Image citation: [6]
Written by Raquel Mattos-Canedo ‘26
Editied by Shivam Kogar '27
Life is everywhere. From the tree outside your home, to the spider in the corner of your office. Yet, life is not everything. The air we breathe, the dirt we stand on, the water we drink. Life is constantly and always surrounded by non-life. How did life emerge? How did the inorganic compounds on a young planet Earth, result in organic life? For years, scientists have attempted to pinpoint the exact conditions that allowed for the very first organic cells to form, and then to evolve into what we know as life today. This has proven extremely difficult, because of the sheer volume of factors, processes, and conditions to take into account. Geologists alone cannot answer the question “where did we come from?”. Nor can evolutionary biologists, chemists, biochemists, physicists. Rather, this question demands investigation and explanation across various scientific disciplines.
Before diving into how life originated, we must first consider the where and what. The late Hadean era, which took place four billion years ago, was characterized by impacts with asteroids and meteorites (1). This bombardment of our early Earth has been proposed to be the source of volatiles. Volatiles are chemical compounds that rapidly vaporize, and are extremely important for maintaining habitable conditions on Earth. Volatiles include compounds like H2O, and NH3 (ammonia). Chondrites, the most common meteor type to strike the Earth, contain amino acids, nucleic acid bases, and other sugar related compounds (2). These compounds could have then been brought to the early Earth via extraterrestrial impacts, along with water, CO2 and other volatiles.
Figure 1 [2]
This theory, however, contains an important inconsistency. There were over 70 amino acids found on the chondrite meteorites (2), while only about 20 amino acids are seen in all biological life and appear almost exclusively in their L-form (3). So why and how could a common ancestor sort and separate 20 left-handed amino acids from all others delivered to the Earth in the same event? The answer could lie in mineral surfaces. Minerals can contain crystalline surfaces that would allow for specific amino acid binding. If these minerals were present in the environment where life originated, then it would explain why all life mainly utilizes the twenty primary amino acids, and all in the L configuration. The mineral surfaces would act as a filter. This type of selectively could not only separate and sort amino acids, but result in a concentration of these compounds, which would be conducive to their reactivity with the surrounding environment (2).
So what exactly was that environment like? While this remains a source of contention, there are some points of widespread agreement. The end of Hadean coincided with a decline in extraterrestrial bombardment (4). The Earth was cooling, and the water brought on meteorites formed from drops and puddles into rivers and oceans; the perfect home for life-to-be. The surface of the Earth during the Hadean could not have conceivably been the location on which life originated. The physical and chemical composition of the surface was incredibly unstable. In contrast, the depths of the Hadean proto-ocean facilitated a much more proactive environment. Harmful radiation and physico-chemical radiations would be filtered by the large amount of water, which resulted in a stable environment, free of radiation (5). Safe from the harms of surface, the composition of the early ocean was incredibly important in the story of life’s origins. Early in investigating the origin of life, scientists ran into what they dubbed the carbon paradox (6). Besides water, CO2 is arguably the most essential molecule to the sustainability of life. Autotrophs, the base of the chain of energy consumption rely on the intake of CO2 to produce and generate energy. And yet, prebiotic chemistry held very little CO2 in its atmosphere. Rather, CO, formamide and cyanide ruled the atmosphere during this era (7). And so, how could life have possibly risen from an atmosphere so desolate, and missing such an essential cog of the biological machine?
The widely agreed upon star of the show are alkaline vents (6). Alkaline vents are fissures found on the seabed of the deep ocean. Geothermally heated fluids discharge from these fissures, allowing them to react with seawater. The amount of H2 and OH- in these fluids make them strongly alkaline, at a pH of about 9-11. These vents have a labyrinth-style composition, with tiny micropores of various sizes interconnecting, similar to the composition of a sponge. The water in these vents exists below a pressure of 20-35 MPa and a temperature of 350 – 450 oC, which corresponds to the thermodynamic supercritical state. The consequences of these physicochemical conditions of this environment is that ionic and polar compound solubility in and around these vents significantly decreases while apolar molecule solubility increases. This likely works in tandem with the selectivity of any crystalline mineral surfaces (2) present as it allows and facilitates the concentration of prebiotic molecules in the alkaline pores, increasing their reactivity. Pressure further increases the reactivity of the deep ocean environment by decreasing the activation energy for certain reactions by lowering the temperature at which they occur, decreasing ΔG0 (5).
The porous labyrinthine structure in alkaline vents plays a crucial role in the story of biogenesis. Within its pores, the warm alkaline hydrothermal fluids mix in with the cold, acidic, seawater, which allows for steep gradients of pH and temperature and a proton gradient (6).
Figure 2 [6]
These gradients are extremely similar to the gradients found in modern day autotrophic cells, which may give a clue as to how biological systems came to be. Proton gradients are extremely important to the pathways that produce energy and sustain life because of their role in ATP synthesis. During cellular respiration protons are transported into a space between the cell’s inner and outer membranes, called the intermembrane matrix. This creates a proton gradient, where protons move from a high concentration to a low concentration, flowing like a river through the intermembrane matrix. The movement of these protons as they move down the gradient powers a protein called ATP synthase (8). ATP synthase has a unique design; as the protons move down the gradient, the top part of ATP synthase spins, similar to how a windmill spins with the wind. This movement allows for the energy necessary to produce large amounts of ATP.
There are key differences however, between the metabolic pathways we observe in vivo today, and the system set up by alkaline vents. There is also an evolutionary inconsistency to consider.
The Acetyl-CoA pathway is believed to be the oldest known pathway, and it is also the only one that doesn’t require input of ATP to fix carbon (9). Carbon fixation is a process that turns inorganic carbon from carbon dioxide, into organic carbon that can be incorporated into living organisms, which is key for any sort of life to form. We would expect to see evolutionary proof of this pathway across both the archaea and the bacteria domains, if they shared a common ancestor. However, The Acetyl-CoA pathway between bacteria and archaea differs in some very foundational areas. The genes required for methyl synthesis in each pathway are deeply distinct, as are the key membrane proteins necessary for active ion pumping (10). Although mechanistically, the pathways in these two domains operate very similarly, the observation of the difference in the “wiring” in their respective foundational building blocks, means that things may not be so simple.
The most probable hypothesis is that early organisms used the proton gradient to drive ferredoxin reduction, which drove CO2 reduction to Acetyl-CoA, suggesting that CO2 can be reduced and driven purely by an H+ gradient, which is readily available in alkaline vents (6). This pathway is consistent with the methylation mechanisms seen in archaea. This hypothesis suggests that the split off that resulted in the bacteria methylation version only arose once there was not enough ferredoxin left to continue CO2 reduction. This meant that a new pathway had to develop, but how?
It turns out that NADH is a side product of CO2 reduction using ferredoxin. Combined with the availability of ATP synthesis using the ion gradient, a new pathway evolved reducing CO2 to methyl (6). This pathway is much more analogous to the one seen in bacteria, while the ferredoxin method continued being used by archaea ancestors (10). In this way, we see two similar, but distinct evolutionary pathways take place. The role of pressure once again comes in, as high pressures can facilitate the stabilizing of macromolecules such as DNA and RNA, meaning that any synthesis of these compounds will remain present and “usable” in further reactions and processes (5).
By combining the hypotheses put forward, we can attempt to explain the origin of life. While the investigation of these concepts begs the conduction of more experimental procedure, the ideas put forward here could help answer the age-old question of what our place is among the stars, and how we came to be a life-concentrated speck in a cold, sterile universe. Looking even further, we could use this knowledge to ensure our own survival on extraterrestrial bodies, or even facilitate the formation of life ourselves. This would be one of the most important pieces in the puzzle biologists and geologists have been trying to solve, and we would be one step closer to understanding our place in the universe.
References
Jeffrey Taylor G. Wandering Gas Giants and Lunar Bombardment [Internet]. Hawai’i Institute of Geophysics and Planetology; 2006. Available from: http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html
Nakashima S, Kebukawa Y, Kitadai N, Igisu M, Matsuoka N. Geochemistry and the Origin of Life: From Extraterrestrial Processes, Chemical Evolution on Earth, Fossilized Life’s Records, to Natures of the Extant Life. Life. 2018 Sep 20;8(4):39.
Vickery HBradford, Schmidt CLA. The History of the Discovery of the Amino Acids. Chem Rev. 1931 Oct 1;9(2):169–318.
Sleep NH. The Hadean-Archaean Environment. Cold Spring Harb Perspect Biol. 2010 Jun 1;2(6):a002527–a002527.
Daniel I, Oger P, Winter R. Origins of life and biochemistry under high-pressure conditions. Chem Soc Rev. 2006;35(10):858.
Sojo V, Herschy B, Whicher A, Camprubí E, Lane N. The Origin of Life in Alkaline Hydrothermal Vents. Astrobiology. 2016 Feb;16(2):181–97.
Fuchs G. Alternative Pathways of Carbon Dioxide Fixation: Insights into the Early Evolution of Life? Annu Rev Microbiol. 2011 Oct 13;65(1):631–58.
Weber J, Senior AE. ATP synthesis driven by proton transport in F 1 F 0 ‐ATP synthase. FEBS Lett. 2003 Jun 12;545(1):61–70.
Martin WF. Older Than Genes: The Acetyl CoA Pathway and Origins. Front Microbiol. 2020 Jun 4;11:817.
Soppa J. Protein Acetylation in Archaea, Bacteria, and Eukaryotes. Archaea. 2010;2010:1–9.
Comments