Rival Theories of the Origin of Life

THE idea of a spontaneous origin of living cells was not much discussed before about 1940, although it was early seen to be a necessity for a thorough going naturalistic view of life. Russian scientists, especially A. I. Oparin, were active in the field in earlier years, ¹ but the balance of power in this particular field of research would now appear to have swung to the West. This article reviews the scientific results of the last twenty years,² asking whether our knowledge of the structure and function of living cells can be consistent with the much-discussed ideas of gradual appearance without the intervention of a Creator.

Basic Requirements of Living Cells

The steps needed in the abiogenic (origin in life less matter) production of the first living cell would be very much easier to list if there was clear information about the contents of the first living cell. Of course there is not. However, it is quite clear that most of the systems common to living cells found on the earth today would need to have been present in any hypothetical cell described as "living." For any cell the following functions are indispensable:

1. Protection. Some kind of cell wall with provision to prevent attack by radiation and atmospheric oxygen (if present) and to admit nutrients and reject waste products and poisonous substances.

2. Enzyme Synthesis. Machinery for constructing polypeptide chains from a pool of individual amino acids.

3. Specifications. Coded instructions to ensure the correct amino-acid sequence in the enzymes needed by the cell as well as a set of "switches" to activate enzyme synthesis only when it is necessary.

4. Energy Production. A system for using enzymes to break down sugars and other nutrients and to convey the energy so released to the parts of the cell where it is needed.

5. Replication. A mechanism for the controlled reproduction of the set of enzyme specifications so that separate sets are available when cell multiplication takes place.

It is difficult to see how any of these functions could be carried out other than by the complex biochemical substances found in living cells today. Consequently, the scientists interested in the problem always list the spontaneous production of the basic components of today's biochemical substances as the first steps to be climbed by nature in the production of living things from inorganic molecules (see Table 1). The basic components necessary for the synthesis of the important biochemical substances in living cells are: ³

1. Amino acids. Enzymes and other protein constituents of cells are .constructed from twenty different amino acids joined end-to-end in a specific sequence in chains usually some hundreds of amino acids long. The enormous variety of enzymes and other proteins found in living cells is obtained with the use of different sequences of the same twenty amino acids.

2. Certain sugar molecules are necessary be cause of their involvement in cell walls and other structures, because they are universally important as nutrients (the metabolism of all cells is based on the breakdown of glucose), and because of the critical position of the sugar ribose in the genetic code substances DNA and RNA.

3. Nucleic acid components (purines and pyrimidines) are vitally important in the structure of the genetic code substances DNA and RNA without which cells are unable to synthesize proteins or reproduce their specifications.

4. Phosphates and polyphosphates used for the cell's internal energy supply.

5. Fats, required in a variety of locations in the cell.

6. Assorted co-enzymes and other biochemical substances, such as porphyrins, carotenes, et cetera, used for important, highly specific tasks in cell metabolism.

Laboratory Experiments on First Steps

There has been much publicity about the success obtained by chemists who have attempted to carry out the first steps in duplicating the presumed atmospheric conditions of a lifeless planet such as the earth is presumed to have been 4.5 billion years ago. The greatest impetus in this direction came when S. L. Miller, a graduate student of the famous scientist H. C. Urey, cycled a mixture of simple gases in a glass tube past a high voltage spark designed to simulate the effects of lightning on atmospheric gases. The gases were ammonia (NHs), methane (ChU), water vapor (HaO), and hydrogen (Ha). (Oxygen was omitted because it was not considered to have been present in the primitive atmosphere.) ⁴

TABLE 1

Steps Necessary for the Spontaneous Appearance of Life on a Sterile Planet

Miller's experiment was successful in showing that amino acids could be produced as the gases were cycled in the spark. The products of the reaction included some of the amino acids found in living things today. The experiment has been repeated many different times under many different conditions and with many different gas mixtures. Its success indicates that the steps leading to the production of the majority of the amino acids found in proteins are feasible in the atmosphere of a lifeless planet, provided that the important gases are present in the proper proportions and provided oxygen is absent.

Very few of the remaining steps of phase one (see Table 1) can be climbed by "primitive-earth atmosphere" experiments, but there is much evidence about the synthesis of purines, pyrimidines, fats, sugars, et cetera, from more complicated molecules (like hydrogen cyanide, formaldehyde, cyano-acetylene, et cetera). Most of the syntheses presuppose highly specific environmental conditions and starting materials and there is much discussion and argument about their appropriateness. Quite a large amount of research, usually very carefully conducted, has been reported in this field, so much, indeed, that the casual observer may be bewitched by it and fail to observe the highly speculative nature of the presuppositions and the magnitude of problems still to be overcome once the first phase has been passed.

TABLE 2

Chemical Substances Found Outside the Earth

Recently, another way of climbing the first steps has been suggested. Much publicity has been given to the discovery that organic compounds of nitro gen, carbon, hydrogen, and oxygen are present in interstellar space. Radio-telescopes are able to detect signals from outer space related to a large range of chemical compounds. Many of them have been fingerprinted and a long list of substances, very interesting in relation to biochemical synthesis, has been compiled. It includes the compounds hydro gen cyanide (HCN), formaldehyde (HCHO), methanol (CHaOH), formic acid (HCOOH), hydrogen cyanate (HCNO), cyano-acetylene (HCC-CN), and other rather active compounds of the elements most important to biochemistry. The search is continuing on an international basis, many people believing that such substances could have accumulated during the formation of planets from stellar and interstellar debris.

In summary, it is possible to imagine that a fair percentage of the basic components of biochemical compounds would have accumulated in reasonable quantities on the primitive earth, given that there were sufficient quantities of the primitive gases used by Miller, and of the kinds of substances discovered in interstellar space. Several very important sub stances have resisted the attempts at their synthesis, but it is not possible to say that their synthesis is not possible; future research may yet discover the secret.

So far as actual evidence is concerned, it is true that plenty of biochemical substances have been extracted from rocks in the earth but these "chemical fossils" comprise only the substances known to be associated with living cells as they now are. There is no evidence of the vast variety of other sub stances other amino acids, other sugars, other isomers, ⁵ other purines and pyrimidines, other hydrocarbons which would certainly have been produced if spontaneous chemical reaction was responsible for the initial buildup of organic chemical substances on the earth. ⁶

Controlled Synthesis

If the first steps are the production of the basic substances listed above, then the next phase is that in which they are combined into functioning bio chemical compounds. There are too many steps in this category for them to be discussed individually, but what has been said above makes it clear that there must be supplies of sugars for nutrients, amino acids linked together in long polypeptide chains capable of functioning as enzymes, combinations of purines and pyrimidines with ribose and phosphate, capable of acting as code symbols to specify amino-acid sequences, combinations of fats and proteins and polysaccharides arranged in layers capable of acting as a protective cell membrane, co-enzyme substances designed to assist enzymes in specific chemical tasks, and so on. (See Table 1.)

The evidence about steps in this phase is very, very limited. The ability of biochemical molecules to function efficiently is dependent on their unique chemical structure, and the probability that they can be formed by random processes, even granting the existence of the correct building blocks, is vanishingly small. Time is no solution of this problem; what is needed is information.

The situation with the synthesis of enzymes illustrates this general statement. To start with, it is not known how amino acids could be combined in solution to form di-, tri-, and poly-peptides, and the method that has been suggested for the linking of dry amino acids into polypeptides by the action of heat poses so many other problems that it can hardly be a solution either. But, granting the existence of a method for the spontaneous production of polypeptides without the enzyme systems found in the living cells, it is still necessary to explain how any precise sequence of amino acids could be obtained.

Ordinarily, enzymes contain hundreds of amino acids in sequence, each link in the chain being occupied by a specific amino acid if the enzyme is ever to perform its unique task. If, as at present, each amino acid at each point in the sequence must be chosen from the unique set of twenty, the probability that a correct set will be found for enzymes with a length of more than one hundred amino acids is so small that the numbers are meaningless to discuss. Even supposing that a polypeptide with as few as twenty amino acids in a precise sequence could function for some specific biochemical task, the chance that this would be produced spontaneously is one in 10 ²⁶ or 100,000,000,000,000,000,000,000,000. ⁷

Numbers like 10²⁶ (or even 10 ¹⁴) are so large that people frequently lose the perspective necessary to discuss them properly. Certainly it is possible, even for probabilities as small as this, for the correct sequence to be formed: but it must also be remembered that there is a concurrent probability, so great that it must be called a certainty, that some 1026 other polypeptides will be formed by the same process that produced the unique one, and some way must be found to eliminate all of these while retaining the one that is to become useful in bio chemical evolution. This is not only true for the enzyme on which we began the discussion it is also true for every other enzyme of the set that must be simultaneously available for the cell to begin the processes of life.

It may be argued strongly that the concessions that speculative scientists frequently make in discussing this problem such as reducing the number of amino acids from twenty to five, or reducing the size of the polypeptide chain expected from hundreds of units in length to a few tens of units in length have not the slightest support from our current knowledge of biochemistry, but even if they are granted, a simple calculation of the probabilities destroys any hope that a few dozen different kinds of polypeptides capable of acting like the crucial enzymes of today's living cells could be produced together for incorporation into the first cell and in the absence of all but the smallest quantities of similar but non-functioning polypeptides with incorrect sequences.

A Double Problem: Enzymes and Enzyme Codes

The problem of the amino acid sequences of the primitive cell's enzymes is two-edged. Not only must the first functioning enzymes be available in an enclosed and protected environment but they must also be capable of being reproduced. Other wise the cell's life rapidly comes to an end. Enzymes become defective both because of attack by other enzymes and because of spontaneous decomposition processes. These would be expected to be much more serious in the unstructured primitive environment than in modern cells with such a plethora of highly developed devices for their protection.

Now the method for the reproduction of enzymes is based on a very complex set of chemical events starting with (a) a chemically coded representation of the protein sequence of the code on a substance known as DNA, continuing with (b) the transcription of the code onto messenger-RNA, continuing further with (c) the alignment of amino acids in correct sequence along the length of coded RNA by twenty different macromolecules, one for each kind of amino acid, and each capable of recognizing the appropriate RNA code symbol; here enzymes designed for the task link them together and (d) a polypeptide is produced.

It is inconceivable that all this complex machinery could have appeared in functional form at the same time as the enzymes it is designed to reproduce, and all enclosed within the same cell wall, also spontaneously produced. But a greater problem remains. The DNA code which specifies the sequence of amino acids in the enzymes to be produced for the cell must have been written down so as to correspond exactly at each point in the sequence of each of the enzymes on which the life of the cell depends! The figure of an explosion in a print shop producing, quite by chance, a finely printed copy of a dictionary falls too short. Two explosions are needed, one producing an English dictionary and another one, in the print shop across the street, a copy of a corresponding dictionary in French.

The machinery for the synthesis of enzymes, the critical step in the speculative pathway from inorganic substances to living cells, occurs after completion of two distinct processes, transcription and translation. The genetic code, held as a sequence of nucleotides in the 'nuclear DNA, is transcribed onto Messenger RNA (m-RNA), which then migrates to ribosomes outside the nucleus. Transcription occurs when the three-letter code words (codons) arranged in sequence along the m-RNA are matched by the appropriate three-letter code words (anti-codons) of the "trans fer RNA" molecules (t-RNA), each bearing the appropriate amino acid. Enzymes then link the amino acids together and the t-RNA molecules depart. Special codons exist to instruct the mechanism where to begin and where to end a particular sequence of amino acids. The exactness of the matching that occurs in translation and transcription depends on the precision of the molecular architecture of DNA, m-RNA, t-RNA, and the associated enzymes and argues for their creation simultaneously rather than by a stepwise process.

All kinds of suggestions have been made to avoid the impasse that this step so obviously creates. The best way is to find some way of having one of these, the genetic code, or the enzymes, precede the other. This would hardly solve the problems involved in either of these separate steps but it is certainly much less improbable than requiring both events simultaneously. Unfortunately, it is very difficult to imagine the appearance of the genetic code before the enzymes because the genetic code and all its associated machinery requires a whole army of enzymes for its construction and maintenance and, especially, to make it function and actually produce enzymes. Enzymes must come first. But they must also keep coming. Their fragility means that the cell could not long survive unless exact copies of them were continually being produced.

Summarizing, it is not merely the technical matter of finding out how nature could spontaneously put the building blocks together to produce DNA and RNA and enzymes. There is the problem of how nature could know to make the sequences of nucleotides in DNA the exact translation, in a different chemical language, of the sequences of amino acids in the enzymes. ⁸

Interdependence of Cell Functions

This discussion by no means exhausts the problems associated with enzyme synthesis. Enzymes are only effective inside cells if the chain of amino acids is correctly coiled and this occurs only if the acid-base balance is correctly maintained. This is the reason for such expenditure of effort inside a cell to keep the correct pH. How such a complex mechanism, nowadays designed into the whole functioning mechanism of the cell, could have appeared spontaneously at the first appearance of enzymes is mysterious. Other problems are the necessity to avoid enzyme poisons and inhibitors, the dependence on co-enzymes, the long lists of enzymes needed for the stepwise breakdown of nutrients, each adapted for a specific task and none useful unless all are present to complete the job, and so on. It is quite unfair to write glibly of enzyme synthesis unless the reader is also given a feeling for the intricacy of design that enables enzymes to operate in living cells. If the merest changes in cell economy today cause the extinction of well-regulated and fully-protected cells, how can their gradual development be contemplated, when every function would be imperfect and every process would violate the conditions under which all the rest would need to operate?

New quandaries are found at each of the steps that lie further up the hillside. Photosynthesis is considered to be far too complicated a process to have appeared within the first living cells. Consequently, some other explanation must be found for the source of their energy. Photosynthetic organisms take radiant energy from the sun and use it to build carbohydrates that can then be transported around and used as an energy supply. But if primitive organisms lacked the ability to trap the energy of sunlight, scientists are forced to assume that these organisms had sugars and other energy-rich sub stances readily available to them in the ocean. Fine, but how did they get there without photosynthesis? Nature today gives no hint that such pleasant accidents occur without the assistance of other living things. Organisms today either manufacture their own nutrients by photosynthesis or depend on other organisms that do.

As it continues, research on living cells emphasizes the importance of all the structures and processes that living cells possess today. It is quite unreasonable to think of primitive cells adding structures and functions one by one. Without proteolytic enzymes, for example, the cell would soon poison itself with its own worn-out proteins. But on the other hand, who dares suggest that proteolytic enzymes were already present in the cell when it produced its first enzymes? (How could they then be first?)

In the past, dilemmas such as this have led to notions such as creative evolution or directed evolution, which let their proponents introduce design and purpose into nature while keeping the Designer outside. Such ideas are mocked by scientists today, but they are hard to avoid if God is to be excluded from the story. The facts are that living things display clear evidence of having been designed as a "going concern," which is quite consistent with the doctrine of instantaneous creation but most difficult to accommodate to a gradual, trial-and-error development.

Certainly there is evidence about how some of the first steps may be mounted by nonbiological means, but many of the remaining steps are too high to be taken in any kind of stride.