The Origin of Life
Ó 1999 David J. Tyler
A major review article has recently appeared in Earth-Science Reviews. I start with the published abstract and then give my own overview of the article, sprinkled with quotations that I find particularly interesting.
The Origin of Life
John H. McClendon,
Earth-Science Reviews, 1999, 47, 71-93.
Microfossil finds have been firmly established at about 3.5 Ga (giga annee=109 years), but no rocks older than about 4.0 Ga have been demonstrated, leaving the history of the first 0.6 Ga missing. This gap has been filled by models of the solar system. The origin of the ocean, atmosphere, and much crustal material apparently lies in a heavy rain of comets, subsequent to the catastrophic Moon-forming event. The earliest microfossils are those of the Apex chert in Australia, about 3.5 Ga old. `Prebiotic' simulations of possible biochemistry have made some progress in recent years, but many obstacles remain, and there is no agreement as to the course of development. The `ribose nucleic acid (RNA) World', aboriginal `clay genes', and catalysis on iron-sulfide precipitates are not ruled out. The search for the `last common ancestor' has reached a point between the Bacteria and the Archaea. It is possible that this organism may have been a thermophile, similar to many modern hot spring organisms. But it is likely to have been an autotroph, and a late development after the true origin of life. Even more speculative are suggestions about the origins of metabolic sequences, in particular the origin of the genetic code. Since all modern organisms share this code (and many other things), there had to be a long history of development during the blank period of Earth history.
This major review of the abiogenesis literature gives particular emphasis to the geological context and also to the lengthy transition from the first replicating cell to the last common ancestor.
The geologic evidence for the early earth having a reducing atmosphere is well discussed. "I have found no recent authors who support the strongly reducing atmosphere model, attractive as it is from a chemical synthesis point of view" (p,78).
The earth is thought to have accreted from dust and planetesimals, largely from dry materials. The water is considered to have come in later - from comets. One view is that the ocean was in place by 4.4 Ga. McClendon says "This is a worry, because it would have diluted any organic matter that was there, making further synthetic reactions less probable." (p.76). Whatever the timing, there are weighty issues concerning the volume of the Hadean ocean. This is because models of earth history are only beginning to grow the continents - and the earth's surface would be submerged. "Ponds might have existed on the sides of early volcanoes, but continents did not exist, and the ocean was apparently massive very early. Miller's experiments assumed ... that the atmosphere was full of methane and ammonia, but the discussion above again puts this in grave doubt. Since we know that life did arise, we are obligated to find mechanisms to accumulate enough organic matter to start life" (p.78). One could point out that this obligation is imposed by a commitment to methodological naturalism.
The chemical simulations are reviewed. "For prebiotic amino acids, the situation is this: these are easy to make if you do not worry about chirality.… Meteorites, in particular the Murchison, contain much the same amino acids as Miller obtained in his spark synthesis in an atmosphere of methane, ammonia and water (Table 1). However, these are all close to racemic, while proteins are only made from L-amino acids. In addition, some of the protein amino acids, such as lysine, histidine and arginine, are not found in detectable amounts. On the other hand, many amino acids are found which have no role in modern proteins, such as norvaline and norleucine ..." (p.81). Later, McClendon draws attention to particular problems created by the scarcity of lysine in primordial synthesis experiments - it is essential for life as we know it.
Discussion of the different proposals is provided. As an example, consider the "protein-first" scenario proposed by Fox:
"As for the amino acid polymers, i.e. proteins, Sidney Fox (Fox and Dose 1977) has demonstrated the ease of polymerisation into 'proteinoids' by hot dehydration, but although some catalytic activity was expressed, no regular structure was present. Chyba and McDonald (1995) were particularly critical of the need for dehydrating conditions for this and other syntheses, as unlikely on the Hadean Earth. The only likely location would be on the sides of volcanoes emergent from the ocean, but maybe this would be important.
"In modern proteins, the order of the monomers in the protein determines its function. In the above proteinoids, more or less random order is obtained, which is further degraded by the occurrence of racemic monomers (giving an irregular chain) and the possibility of cross-links beteen the double function amino acids: aspartic and glutamic acids and lysine. The only way out of this dilemma is the use of surface catalysts to determine the structure of the polymer. " (p.82)
Of greatest value is the delineation of what must be present for anything to be called the first replicating cell. (Failure to do this has led some, Fox in particular, to make bold claims about making proto-cells in the laboratory). These points are summarised as McClendon draws things to a conclusion: "Following the assembly of the most primitive cell, i.e., one with a membrane and a genetic/protein synthetic mechanism, Darwinian evolution could begin, gradually developing into the last common ancestor." (p.84).
In the discussion of the development to the last common ancestor, McClendon makes this observation: "One principle must be emphasised. That is, no complex system could develop instantaneously. There had to be a period of sequential development". (p..87). This is, of course, fundamental to Darwinism - leaving this approach vulnerable to the kind of analysis given by Mike Behe.
Some extracts from the conclusions:
"The biological fossil record goes back as far as we can reasonably expect it. Perhaps new discoveries will extend the record into the Hadean, but that seems unlikely. Unfortunately, this history apparently begins with metabolically advanced organisms which emit elemental oxygen. Thus, much of the origin of life is still hidden from us. Perhaps exploration of Mars will give us more clues." (p.88). (I had not before appreciated this aspect of the search for life on Mars. There is really a sense of desperation here - as the Earth is not releasing its secrets!).
"The synthetic organic chemistry of 'prebiotic' compounds has come to an impasse. Although many monomers have been shown to occur in simulations, nucleic acid-type polymers resist our efforts. Proteins of regular structure are likewise missing." (p.89).
Regarding the RNA world: "Nevertheless, the gap between simple monomers and RNA remains" (p.89). (I like to see advocates of naturalism using the word "gap"!). It is an interesting and well-written review. I am sure McClendon can be challenged on some points, but his essay is a fair and thorough review of the abiogenesis literature. This article can be downloaded for free from the web in PDF format. Follow links from: http://www.elsevier.com/locate/earscirev/
By way of a conclusion, I would like to interact a little with some words written by Howard Van Till in "Three views on Creation and Evolution", Moreland, J.P. and Reynolds, J.M. (eds), Zondervan Publishing House, Grand Rapids, 1999.
Van Till is very strong on the view that "Informed scientific judgment is best done by people whose professional training and experience is in the natural sciences" (p.193). When there are deep differences between scientists, it would be best to suspend judgment. But "if it is a matter of a small number of persons contesting a judgment held by the vast majority of scientists, I would consider the majority position far more likely to be correct." (p.193)
On pages 184-185, Van Till has a list of the elements that constitute "creation's formational economy". This summarises his perception of the "fully gifted" creation. Among the set is this entry:".. .. biologically important molecules have the capabilities for self-organization into complex molecular assemblies. (In the judgment of the vast majority of scientists, some of these molecular systems achieved the attributes of living systems in the course of the earth's formational history.)" (p.185)
This point is essential to Van Till's position that creation's formational economy was sufficiently robust to organize and transform itself from elemental forms of matter into the full array of physical structures and life-forms that we are aware of.
The point I wish to draw attention to is this: what has influenced the "vast majority of scientists" to say that abiogenesis occurred? Has the data emerging from science been so compelling that few could resist drawing this conclusion? I would suggest from McClendon's review (which I think is very fair to the researchers involved) that this conclusion DOES NOT emerge from the research. On the contrary, the research has come to "an impasse". It has NEVER provided scientists with any well-grounded evidence that abiogenesis has occurred.
So what are we to make of Van Till's confidence in the judgment of scientists? Why do these people take this view? Could it be that these scientists have imbibed a philosophy which drives their thinking? When McClendon writes: "Since we know that life did arise, we are obligated to find mechanisms to accumulate enough organic matter to start life" - is this obligation driven by the quest for truth or is it driven by a philosophical commitment to "mechanism"?