Part 18 of 22 in Evolution Basics

At the Frontiers of Evolution: Abiogenesis and Christian Apologetics

Weizsäcker’s book The World View of Physics is still keeping me very busy. It has again brought home to me quite clearly how wrong it is to use God as a stop-gap for the incompleteness of our knowledge. If in fact the frontiers of knowledge are being pushed back (and that is bound to be the case), then God is being pushed back with them, and is therefore continually in retreat. We are to find God in what we know, not in what we don’t know; God wants us to realize his presence, not in unsolved problems but in those that are solved.

Dietrich Bonhoeffer, Letters and Papers from Prison

When we began this series, we started with a discussion of what a theory is in the scientific sense: a broad explanatory framework supported by experimental evidence, that makes accurate predictions, that has not (yet) been falsified through experimentation. Since that time we have traced the outlines of evolution as a scientific theory – discussing its origin with Darwin’s travels, discussing its mechanisms, and tracing the history of life on earth, including our own. We have also seen how evolution has withstood new evidence provided through scientific advancements, such as in paleontology and genetics. Though scientific theories always remain provisional, evolution as a scientific theory is so well established that its basic contours (species are related through common ancestry, natural selection is a significant player in speciation, et cetera) are no more likely to be overturned than those of other well-established scientific theories.

All theories, however, have frontiers – where their theory-like structure slowly gives way to a more hypothesis-like one. Demarcating the line between theory and hypothesis is also an attempt to draw a line on a gradient, but nonetheless it is a feature of all theories to expand into areas where less is known. As theories expand, some of what was once at the hypothetical frontier moves towards the well-established core – namely, those hypotheses that were accurate, or at least accurate enough to be refined through experimentation. In this series, we have seen historical examples of this process occurring. For example, in Darwin’s time, the idea that humans are related to other forms of life through common ancestry was much less well established than it is today, and was very much towards the “hypothesis” side of the equation. Human common ancestry was clearly predicted by Darwin’s work with other species, and supported by the lines of available evidence (such as the anatomy and physiology of humans compared to living great apes). Despite these evidences, however, in 1859 the science of human evolution had a long way to go – and it would be decades in coming, as we have seen. Eventually, the idea that humans are a lineage nested within the great apes would become entirely uncontroversial for scientists, given the accumulated evidence. This idea, then, moved from what was once the frontier of evolutionary theory to the core – a natural progression for an accurate hypothesis.

So, it is not a surprise that a scientific theory will address areas that are poorly understood: indeed, it is expected that theories, as they expand, will naturally have a frontier where the science is far from settled. Accordingly, we expect evolutionary theory to have its areas that are being actively researched and thus are more hypothetical than theoretical (in the scientific sense of those terms). For evolution, there are many such areas of active inquiry, where no single hypothesis has yet outcompeted its rivals – and no survey of evolutionary theory would be complete without at least a sketch of some of these areas.

#Frontier Science, Abiogenesis and Christian Apologetics

One challenge that faces us when examining frontier areas of evolution is that many Christians have had exposure to such topics exclusively in the context of antievolutionary apologetics. In such cases, it is common for the arguments to have the following basic structure: discuss a genuine scientific controversy from a frontier area of evolution, and then inappropriately use it in an attempt to cast doubt on evolution as a whole. This approach, though sadly common, misses the mark for two reasons: it fails to appreciate that a field of science is expected to have areas that are well supported as well as areas that are more speculative; and that in speculative areas, the presence of competing hypotheses does not imply that the more theoretical base that allows the hypotheses to be made in the first place is somehow suspect.

Nowhere in Christian antievolutionary apologetics is this approach more prominent than for the first frontier area of evolution that we will examine: abiogenesis, or the proposed transition between nonliving matter and the first life on earth. Strictly speaking, abiogenesis is not part of evolutionary theory, in that evolution is the theory of how life changes over time, not how life may have arisen from non-life. As we will see, however, there is good evidence that this distinction is yet another attempt to draw a line on what is in fact a gradient between “non-living” and “living”. Regardless of these careful distinctions that a scientist might make, however, in the popular Christian antievolutionary literature the mystery of abiogenesis is reason enough to doubt evolution as a whole. Hopefully, the scientific problem with this approach is by now obvious – unsolved problems at the frontier are expected, and the natural result of a productive theory. Of course, there is also an apologetics problem with this approach: should a hypothesis at the frontier find experimental support, it will shift towards the theoretical core over time. If an apologetics argument is based on the expectation that such a hypothesis is false, then that argument will lose even what meager force it may have once had, to the detriment of the apologetic it was designed to support. Bonhoeffer famously rejected this approach, and we would do well to follow suit.

#Abiogenesis and the Question of Naturalism

Previously, we left behind the comfortable confines of evolution as a theory (in the scientific sense) and headed out into one of its “frontier” areas – abiogenesis, the hypothesized transition from chemical “non-life” to life. For an American audience, “frontier” immediately calls to mind images of the Wild West – where law and order did not (yet) hold sway, outlaws and renegades were behind every barrel, and justice was dealt from the sheriff’s six-shooter (well, at least that’s how I remember the Hollywood version I saw on television as a child). In short, the frontier was a rough-and-tumble land of conflict and courage a world away from the law and order of the city – and their respective inhabitants had personalities to match.

For abiogenesis, the frontier analogy is surprisingly appropriate. When I read the scientific literature on the topic, I am instantly struck with how, well, speculative it is. This is not the scientific equivalent of quaint New England streets – this is a rough map with large swaths of uncharted territory with perhaps a few wagon tracks through it, at best. Multiple, competing hypotheses abound – was the first life a self-replicating RNA enzyme? Did life start as a metabolic pathway that only later added a heritable molecule of some sort? Did life start near deep sea vents? Did life (or some precursors to life) start somewhere else (as in not even on this planet)? All of these ideas have at least some traction in the scientific literature, and not one of them holds a majority position. It’s the scientific equivalent of the Wild West, complete with interesting personalities “dueling” each other in the scientific literature. The truth of the matter is that we just don’t know.

Let me say that again: when it comes to abiogenesis, we have almost no clue how it might have occurred.

Of course it is at this juncture, as we discussed in the last post, that a great many Christians seize on this professed ignorance to proclaim the failure of evolution as a whole. I too once did so as well, when I held antievolutionary views, so I know full well this temptation. I now realize that this makes about as much sense as claiming the map of New England to be a hopeless mess because Oregon has not yet been fully charted. Unknowns are expected at the frontier – that’s why you go there.

But… God?

But Dennis, you might say, aren’t you assuming that life had a “natural” origin? Aren’t you discounting the possibility that God might have brought life about through supernatural means? Haven’t you capitulated to “naturalism” from the outset?

There is of course much to discuss here, and the topic goes well beyond the scope of this article (or even this series as a whole). Still, the question is a valid one that merits at least a brief reply. Since abiogenesis is a frontier area of science, in principle it could have either what Christians would call a “supernatural” or a “natural” explanation – we simply do not know. Of course, any explanation in science might actually have a “supernatural” explanation, but merely appear to us as the regular outworking of “natural” law. Most Christians avoid such arguments, of course, if the science is well settled – not many of us hold out for a supernatural explanation of weather patterns or human reproduction, for example – choosing rather to see these “natural” events as part of the providence of God. No, it is typically only where less is known scientifically that Christians tend to favor supernatural explanations.

Personally, I am reluctant to ascribe to miracle what is not yet well studied scientifically, since more work may reveal a “natural” explanation – “natural”, of course, meaning part of the reproducible structure of the cosmos that God has put in place and continues to uphold that allows us to investigate it using science. In this sense, “natural laws” can be seen as part of God’s covenant faithfulness to His creation, something that my colleague Arnold Sikkema has written on (PDF) and I have found helpful. So, whether the origin of life was “natural” or “supernatural” it was of God, and there is nothing to be lost by attempting to investigate it through the scientific method. Perhaps another way to put it is that, as a scientist, I am curious just how far this regular, reproducible structure of the cosmos extends – does it extend all the way to a transition between non-life and life? Did God, in His wisdom, fashion the cosmos in such a way that chemicals could become alive? Just how deep does the rabbit hole go? While I suspect that life had a “natural” origin in the sense above, I recognize that not all Christians feel the same way. The reason for this hunch – and it is a hunch – is that I see pointers in what we doknow that suggest it to be the case.

Hints & whispers

One such “pointer” to a possible chemical past for life on earth is a feature of all living things at the very heart of what it means to be alive in a molecular sense. One of the first things one learns as a biologist is that macromolecules have divided the labor between heredity and enzymatic function: DNA is for genes, and proteins are for catalyzing reactions. Then, one learns about the various forms of RNA – a class of molecules that, interestingly, in some cases have both hereditary and enzymatic function simultaneously. Then one learns that the key enzyme at the center of the cellular machinery is in fact not a protein enzyme, as one would expect, but rather an RNA molecule – the ribosome. Ribosomes are responsible for using RNA templates to direct protein synthesis, and proteins go on to complete the loop by copying the cell’s DNA, which encodes the information for making RNA. In a significant sense, it’s all about RNA: RNA enzymes using RNA templates to make proteins that copy the cell’s DNA (which contains the crucial RNA information in a more stable form). When the chemical structure of the ribosome was determined in the early 2000s it was demonstrated that the few proteins associated with it are not part of its enzymatic function – which was a significant surprise for many molecular biologists at the time. To many of them, this absolutely crucial RNA enzyme using RNA templates at the center of cellular life was suggestive – suggestive that life once passed through a stage where RNA was the major player in heredity and enzymatic function, rather than the DNA/protein world of the present.

Next, we’ll explore this proposed “RNA world” – a hypothesis that has gained some experimental support in recent years.

#The RNA World Hypothesis

Previously, we introduced the “RNA world” hypothesis – the idea that life was RNA based prior to DNA-based biochemistry. As we noted, this hypothesis received a great boost in the early 2000s, when it was determined that the ribosome – the enzyme responsible for making proteins from information coded in DNA and RNA – was in fact a ribozyme: an enzyme constructed out of RNA.

We can better appreciate ribozymes for the marvels that they are with a brief review of some basic cell biology we covered earlier in the series (and those requiring a more thorough refresher can use the links provided). You might recall that DNA is a polymer formed from monomers, and that the information in DNA is transferred to proteins, which have a three-dimensional shape that can perform structural and enzymatic functions. In this way, DNA functions as a hereditary molecule and proteins do the day-to-day work in the cell. “Messenger RNA”, as we discussed, is the intermediate between DNA and protein. As a “working copy” of a gene, messenger RNA is used as a template for directing the order of monomers in the resulting protein. What we did not discuss, however, is the key role that a different type of RNA plays in this process – the RNA that makes up the enzymatic core of the ribosome, the enzyme that connects protein monomers together as directed by the messenger RNA.

These RNA molecules, called “rRNA” for “ribosomal RNA” are strings of monomers similar to DNA monomers. Yet these molecules also have an enzymatic function that depends on their three-dimensional shape – their sequences direct them to fold up into a structure that can perform an enzymatic function. In this way, they are similar to proteins, which also fold up into functional shapes to do their jobs. Despite this shape, they remain a polymer that in principle can be used as a template for replicating themselves, much like DNA. While it is not possible to show a complete structure of rRNA here (since it is such a large molecule), a smaller ribozyme can be used to illustrate its features: a string of nucleotide monomers that folds up to form an active enzyme based on its three-dimensional shape:

Image credit: William G. Scott. [CC BY-SA 3.0]

These RNA molecules, called “rRNA” for “ribosomal RNA” are strings of monomers similar to DNA monomers. Yet these molecules also have an enzymatic function that depends on their three-dimensional shape – their sequences direct them to fold up into a structure that can perform an enzymatic function. In this way, they are similar to proteins, which also fold up into functional shapes to do their jobs. Despite this shape, they remain a polymer that in principle can be used as a template for replicating themselves, much like DNA. While it is not possible to show a complete structure of rRNA here (since it is such a large molecule), a smaller ribozyme can be used to illustrate its features: a string of nucleotide monomers that folds up to form an active enzyme based on its three-dimensional shape:

Given that RNA can have both DNA-like and protein-like attributes, it’s not surprising that researchers have proposed that RNA in fact precedes both. From the beginning of this hypothesis, one of the key goals of researchers investigating the RNA world has been to identify a self-replicating RNA ribozyme. Such a molecule would have the essential ingredients for evolution: a genome subject to mutation, thereby producing genetically different “offspring” that would be subject to natural selection.

Challenges and difficulties

One of the main problems with the RNA world hypothesis, among many (after all, this is a frontier area), is that as far as we can tell, a self-replicating RNA ribozyme needs to be quite complex. To date, scientists have not succeeded in identifying an RNA sequence that is capable of being a general RNA replicator, and RNA molecules that do have at least some ability to replicate RNA tend to be quite long (i.e. comprised of many building blocks). The probability that such a molecule would arise spontaneously from a pre-biotic mixture of chemicals is vanishingly slim, even if one grants an environment where the required chemicals are common (a problem in itself that we will discuss further below).

One conjecture that addresses this issue is the idea that the original self-replicating RNA ribozyme was not a single molecule, but rather a collection of molecules – a sort of molecular ecosystem where a number of smaller RNA molecules contribute to the replication of the entire set. Such a system might be easier to hit upon by chance (since each individual molecule is less complex), or, conversely, such a system might be easier to develop from some as-yet-unknown precursor system.

While seemingly farfetched, this idea recently received some empirical support. In this paper, a research group reports their findings that self-sustaining catalytic networks of small RNA molecules can spontaneously arise from mixed populations of RNA precursors, and that such networks can evolve increased complexity over time. While far from solving all of the problems with the RNA world hypothesis, these results indicate that the first RNA-dependent RNA replicating enzyme was in fact a population of small, simpler RNA molecules rather than one large and complex one.

Further challenges and difficulties

Despite these recent advances, one of the longstanding challenges to the RNA world hypothesis remains: the difficulty of the required precursors arising directly through pre-biotic chemistry. RNA ribozymes, while simpler than cellular life, are themselves quite complex, and formed from relatively complex precursors. While the evidence we have is suggestive that life went through an RNA world stage, recent work has not demonstrated an easy path by which such a world could form directly from nonliving components. Accordingly, some researchers have begun to search for other “worlds” – simpler ones that predate the RNA world, and might have served as an intermediate stepping-stone on the gradient between non-living chemistry and RNA-based life. While this work is by its nature quite speculative (it is, after all, the frontier of a frontier) recent work has supported the hypothesis that RNA could have been preceded by simpler chemistry more amenable to spontaneous assembly in a pre-biotic chemical mixture. In the coming years it will be interesting to see if any of these hypotheses gain additional support. After all, for a research scientist, the frontiers are the exciting areas – and few areas in evolution are more at the frontier than work on abiogenesis.

So, will science ever solve the problem of abiogenesis? Perhaps not – though when I reflect on the fact that we are only 400 years removed from the time of Galileo, I am reminded that many seemingly unsolvable scientific problems have indeed been solved. And along with Bonhoeffer, I delight in these scientific advances that give us an ever-larger picture of God and his faithfulness to his creation.

In the next post in this series, we’ll move on to another frontier area of evolutionary biology – the ongoing debate between those who view evolution as a primarily convergent process, and those who see evolution as primarily driven by chance events (i.e. as a contingent process).

Notes & References

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