This page is excerpted from Origin of Life & Evolution in Biology Textbooks - A Critique
by Gordon Mills, Malcolm Lancaster, & Walter Bradley
The American Biology Teacher, Volume 55, No.2, February 1993, 78-83 {entire paper}
{ Walter Bradley is co-author of The Mystery of Life's Origin, a comprehensive book about the origin of life. }

Origin Of Life Hypotheses:
Credible Or Beyond Credibility?

Despite the abundant use of leading questions and tentative terminology in their origin of life discussions, the majority of textbooks exude confidence that confirmation of a naturalistic model of life's origin is inevitable. The treatment in these textbooks stands in marked contrast to a recent review article by Klaus Dose summarizing origin of life research. In this thorough review, a strikingly different picture emerges of the current state of affairs regarding the origin of life. Dose, one of the best known origin of life researchers for the past 20 years, in The Origin of Life: More Questions than Answers (Dose 1988, p. 348) provides the following summary:

More than 30 years of experimentation on the origin of life in the fields of chemical and molecular evolution have led to a better perception of the immensity of the problem of the origin of life on Earth rather than to its solution. At present all discussions on principal theories and experiments in the field either end in stalemate or in a confession of ignorance.

First, we will consider the validity of the atmospheric models used for origin of life experiments, followed by whether data from these experiments are properly evaluated and interpreted.

 
Clinging to Outdated Atmospheric Models

Comments like that quoted above and the objective tone of the entire review article by Dose stand in sharp contrast to the optimism that colors the treatment of life's origin in most of the biology textbooks. The latter generally give the impression that the origin of life problem is nearly solved, since amino acids and other small building blocks have been produced using simulated atmospheres. In regard to composition of the early atmosphere, the following statements illustrate inaccuracies or overstatements in some texts. "The atmosphere had no free oxygen as it does today. Instead, the air was probably made up of water vapor, hydrogen, methane and ammonia" (Biggs et al. 1991, p. 227). "The Earth's first atmosphere most likely contained water vapor (H₂0), carbon monoxide (CO) and carbon dioxide (CO₂), nitrogen (N₂ ), hydrogen sulfide (H₂5) and hydrogen cyanide (HCN)" (Miller & Levine 1991, p. 343). It is unfortunate that only a few of the books acknowledge that it is not likely that the earth ever contained an atmosphere comparable to those used in simulation experiments (Dose 1988, p. 351). The assumption that there was no oxygen in the early atmosphere is of crucial importance to the success of simulation experiments, yet there is no proof that oxygen was absent from that atmosphere.

 
Overstating the Experimental Results

In several of the textbooks, inconsistencies and overstatements regarding the nature of compounds produced in simulation experiments pose a second problem. In some cases false impressions are given because of what students are not told. Most texts fail to note that the compounds produced are markedly dependent upon starting materials and experimental conditions. Some quotes follow: "They found amino acids, sugars and other compounds just as Oparin had predicted" (Biggs et al. 1991, p. 228). "Nucleic acids and ATP also have been formed" (Biggs et al. 1991, p. 228). "Their experiments have produced a variety of compounds, including various amino acids, ATP and the nucleic acids in DNA" (Towle 1991, p. 210). "Similar mechanisms might have led to the formation of carbohydrates, lipids and nucleic acids." (Towle 1991, p. 210). "Thus, over the course of millions of years, at least some of the basic building blocks of life could have been produced in great quantities on early Earth" (Miller &: Levine 1991, p. 344). The texts fail to note that most of the compounds produced in Miller and Urey's original simulation experiment have no relevance to compounds found in living cells; that amino adds produced are always racemic (that is, D-, L-) mixtures; that carbohydrates and amino acids are never produced in the same experiment (they require different starting materials and different conditions); or that no one has produced any ATP or true nucleic acids using reasonable starting materials. As Dose (1988, p. 352) notes:

Substantial amounts of biologically relevant sugars, including D, L-ribose, have never been produced in realistic prebiotic simulation experiments.

They also neglect entirely the fact that compounds in cells have specific intramolecular bonds. Amino acids, carbohydrates, purines and pyrimidines all have many possible isomers, and in most cases only one, or at most very few of these isomers are found in living cells. In simulation experiments mixtures of isomers would usually be produced.

In regard to formation of proteins from amino acids, several quotations follow: "Other scientists have shown that amino acids will link up when heated in the absence of oxygen gas" (Towle 1991, p. 210). Also, ". . . amino acids tend to link together spontaneously to form short chains" (Miller & Levine 1991, p. 344). Neither of these texts notes that linkages occur only when amino acids are heated in the dry state; amino acids do not link together spontaneously in aqueous solution. Nor do these texts note that heating in the dry state produces some linkages that are not found in protein molecules, linkages that would prevent the formation of useful amino add sequences.

Several quotations from the texts relating to membrane enclosures and/or cell formation are alive with expectation: "One process that must have occurred on the earth was the enclosure of nucleic acids in membranes. Once DNA was separated from the environment by some kind of boundary, it would be protected, and might be able to carry out the precise reactions of replication" (Towle 1991, p. 211). "Some of these droplets grow all by themselves, and others even reproduce" (Miller & Levine 1991, p. 344). These statements are pure speculation. Cell membranes usually contain lipids of various types, but they also contain proteins and carbohydrates. More importantly, membranes have very little to do with precise reactions of replication. Students are in no position to know it, but growth and division of coacervate droplets have no similarity to growth and reproduction of living cells.

The effect of the discussions in most of these texts is to make the emergence of life on Earth by chance appear to be highly probable. The following summary statement illustrates this:

If we just said that life did arise from nonlife billions of years ago, why couldn't it happen again? The answer is simple: Today's Earth is a very different planet from the one that existed billions of years ago. On primitive Earth, there were no bacteria to break down organic compounds. Nor was there any oxygen to react with the organic compounds. As a result, organic compounds could accumulate over millions of years, forming that original organic soup. Today. However, such compounds cannot remain intact in the natural world for a long enough period of time to give life another start {Miller &: Levine 1991. p. 346).

It is not mentioned that degradation of organic compounds would occur in an early atmosphere as a result of electrical discharges, heat, ultraviolet light, etc.. opposing any accumulations of relevant organic compounds. Nor is it mentioned that no geological evidence of an organic soup has ever been found. Coal, oil and natural gas are all considered to be produced from ancient trees or organisms. For a critical evaluation of origin of life hypotheses, the reader is referred to two recent books that deal extensively with this topic [Thaxton et al. (1984) and Shapiro (1986)].

In closing this section, it should be noted that not all of the texts are equally careless in their statements regarding life's origin. Although all of the biology texts give the dear impression that the spontaneous origin of life on the early Earth is very plausible, the degree to which erroneous statements are made in support of that view varies widely.

 
Neglect of the Central Problem, Genetic Information

Although most of the texts deal with complex biochemical processes quite well in other chapters, none mention the problem of the origin and transfer of genetic information in dealing with origin of life studies. Moreover, the texts fail entirely to note that even if some complicated molecules were formed by chance, all of the machinery required to exactly reproduce these molecules must also be present in order for cells to survive and reproduce. Indeed, Harold Klein, chairman of a National Academy of Sciences committee which recently reviewed origin of life research, notes that the simplest bacterium is so complicated from the point of view of a chemist that it is almost impossible to imagine how it happened (Horgan 1991, p. 120).

Instead, the textbook's origin of life chapters uniformly disregard recent studies related to the complexity of origin of life requirements. Proteins in cells are made up of 20 different L-amino acids. The texts fail to note that unique linear sequences of these L-amino acids are required in protein molecules in order for those proteins to function. These unique amino acid sequences are required whether the protein is an enzyme, a structural component, or is used for some other function. The unique sequence, in turn, is responsible for the three-dimensional structure of the protein, which is also essential to its function. Even though there may be some variability in amino acid sequence in some positions of a protein molecule, calculations with cytochrome c, a protein 104 amino acids long, indicate that the probability of achieving the linear structure of this one protein by chance is 2 x 10^-65 (Yockey 1977). Consequently, it is not surprising that the means of assembling such unique sequences during the process of protein synthesis in living cells is extremely complex. The genetic information for these unique linear sequences is initially carried in sequences of nucleotides in DNA of a gene in the nucleus of the cell. From there it is transferred to a nucleotide sequence in messenger RNA (a process called transcription) and from the mRNA to the sequence of amino acids in the final product, a protein molecule (a process called translation). The latter process is so complex that even in the simplest organisms, as many as 200 different protein molecules are required. Altogether, the result of these different processes is an amazingly accurate transfer of information from the nucleotide sequence in DNA to the amino acid sequence in the protein.

In addition, the texts fail to note that most of the more complex biochemical reactions of cells require not only a protein enzyme, they also require an additional component (coenzyme, prosthetic group, etc.). Examples of these groups are heme of various heme proteins and also the different vitamin coenzymes. These groups, which are often complex molecules, may be an integral part of the enzyme molecule (covalently bound), or they may freely dissociate from the protein. In the majority of cases, these organic components are absolutely essential to the catalytic function of the protein molecule. As a consequence, postulated scenarios for the origin of life must provide for the simultaneous formation of the essential coenzyme or prosthetic group and assembly of a specific linear amino acid sequence in the enzyme protein. They must, of course, also provide for the formation of many other complex macromolecules (nucleic acids, carbohydrates, lipids, etc.) that are essential to the function and reproduction of the living cell. The failure to address these requirements shows even more fully the implausibility of the origin of life scenarios presented in the texts.

Of the important problems for origin of life models, Dose (1988, p. 355) discusses the source of genetic information last, closing with a summary of few words: "The difficulties that must be overcome are at present beyond our imagination." In regard to the chance hypothesis for the origin of genetic information, Kuppers (1990, p. 60) notes:

The expectation probability for the nucleotide sequence of a bacterium is thus so slight that not even the entire space of the universe would be enough to make the random synthesis of a bacterial genome probable.

Compare these statements with the easy confidence noted in the textbooks that a naturalistic explanation of life's origin is soon to be found. It is this confident tone, coupled with what students are not told, that makes origin of life chapters in the texts fall short of the guidelines "examining alternative scientific evidence and ideas to test, modify, verify or refute scientific theories."
 

 
For more information (from many perspectives) you can read an overview-with-links about The Origin of Life: Abiogenesis by Chemical Evolution?