Do Present Proposals on Chemical Evolutionary Mechanisms Point Toward the First Life?
Editor’s note: We are delighted to present a series of excerpts from chapters in the recent book, Science and Faith in Dialogue, edited by Frederik van Niekerk and Nico Vorster. You can download a full copy of the book for free by going here.
Abiogenesis is the prebiotic process wherein life, such as a cell, arises from non-living materials such as simple organic compounds. Long before evolution could even begin, the origin of the first life, that first cell, would have arisen from some simpler non-living molecules. On Earth, the essential molecules for life as we know it are carbohydrates (also called sugars or saccharides), nucleic acids, lipids, and proteins (polymers of amino acids). Described in this chapter is the process by which organic synthesis is performed and the considerations that are generally required to synthesize a complex system where many molecular parts come together to operate concertedly. This will be demonstrated in the synthesis of nanomachines.
Then some proposals that others have espoused for the synthesis of carbohydrates and carbohydrate- bearing nucleotide bases will be considered, from a prebiotic milieu. The obstacles to the much more difficult task of having the molecular building blocks assemble into a functional system will be briefly mentioned. The scientifically unknown entities that have been proposed to have seeded life on Earth, such as a design agent or panspermia, are not considered. An opinion showing that the strongest evidence against the proposals of current prebiotic research is the researchers’ own data will be rendered. The current proposals can prevent the discovery of scientific solutions in the field as they seem to be directing researchers down paths of futility despite hyperbolic claims to the contrary.
Paths of Futility
Any account of the origin of the first form of life must include a mechanism for the generation of the chemicals needed for life and then for how life arose from those pre-existing non-living chemicals. Abiogenesis proposals attempt to explain how chemical processes transformed pre-existing non-living chemicals into more complex information-bearing molecules such as DNA, RNA, and proteins. For an account of the origin of life to be realistic, there must be chemical processes that can successfully arrange simple organic compounds into complex biologically relevant macromolecules and living cells. Life requires carbohydrates, nucleic acids, lipids, and proteins. But what is the chemistry behind their origin? What is the origin of metabolism, or of the information-storage and processing systems that depend on these complex biochemical compounds?
Working in synthetic chemistry, building relatively simple nanomachines, has led to being sceptical of proposals for the origin of the requisite chemical building blocks necessary for life. Some biologists seem to think that there are well-understood prebiotic molecular mechanisms for the synthesis of carbohydrates, proteins, lipids, or nucleic acids. They have been grossly misinformed. Others think that, if not yet known, such chemical pathways will soon be identified. To me, these biologists are naively optimistic. What they hope for will not happen anytime soon.
And no wonder: few biologists have ever synthesized a complex molecule ab initio. Experience with organic synthesis leads to suggesting that chemistry acting on its own simply does not do what it would need to do to generate the biologically relevant macromolecules, let alone the complex nanosystems in a living cell. The reasons for this skepticism are further explained in more detail.
Lessons from Synthetic Chemistry
The process of molecular design and synthesis in general, what it takes to successfully build a molecule to perform a particular function, is discussed at the start. The initial design is important. Sometimes molecular designs are computer-assisted, but more often than not, the initial steps are noted on paper. A target must first be drawn or otherwise designated. This is no trivial task. In some cases, chemists have seen the target in a related system; in other cases, they guess the target’s properties on the basis of its molecular weight, its shape, the moieties appended to the main backbone and its functional capacities.
Once a target is selected, retrosynthesis is next, whether on paper or on a computer screen. Placing the target at the top, the chemist draws an inverted tree (or graph), one step down at a time, into multiple branch points, until a level where starting materials are at hand is reached. The inverted tree is then pruned. Certain branches lead to dead-ends. They are lopped off. Further refinement of various routes leads to a set of desired paths; these are the routes that can be attempted in the laboratory.
A Target and a Path
Given a target and a path to get there, the synthetic chemist must now try a number of chemical permutations. Each step may need to be optimized, and each step must be considered with respect to specific reaction site modifications and different reaction rates.
What is desired is often ever so slightly different in structure from what is not. If Product A is a mirror image of Product B, one left-handed and the other right-handed, separation becomes a time-consuming and challenging task, one requiring complementary mirror-image structures. Many molecules in natural biological systems are homochiral, meaning only left-handed or right-handed molecules are used, not both. Their mirror images cannot do their work. In addition, few reactions ever afford a 100 percent yield; few reactions are free of deleterious by-products. Purification is essential. If by-products are left in the reaction, they result in complex mixtures that render further reactions impossible to execute correctly.
After purification, a number of different spectroscopic and spectrometric methods must be used to confirm the resulting molecular structures. In case the wrong molecular intermediate is made, the synthetic chemist quickly learns, and all subsequent steps are compromised.
Finally, intermediate products are often unstable in air, sunlight or room light, or water. Synthetic chemists must work in seconds or minutes to prevent destructive natural processes or chemical reactions from taking over.
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