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What Were Conditions Really Like on Early Earth?

By Guest Writer - August 23, 2018
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By: Nicholas Leadbeater

As a chemist, I am fascinated by the complexity of the molecules that make up life. Life can do with ease what it takes chemists in a lab a lifetime to accomplish. And what they do achieve still does not even come close to the efficiency and speed of life’s designs. Proponents of evolution claim that the chemicals of life came from simpler precursors, which themselves came from basic molecules made up of just a few atoms. How we got from there to here—from simple molecules to systems of such amazing complexity that we see today—has been the subject of a lot of discussion and debate.

If I want to probe a reaction, I go into the laboratory and perform some experiments. However, it surprises me to see that when it comes to the discipline of prebiotic chemistry, people quite often take assumptions as fact and don’t thoroughly test them. Researchers conduct prebiotic chemistry reactions under conditions that are designed to give the greatest chance of success. For example, rather than assess whether a reaction would take place under the conditions expected on early Earth, they instead carefully control things like the temperature and pH, as well as the concentrations of the reagents.

That is why I was so pleased to see a research article from a collaborative team from Japan and the US that reported a series of experiments to probe some reactions based around compounds containing the element sulfur under conditions that would be expected on early Earth.1 And what happened? They found that a class of compounds called thioesters, which are at the heart of many origin-of-life theories, are in fact unlikely primordial contributors.

I wanted to share a bit more about the work they performed and how, in my mind, it provides yet another example of evidence for a creator.


Testing the Conditions on Early Earth

Many of the reactions that form biological matter require enzymes that would not have existed in prebiotic times. However, the high temperatures close to volcanoes and in hydrothermal vents can plausibly overcome reaction barriers without the need for enzymes, making them widely considered likely sites for the origin of life. These reactions involve compounds like carbon monoxide and carbon dioxide but also include thioesters and thioacids, classes of compounds that contain the element sulfur. Thioesters and thioacids play an important role in origin-of-life research because they are central to many of the proposed mechanisms for primitive self-complexifying chemical cycles and the polymerization of biomolecules. Researchers think they act like an enzyme surrogate to allow for some quite complex chemical reactions to occur. However, while the chemistry of carbon monoxide and carbon dioxide has received detailed attention in the context of hydrothermal vent chemistry, even the simplest thioacids and thioesters have received considerably less.

To probe the reaction chemistry of thioesters, one would have to do so under the conditions found in hydrothermal vents. This process would also include making the thioesters and their closely related cousins, thioacids, under these conditions. The simplest thioester is a compound called methyl thioacetate. The generation of this compound was the subject of the first part of the 2016 study.2 Methyl thioacetate can be made by the reaction of an acetate group with a sulfur group. Despite laboratory experiments demonstrating the production of methyl thioacetate from carbon monoxide (which goes on to make the acetate piece) and methanethiol (which goes on to make the sulfur-containing piece), evaluation of modern hydrothermal vent effluents has not provided evidence for abiotically derived (nonbiological) acetate or methanethiol. Indeed, methanol (the oxygen analog of methanethiol) does not appear to be a stable form of carbon under most hydrothermal vent conditions. This does not bode well for those who propose that methanethiol is around in significant concentrations under the same conditions.

The addition of thiols (such as methanethiol) to a class of compounds called aldehydes, followed by an oxidation reaction, is another route to thioesters like methyl thioacetate. But there is still the issue of thiol concentration (as mentioned above). Plus, the presence of significant concentrations of aldehydes is unlikely because they are not stable under the temperature and aggressive reaction chemistry conditions found in hydrothermal vents. Indeed, researchers have been unable to detect abiotic aldehydes in natural underwater hydrothermal environments. Similar arguments—namely, the inability to detect concentrations of aldehydes in these proposed origin-of-life sites–can be made for thioacids, the cousins of thioesters, the simplest example being thioacetic acid.


Getting Unexpected Results

When it comes to the reaction chemistry of thioacids and thioesters—should they be formed in significant enough quantities—another problem arises. Both classes of compounds are prone to rapid hydrolysis under the conditions found in hydrothermal vents. This means that they very rapidly react with water to make acetic acid. But if the thioacids and thioesters were supposed to be analogs of an enzyme that makes activated acetate species, they need to be around for long enough to be able to participate in those reactions, rather than to just make acetic acid.

In previous work, a 0.5% activated acetate yield was observed based on input from methanethiol and carbon monoxide. But on closer examination, and part of the critique presented in this 2016 article, the concentrations of both methanethiol and carbon monoxide used in these earlier experiments turn out to be much greater than those measured to date in natural hydrothermal vents. And not just a bit greater, but a lot greater—some 500 times for methanethiol and 3,700 times for carbon monoxide.3 In the 2016 study, even examining the behavior of thioacetic acid and methyl thioacetate concentrated far beyond that which is plausible in natural hydrothermal environments, only the hydrolysis products were observed.4

The difficulty in preparing thioesters and thioacids under hydrothermal vent conditions, and their subsequent reaction chemistry under these conditions, makes it hard to believe that they play a role in the origins of more advanced chemistry or in jumpstarting early metabolism. Modern organisms are capable of generating highly reactive compounds like thioesters internally under relatively extreme conditions by coupling their generation to other reactions in the cell and shunting them for use in other biochemical pathways before they have time to decompose. However, until thioesters were capable of being generated at reasonably steady states by relatively complex chemical assemblages, it is unlikely such compounds could have contributed to the origin or maintenance of these assemblages.

It is becoming apparent that more and more of the simple organic compounds detected in modern hydrothermal vents and, by inference, the hydrothermal vents of early Earth, are of biological origin—suggesting the work of a creator rather than random chemistry. Indeed, an increasing number of these organics have been convincingly determined to be of biological origin by a range of techniques. This 2016 report is just another chink in the armor when it comes to a nontheistic view of chemical evolution. Modern science continues to show evidence for a creator, and that is what drives me as a chemist to use my scientific background to look objectively at discoveries such as these. When I do, I see more and more evidence that I was created by God, and that I am not the result of a series of random, and sometimes dubious, chemical reactions.

To find out more about visiting scholar Nicholas Leadbeater, please check out their biography.

Endnotes
  1. Kuhan Chandru et al., “The Abiotic Chemistry of Thiolated Acetate Derivatives and the Origin of Life,” Scientific Reports 6 (July 2016): id. 29883, doi:10.1038/srep29883.
  2. Chandru et al., “The Abiotic Chemistry.”
  3. Chandru et al., “The Abiotic Chemistry.”
  4. Chandru et al., “The Abiotic Chemistry.”

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