Hello Everyone!

It’s funny — despite the fact that we study life so intensively (I mean, it’s the purpose of the entire field of biology), we still can’t agree on what life is, or how it came to be. ^[1] 1. McKay, C. P. What Is Life—and How Do We Search for It in Other Worlds? PLoS Biol. 2, e302 (2004) ^[2] 2. Tsokolov, S. A. Why is the definition of life so elusive? Epistemological considerations. Astrobiology 9, 401–412 (2009). Fortunately, we don’t necessarily need answers about the origin of life to study life, but exploring this question is an essential (and super interesting) part of understanding the universe, and life’s place in it.

That brings us to today’s topic: I came across a super interesting review article that discussed one prevailing hypothesis on how life may have begun. As it turns out, hydrothermal vents in the bottom of the ocean, and especially a specific hydrothermal vent system called the Lost City Hydrothermal Field (LCHF) create unique chemical environments that may have been perfect for abiogenesis (the formation of life).^[3] 3. Martin, W., Baross, J., Kelley, D. & Russell, M. J. Hydrothermal vents and the origin of life. Nat. Rev. Microbiol. 6, 805–814 (2008).

In this post, I’ll go over some details about how life is defined and how it works, what sorts of processes may have been required for abiogenesis to occur, and why the LCHF may have been the perfect environment for abiogenesis to happen in.

What is life, and how does it work presently?

Despite the fact that we study life so closely, it’s quite difficult to be specific about what “life” actually is ^[4] 1. McKay, C. P. What Is Life—and How Do We Search for It in Other Worlds? PLoS Biol. 2, e302 (2004) ^[5] 2. Tsokolov, S. A. Why is the definition of life so elusive? Epistemological considerations. Astrobiology 9, 401–412 (2009). . Around 123 different definitions of life have been proposed, most of which focus on what life does. ^[6] 4. Trifonov, E. N. Vocabulary of Definitions of Life Suggests a Definition. J. Biomol. Struct. Dyn. 29, 259–266 (2011) One popular definition you may have heard is that life exhibits the traits of Homeostasis, Organization, Metabolism, Growth, Adaptation, Response to Stimuli, and Reproduction. ^[7] 5. Koshland, D. E. Special essay. The seven pillars of life. Science 295, 2215–2216 (2002).
Another noteworthy definition used by NASA is that life is a “self-sustaining chemical system capable of Darwinian evolution”. ^[8] 6. Voytek, M. A. NASA Astrobiology. NASA.gov https://astrobiology.nasa.gov/research/life-detection/about/ (2021). For this discussion, it’s useful to use a definition that’s common in biophysics, which is that life is a process that goes against entropy. ^[9] 1. McKay, C. P. What Is Life—and How Do We Search for It in Other Worlds? PLoS Biol. 2, e302 (2004). ^[10] 7. Ulanowicz, R. E., Hannon, B. M. & May, R. M. Life and the production of entropy. Proc. R. Soc. Lond. B Biol. Sci. 232, 181–192 (1987).

In case you need a refresher, “entropy” is basically a formal way of describing randomness or disorder. As a system becomes more disordered or chaotic, its entropy increases; as it becomes more ordered, its entropy decreases. When molecules are broken down (like when we metabolize glucose into pyruvate during glycolysis), entropy goes up; when large molecules are built (like when we use amino acids to make proteins), entropy goes down.

The second law of thermodynamics states that the entropy of the universe is always increasing; spreading things out, breaking things down, and generally making everything more disordered. ^[11] 8. LibreTexts. The Four Laws of Thermodynamics. Chemistry LibreTexts https://chem.libretexts.org/@go/page/1918 (2019).
Life, on the other hand, tends to go against entropy; it builds complex molecules, carries out ordered and repeated chemical reactions, and generally defies what the universe does by default.

So, currently, how does life do this? There are two important factors at play: first, it uses enzymes to lower the energy requirements of chemical reactions; second, it takes advantage of favorable processes to allow unfavorable processes to occur. Let’s talk a little bit about how living systems use these concepts to defy entropy:

Enzymes can’t make an unfavorable reaction happen, but they can make a favorable happen more easily by lowering its activation energy — the amount of energy necessary to make a reaction occur spontaneously. ^[12]9. LibreTexts. Gibbs (Free) Energy. Chemistry LibreTexts … Continue reading Enzymes do this through a number of mechanisms: they make it easier for the transition state (a very short-lasting intermediate) of the reaction to form; they bring the molecules close together and orient them optimally; Sometimes they even provide alternate reaction pathways by temporarily binding with reactants. ^[13]10. LibreTexts. 9.8: ATP Synthase. Chemistry LibreTexts … Continue reading

So how can living systems make unfavorable reactions happen? They can do this by coupling an unfavorable reaction with a favorable one. For example, adenosine triphosphate (ATP), acts as the universal energy currency in living systems because its hydrolysis (a type of chemical decomposition) releases energy. Enzymes will often use the energy released by the hydrolysis of ATP to power chemical reactions that wouldn’t otherwise occur. Something that is conceptually similar can happen when there is an uneven distribution of charged molecules across some sort of barrier — which we will discuss in an upcoming section.

These two processes are extremely important for life as we know it, and one of the biggest challenges that researchers face when they are trying to describe possible origins of life is finding out how life could have occurred without these processes in place.

What happens in the LCHF, and why might it have been the origin of life?

Broadly speaking, it’s for three reasons:

1. There’s evidence that the chemical reactions occurring there are sufficient for producing Acetyl-CoA, which is essential for most foundational biochemical pathways11.
2. The highly alkaline environment in the vents creates a gradient from the vent to the ocean water, which can be used as an energy source.
3. The physical structure of the vents may have allowed certain reactions to become sufficiently concentrated.

Before we get into this, it’s important that we mention that what the beginnings of life actually looked like are still debated. We can discuss the various strengths and weaknesses of each of the various hypotheses in future videos, but for now, you just need to know that this primarily supports the “autotrophic origins hypothesis”. This hypothesis claims that the first life forms to exist produced their own energy from carbon they drew from their environment. In this model for the origin of life, the most important starting material for life is acetyl-CoA, so if you can show that acetyl-CoA can be spontaneously produced in an environment, you’ve made a very good case for this hypothesis for the origin of life. ^[14] 3. Martin, W., Baross, J., Kelley, D. & Russell, M. J. Hydrothermal vents and the origin of life. Nat. Rev. Microbiol. 6, 805–814 (2008). ^[15] 11. Martin, W. & Russell, M. J. On the origin of biochemistry at an alkaline hydrothermal vent. Philos. Trans. R. Soc. B Biol. Sci. 362, 1887–1926 (2007).

Acetyl-CoA is a somewhat complex molecule, and there are many reactions that occur at the LCHF that potentially produce its chemical ingredients. The strength of the evidence for each of these reactions varies, but one thing that that is well-established is that the LCHF can form large quantities of diatomic hydrogen, methane, and reduced carbon species (such as formate and acetate) through a process called serpentinization, which occurs when the mineral olivine reacts with water and various carbon compounds. ^[16] 11. Martin, W. & Russell, M. J. On the origin of biochemistry at an alkaline hydrothermal vent. Philos. Trans. R. Soc. B Biol. Sci. 362, 1887–1926 (2007). ^[17] 12. Proskurowski, G. et al. Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field. Science 319, 604–607 (2008). This reaction can occur at many hydrothermal vents, but the relatively low temperatures of the LCHF allow for greater production of reduced carbon. ^[18] 13. Russell, M. J., Hall, A. J. & Martin, W. Serpentinization as a source of energy at the origin of life. Geobiology 8, 355–371 (2010).

Another property of the LCHF that is promising for this hypothesis is the highly alkaline environment of the vents, which creates a pH gradient from the vents to the ocean. As you may know, a pH gradient forms when protons (H+) are distributed unevenly through a system. Due to the laws of diffusion, the protons “want” to move to distribute themselves evenly; biological systems can harness this movement as a way of powering reactions that wouldn’t otherwise be energetically favorable. A notable example of this is how mitochondria produce ATP —they pump protons across their inner membrane, then allow them to flow back, harnessing the energy released to power ATP synthase and generate ATP for the cell. ^[19]10. LibreTexts. 9.8: ATP Synthase. Chemistry LibreTexts … Continue reading The vents of the LCHF have a strong and consistent enough pH gradient that it may have been used to power certain early biological reactions before they could be “harnessed” in a more controlled way. ^[20] 3. Martin, W., Baross, J., Kelley, D. & Russell, M. J. Hydrothermal vents and the origin of life. Nat. Rev. Microbiol. 6, 805–814 (2008).

Finally, the vents in the LCHF are full of microscopic pores. Cells have membranes or walls, which allow them to concentrate molecules enough where they become biologically useful. Before membranes existed, these pores may have been the perfect microenvironments to fulfill the same role, allowing more complex and extended reactions to occur. Additionally, they contain iron monosulfide, which can act as a catalyst for some of the previously-mentioned reactions.

How likely is all of this?

Currently, the biggest unanswered questions are surrounding the synthesis of acetyl-CoA’s ingredients. Although the production of reduced carbon, diatomic hydrogen, and methane has been well-studied, there are many more synthetic pathways to be explored. Essentially, the current research has gotten us the “acetyl” part, but the “CoA” part is much more complex; it’s built from molecules such as acetyl thioesters (which are made from carbon, hydrogen, and sulfur), as well as an adenosine diphosphate nucleotide (which is made from a carbohydrate called “ribose” and a nitrogenous base; both typically require complex biochemistry to synthesize). There has been some speculation on how these molecules could be made in the LCHF, but nothing definitive. ^[21] 11. Martin, W. & Russell, M. J. On the origin of biochemistry at an alkaline hydrothermal vent. Philos. Trans. R. Soc. B Biol. Sci. 362, 1887–1926 (2007).   In fact, a 2016 review asserted that acetyl thioesters probably weren’t present in high enough concentrations to be biologically meaningful. ^[22] 14. Chandru, K., Gilbert, A., Butch, C., Aono, M. & Cleaves, H. J. The Abiotic Chemistry of Thiolated Acetate Derivatives and the Origin of Life. Sci. Rep. 6, 29883 (2016). In order for this hypothesis to hold water, researchers first need to demonstrate that each of acetyl-CoA’s ingredients can be made in adequate amounts in the LCHF, and then show how these basic molecules go on to form life, rather than just a soup of chemicals.

Conclusion

The origin of life is one of the greatest unsolved mysteries on Earth, and we still have a long way to go before we can be sure about how it occurred. In general though, scientists usually agree that some sort of simple, Abiotic, molecule building was necessary as a first step. Hydrothermal vents, and the vents in the LCHF in particular, are fascinating environments that, through their unique chemical and geological properties, may have been the perfect place for these types of chemical processes to occur billions of years ago.

That’s all for today, everyone! I hope you found it as interesting as I did!

Nicholas Milazzo

Writer, Copy editor 

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