of the cells were confined.

The difference between these two kinds of cells was that the blue-green algae were capable of photosynthesis (the use of the energy of sunlight to convert carbon dioxide and water into tissue components) and the bacteria were not. Bacteria, without photosynthetic ability, were forced to break down already existing organic compounds for energy (or, in some cases, to take advantage of other types of chemical changes for the purpose).

Although the blue-green algae made use of the energy of sunlight to form their tissue components, they thereafter made use of chemical changes similar to those the bacteria used. These chemical changes did not supply much in the way of energy, so that the growth and multiplication of living things—to say nothing of its evolution into various more advanced species—was extremely slow. The reason for this is that the chemical changes that yield considerable energy to living things on our Earth of today all involve the utilization of molecular oxygen, and in the early days of life on Earth there was virtually no oxygen in the atmosphere.

The blue-green algae did produce small quantities of oxygen in the course of their photosynthesis, but the sparse distribution and feeble activity of the tiny cells made those quantities very small indeed.

But even though evolution progresses very slowly, it progresses. About 1,500,000,000 years ago, when Earth had been the abode of life for over 2 billion years, the first cells with nuclei appeared. These were large cells of the type that exist today, with more efficient chemistries, that were capable of conducting photosynthesis at greater rates than before.

This meant that oxygen began entering the atmosphere in perceptible quantities and carbon dioxide began declining. By 700 million years ago, after Earth had been the abode of life for nearly 3 billion years, the atmosphere was some 5 percent oxygen.

By this time, those forms of burgeoning animal life, still made up of single cells, which like bacteria made use of chemical changes rather than sunlight as a source of energy, began to develop means of making use of the free oxygen of the atmosphere. Combining organic compounds with oxygen releases twenty times as much energy for a given mass of such compounds as does the breakdown of organic compounds without the use of oxygen.

With a flood of energy at their disposal, animal life (and plant life, too) was able to move more quickly, live more briskly and efficiently, reproduce more copiously, evolve in more different directions. It could even make use of energy in what would have been a wasteful fashion by earlier standards. It evolved into organisms in which cells clung together and specialized. Multicellular organisms were developed, and rigid tissues had to be formed to support them and serve as anchors for muscles.