by Isaac Asimov
The proponents of spontaneous generation were not convinced. They maintained that boiling destroyed some vital principle and that, as a result, no microscopic life developed in Spallanzani’s boiled, sealed flasks. It remained for Pasteur to settle the question, in 1862, seemingly once and for all. He devised a flask with a long swan neck in the shape of a horizontal S (figure 13.7). With the opening unstoppered, air could percolate into the flask, but dust particles and microorganisms could not, for the curved neck would serve as a trap, like the drain trap under a sink. Pasteur put some broth in the flask, attached the S-shaped neck, boiled the broth until it steamed (to kill any microorganisms in the neck as well as in the broth), and waited for developments. The broth remained sterile. There was no vital principle in air. Pasteur’s demonstration apparently laid the theory of spontaneous generation to rest permanently.
Figure 13.7. Pasteur’s flask for the experiment on spontaneous generation.
All this left a germ of embarrassment for scientists. How had life arisen, after all, if not through divine creation or through spontaneous generation?
Toward the end of the nineteenth century some theorists went to the other extreme and made life eternal. The most popular theory was advanced by Svante Arrhenius (the chemist who had developed the concept of ionization). In 1907, he published a book entitled Worlds in the Making, picturing a universe in which life had always existed and migrated across space, continually colonizing new planets. Life traveled in the form of spores that escaped from the atmosphere of a planet by random movement and then were driven through space by the pressure of light from the sun.
Such light pressure is by no means to be sneered at as a possible driving force. The existence of radiation pressure had been predicted in the first place by Maxwell, on theoretical grounds and, in 1899, had been demonstrated experimentally by the Russian physicist Peter Nicolaevich Lebedev.
Arrhenius’s views held, then, that spores traveled on and on through interstellar space, driven by light radiation this way and that, until they died or fell on some planet, where they would spring into active life and compete with life forms already present, or inoculate the planet with life if it was uninhabited but habitable.
At first blush, this theory looks attractive. Bacterial spores, protected by a thick coat, are very resistant to cold and dehydration and might conceivably last a long time in the vacuum of space. Also, they are of just the proper size to be more affected by the outward pressure of a sun’s radiation than by the inward pull of its gravity. But Arrhenius’s suggestion fel~before the onslaught of ultraviolet light. In 1910, experimenters showed that ultraviolet light quickly kills bacterial spores; and in interplanetary space, the sun’s ultraviolet light is intense-not to speak of other destructive radiations, such as cosmic rays, solar X rays, and zones of charged particles like the Van Allen belts around the earth. Conceivably, there may be spores somewhere that are resistant to radiation, but spores made of protein and nucleic acid, as we know them, could not make the grade. To be sure, some particularly resistant microorganisms were exposed to the radiation of outer space on board the Gemini 9 capsule in 1966 and survived six hours of harsh unfiltered sunlight. But we are talking of exposures not of hours, but of months and years.
Besides, if we suppose Earth to bear life only because it was seeded by bits of life that originated elsewhere, we would have to wonder how it originated elsewhere. Thus, such seeding is not a solution to the problem but only shifts the problem elsewhere.
CHEMICAL EVOLUTION
Although some scientists, even today, find the possibility of seeding attractive, the large majority feel it appropriate to work out reasonable mechanisms for the origin of life right here on Earth.
They are back to spontaneous generation, but with a difference. The pre-Pasteur view of spontaneous generation was of something taking place now and quickly. The modern view is that it took place long ago and very slowly.
It could not take place now, for anything that even approached the complexity required of the simplest conceivable form of life would promptly be incorporated, as food, into one of the innumerable bits of life that already exist. Spontaneous generation, therefore, had to take place only on a planet on which life did not already exist. On Earth, that would be over three and a half billion years ago.
Then, too, life could not take place in an atmosphere rich in oxygen. Oxygen is an active element that would unite with the chemicals that were building up into near-life, and break them down again. However, as I said in chapter 5, scientists believe that Earth’s primordial atmosphere was a reducing one and did not contain free oxygen. In fact, one possibility is that Earth’s original atmosphere was composed of hydrogen-containing gases such as methane (CH4), ammonia (NH3) and water vapor (H2O), with perhaps some hydrogen (H2) as well.
Such a highly hydrogenated atmosphere we might call Atmosphere I. Through photodissociation, this would slowly turn into an atmosphere of carbon dioxide and nitrogen (see chapter 5), or Atmosphere II. After that an ozone layer would form in the upper atmosphere, and spontaneous change would halt. Can life then have formed in one or the other of the early atmospheres?
H. C. Urey felt life started in Atmosphere I. In 1952, Stanley Lloyd Miller, then a graduate student in Urey’s laboratories, circulated water, plus ammonia, methane and hydrogen, past an electric discharge (to simulate the ultraviolet radiation of the sun). At the end of a week, he analyzed his solution by paper chromatography and found that, in addition to the simple substances without nitrogen atoms, he also had glycine and alanine, the two simplest of the amino acids, plus some indication of one or two more complicated ones.
Miller’s experiment was significant in several ways. In the first place, these compounds had formed quickly and in surprisingly large quantities. One-sixth of the methane with which he had started had gone into the formation of more complex organic compounds; yet the experiment had only been in operation for a week.
Then, too, the kind of organic molecules formed in Miller’s experiments were just those present in living tissue. The path taken by the simple molecules, as they grew more complex, seemed pointed directly toward life. This pointing-toward-life continued consistently in later, more elaborate experiments. At no time were molecules formed in significant quantity that seemed to point in an unfamiliar nonlife direction.
Thus, Philip Abelson followed Miller’s work by trying a variety of similar experiments with starting materials made up of different gases in different combinations. It turned out that as long as he began with molecules that included atoms of carbon, hydrogen, oxygen, and nitrogen, amino acids of the kind normally found in proteins were formed. Nor were electric discharges the only source of energy that would work. In 1959, two German scientists, Wilhelm Groth and H. von Weyssenhoff, designed an experiment in which ultraviolet light could be used instead, and they also got amino acids.
If there was any doubt that the direction-toward-life was the line of least resistance, there was the fact that, in the late 1960s, more and more complicated molecules, representing the first stages of that direction, were found in gas clouds of outer space (see chapter 2). It may be, then, that at the time the earth was formed out of clouds of dust and gas, the first stages of building up complex molecules had already taken place.
The earth, at its first formation, may have had a supply of amino acids. Evidence in favor of this theory came in 1970. The Ceylon-born biochemist Cyril Ponnamperuma studied a meteorite that had fallen in Australia on 28 September 1969. Careful analyses showed the presence of small traces of five amino acids: glycine, alanine, glutamic acid, valine, and proline. There was no optical activity in these amino acids, so they were formed not by life processes (hence their presence was not the result of earthly contamination) but by the nonliving chemical processes of the type that took place in Miller’s flask.
In fact, Fred Hoyle and an Indian colleague, Chandra Wickramasinghe, are so impressed by this finding that they feel that the syntheses may go
far beyond what has been detected. Very small quantities of microscopic bits of life may be formed, they feel-not enough to be detected at astronomical distances, but large in an absolute sense; and these may be formed not only in distant gas clouds but in comets of our own solar system. Life on Earth may therefore have originated when spores were carried to Earth by comet tails. (It is only fair to say that almost no one takes this speculation seriously.)
Could chemists in the laboratory progress beyond the amino acid stage? One way of doing so would be to start with larger samples of raw materials and subject them to energy for longer periods. This process would produce increasing numbers of ever more complicated products, but the mixtures of these products would become increasingly complex and would be increasingly difficult to analyze.
Another possibility would be for chemists to begin at a later stage. The products formed in earlier experiments would be used as new raw materials. Thus, one of Miller’s products was hydrogen cyanide. The Spanish-American biochemist Juan Oro added hydrogen cyanide to the starting mixture in 1961. He obtained a richer mixture of amino acids and even a few short peptides. He also formed purines-in particular, adenine, a vital component of nucleic acids. In 1962, Oro used formaldehyde as one of his raw materials and produced ribose and deoxyribose, also components of nucleic acids.
In 1963, Ponnamperuma also performed experiments similar to those of Miller, using electron beams as a source of energy, and found that adenine was formed. Together with Ruth Mariner and Carl Sagan, he went on to add adenine to a ribose solution; and under ultraviolet light, adenosine, a molecule formed of adenine and ribose linked together, was formed. If phosphate was also present, it, too, was hooked on to form the adenine nucleotide. Indeed, three phosphate groups could be added to form adenosine triphosphate (ATP), which, as was explained in chapter 12, is essential to the energy-handling mechanisms of living tissue. In 1965, he formed a dinucleotide, two nucleotides bound together. Additional products can be built up if substances such as cyanamide (CNNH2) and ethane (CH3CH3)—substances which may well have been present in the primordial era—are added to the mixtures employed by various experimenters in this field. There is no question, then, but that normal chemical and physical changes in the primordial ocean and atmosphere could have acted in such a way as to build up proteins and nucleic acids.
Any compound that formed in the lifeless ocean would tend to endure and accumulate. There were no organisms, either large or small, to consume them or cause them to decay, Moreover, in the primeval atmosphere there was no free oxygen to oxidize and break down the molecules. The only important factors tending to break down complex molecules would have been the very ultraviolet and radioactive energies that built them up. But ocean currents might have carried much of the material to a safe haven at mid-levels in the sea, away from the ultraviolet-irradiated surface and the radioactive bottom. Indeed, Ponnamperuma and his co-workers have estimated that fully I percent of the primordial ocean may have been made up of these built-up organic compounds, If so, this would represent a mass of over a million billion tons. This is certainly an ample quantity for natural forces to play with; and in such a huge mass, even substances of most unlikely complexity are bound to be built up in not too long a period (particularly considering a billion years are available for the purpose).
There is no logical barrier, then, to supposing that out of the simple compounds in the primordial ocean and atmosphere there appeared, with time, ever higher concentrations of the more complicated amino acids, as well as simple sugars; that amino acids combined to form peptides; that purines, pyrimidines, sugar, and phosphate combined to form nucleotides; and that, gradually over the ages, proteins and nucleic acids were created. Then, eventually, must have come the key step—the formation, through chance combinations, of a nucleic acid molecule capable of inducing replication. That moment marked the beginning of life,
Thus a period of chemical evolution preceded the evolution of life itself.
A single living molecule, it seems, might well have been sufficient to get life under way and give rise to the whole world of widely varying living things, as a single fertilized cell can give rise to an enormously complex organism. In the organic “soup” that constituted the ocean at that time, the first living molecule could have replicated billions and billions of molecules like itself in short order. Occasional mutations would create slightly changed forms of the molecule, and those that were in some way more efficient than the others would multiply at the expense of their neighbors and replace the old forms. If one group was more efficient in warm water and another group in cold water, two varieties would arise, each restricted to the environment it fitted best. In this fashion, the course of organic evolution would be set in motion.
Even if several living molecules came into existence independently at the beginning, it is very likely that the most efficient one would have outbred the others, so that all life today may very well be descended from a single original molecule. In spite of the great present diversity of living things, all have the same basic ground plan. Their cells all carry out metabolism in pretty much the same way. Furthermore, it seems particularly significant that the proteins of all living things are composed of L-amino acids rather than amino acids of the D type. It may be that the original nucleoprotein from which all life is descended happened to be built from L-amino acids by chance; and since D could not be associated with L in any stable chain, what began as chance persisted by replication into grand universality. (This is not to imply that D-amino acids are totally absent in nature. They occur in the cell walls of some bacteria and in some antibiotic compounds. These, however, are exceptional cases.)
THE FIRST CELLS
Of course, the step from a living molecule to the kind of life we know today is still an enormous one. Except for the viruses, all life is organized into cells; and a cell, however small it may seem by human standards, is enormously complex in its chemical structure and interrelationships. How did that start?
The question of the origin of cells was illuminated by the researches of the American biochemist Sidney Walter Fox. It seemed to him that the early earth must have been quite hot, and that the energy of heat alone could be sufficient to form complex compounds out of simple ones. In 1958, to test this theory, Fox heated a mixture of amino acids and found they formed long chains that resembled those in protein molecules. These proteinoids were digested by enzymes that digested ordinary proteins, and could be used as food by bacteria.
Most startling of all, when Fox dissolved the proteinoids in hot water and let the solution cool, he found they would cling together in little microspheres about the size of small bacteria. These microspheres were not alive by the usual standards but behaved as cells do, in some respects at least (they are surrounded by a kind of membrane, for instance). By adding certain chemicals to the solution, Fox could make the microspheres swell or shrink, much as ordinary cells do. They can produce buds, which sometimes seem to grow larger and then break off. Microspheres can separate, divide in two, or cling together in chains.
Perhaps in primordial times, such tiny not-quite-living aggregates of materials formed in several varieties. Some were particularly rich in DNA and were very good at replicating, though only moderately successful at storing energy. Other aggregates could handle energy well but replicated only limpingly. Eventually. collections of such aggregates might have cooperated, each supplying the deficiencies of the other, to form the modern cell, which was much more efficient than any of its parts alone. The modern cell still has the nucleus—rich in DNA but unable of itself to handle oxygen—and numerous mitochondria—which handle oxygen with remarkable efficiency but cannot reproduce in the absence of nuclei. (That mitochondria may once have been independent entities is indicated by the fact that they still possess small quantities of DNA.)
To be sure, in the last few years, there is an increasing tendency to suspect that Atmosphere I did not last very long, and that Atmosphere II was present almost at the b
eginning. Both Venus and Mars have Atmosphere II (carbon dioxide and nitrogen), for instance; and Earth may have had one, too, at a time when, like Venus and Mars, it bore no life.
This is not a fatal change. Simple compounds can still be built up from carbon dioxide, water vapor, and nitrogen. The nitrogen could be converted to nitrogen oxides or cyanide or ammonia by combination with carbon dioxide or water, or both, under the influence of lightning discharges perhaps; and molecular changes would then continue upward toward life under the lash of ( sunlight and other energy sources.
ANIMAL CELLS
Throughout the existence of Atmospheres I and II, primitive life forms could only exist at the cost of breaking down complex chemical substances into simpler ones and storing the energy evolved. The complex substances were rebuilt by the action of the ultraviolet radiation of the sun. Once Atmosphere II was completely formed and the ozone layer was in place, the danger of starvation set in, for the ultraviolet supply was cut off.
By then, though, some mitochondrialike aggregate was formed which contained chlorophyll—the ancestor of the modern chloroplast. In 1966, the Canadian biochemists C. W. Hodson and B. L. Baker began with pyrrole and paraformaldehyde (both of which can be formed from still simpler substances in Miller-type experiments) and demonstrated the formation of porphyrin rings, the basic structure of chlorophyll. after merely three hours gentle heating.
Even the inefficient use of visible light by the first primitive chlorophyll-containing aggregates must have been much preferable to the slow starvation of nonchlorophyll systems at the time when the ozone layer was forming. Visible light could easily penetrate the ozone layer, and the lower energy of visible light (compared with ultraviolet) was enough to activate the chlorophyll system.