If you were to take apart a cell and examine each of its components—from the mitochondrial powerhouse to the Golgi packaging center—none of them, on their own, could be classified as living. Zoom in even closer, and you’ll find an unfathomably complex network of chemistry: clusters of macromolecules whizzing past each other, transforming in spectacular collisions to reach more energetically favorable states.
Life is an emergent property. Every living thing consists of billions upon billions of nonliving things, engaging in highly specific interactions that sustain the larger system. How, exactly, they do so is at the heart of the origin of life problem—the transition from the abiotic to the biotic, from macromolecules to the cell.
Forming a living system requires a vast array of chemical reactions, which in turn need to be propelled by a robust energy source. In a groundbreaking experiment in 1952, Stanley Miller and Harold Urey electrified an “organic soup” of methane, ammonia, and hydrogen, elements thought to have been present in the early Earth’s atmosphere. In this miniature simulation of the ancient world, simple organic molecules and naturally occurring amino acids emerged, demonstrating how the most fundamental building blocks of life could have come to be. However, their system could not form polymers—the more complex chains of molecules that make up essential structures in living organisms. This provoked questions of which environmental conditions would be most suitable for polymerization, and, importantly, which among these systems could have existed on the Archaean Earth.
There are two leading theories in the abiogenesis, or origin of life, debate: some researchers conjecture that life originated in warm little ponds, while others argue that it was born out of hydrothermal vents—cracks in the deep-sea floor where tectonic plates diverge. The former theory is popular among scientists since the “wet and dry” seasonal cycles available to warm little ponds have been shown to polymerize long chains of nucleotides, while the latter set of conditions has experimentally produced much shorter chains. However, up to now, the debate has favored the hydrothermal vents theory, largely because of a glaring Achilles’ heel in the warm little pond theory: geologists have had good reason to suspect that the ancient Earth was completely covered in water.
Reimagining the Water World
An article published in Nature Geoscience by Yale professor Jun Korenaga and Juan Carlos Rosas, a former postdoctoral researcher at Korenaga’s lab and currently a researcher at the Ensenada Center for Scientific Research and Higher Education in Mexico, challenges the hydrothermal vent paradigm. By modeling changes to the depth of the seafloor due to radiogenic heating, or the heat produced by radioactive decay in the Earth’s mantle, the researchers found a surprising result: the internal heat may have caused seafloor shallowing, allowing for the emergence of landmasses from under the sea. The presence of these islands implies that the warm little ponds may have existed as theorized, containing the “organic soup” that eventually birthed life.
The existence of subaerial landmasses, or land exposed to the atmosphere, is subject to the dynamics of plate tectonics. Korenaga explained that when the layers of earth beneath the crust of two tectonic plates converge, the older, heavier plate can subduct under the other into the mantle, bringing water along with it. Therefore, over time, the net effect is that the earth absorbs water. Reversing that process, scientists predict that oceans during the Archaean era had much greater volumes of water than they do today, submerging all continental landmasses and providing the rationale for the “water world” view of ancient Earth.
Overcoming Seafloor Subsidence
Korenaga and Rosas approached the problem from a different layer—the seafloor topography. Presently, geologists observe a phenomenon known as seafloor subsidence—the lowering of the seafloor as its tectonic plate becomes colder and denser, moving away from the mid-ocean ridge where it originated. This is the process by which volcanic islands are gradually submerged and become seamounts. However, the Yale team predicted that sufficient internal heating could overcome the cooling effect, leading seamounts to resurface as the seafloor shallows.
It so happens that there was a source of additional heat under the Earth’s surface: the radioactive decay of trace elements in the mantle. Some elements radioactively decay naturally with time, releasing atomic energy and a small amount of heat in the process. Currently, the concentration of radioactive elements in the mantle is relatively low, such that only a moderate amount of heat is produced. However, Korenaga and Rosas found that, by reversing the clock on the present-day mantle concentrations of potassium, uranium, and thorium, the much higher concentrations that existed during the Archaean would have sufficiently increased radiogenic heating to induce seafloor shallowing.
The Case for Warm Little Ponds
The argument for hydrothermal vents is based on the extremely reactive conditions formed at mid-ocean ridges where tectonic plates diverge. The high temperatures from magma heating provide a robust energy source around which rich ecosystems and complex reactions can thrive. Nevertheless, it has been suggested that the concentrations of chemicals necessary for reactivity would have been extremely low given the massiveness of oceans. “The ocean is, intrinsically, a difficult place for life to emerge,” Korenaga said. Additionally, because the formation of peptide bonds between amino acids to form proteins is a dehydration reaction, this would be difficult to accomplish with water as the medium. Therefore, some scientists would consider dry land essential to initiate life.
The evidence for land above sea level provides the necessary geological environment for warm little ponds to exist. In addition to concentrating chemical compounds more effectively, these ponds would be exposed to a variation in annual rainfall. The seasonal wet and dry conditions could have driven bond formation in RNA, which is widely accepted to have been the original information-carrying molecule in all organisms, before DNA had evolved. Its ability to store and pass on information would subject it to the Darwinian theory of natural selection, leading to the evolution of organisms and life as we know it.
Building Bridges, Traversing the Divide
While Korenaga and Rosas’ work fits another piece into the puzzle of the origin of life, the specifics of the prebiotic chemistry involved remain to be understood. Abiogenesis lies at the intersection of Earth sciences and inorganic chemistry. According to Korenaga, his research focus lies within the more straightforward of the two problems, involving building models of the Archaean Earth using physical and chemical principles that are already well-understood. “The abiological to biological bridge remains a huge question,” Korenaga said. He explained that once the first organism had been formed, one could trace a line down to the currently existing species by Darwin’s theory of evolution.
Korenaga shared that one of the limitations of the field is a lack of communication between scientists from the geological and chemical disciplines. “As science matures, people start to specialize in a very narrow discipline,” he said. “Many of the people working on the origin of life problem are not very aware of our contemporary understanding of geology,” he said. He plans to continue refining the understanding of early Earth conditions by accounting for more components and interactions and testing model predictions against geological records. Communication with prebiotic chemists is also a priority, since an understanding of the early Earth would provide knowledge of the relevant environmental conditions.
Korenaga spoke about the loftiness of the abiogenesis project and how filling the gap between physical and biological systems seems inconceivable at the moment. “I probably won’t see the result in my lifetime,” he said. Nevertheless, by opening up a new paradigm for islands on the ancient Earth, their research contributes to the larger international endeavor. “Once we understand the transition from abiological to biological stuff, it’s probably as big as Darwin’s discovery of biological evolution,” he said.
Citations:
Da Silva, Laura, Marie-Christine Maurel, and David Deamer. 2015. “Salt-Promoted Synthesis of RNA-like Molecules in Simulated Hydrothermal Conditions.” Journal of Molecular Evolution 80 (2): 86–97.
Martin, William, John Baross, Deborah Kelley, and Michael J. Russell. 2008. “Hydrothermal Vents and the Origin of Life.” Nature Reviews. Microbiology 6 (11): 805–14.
Orgel, L. E. 1998. “The Origin of Life–a Review of Facts and Speculations.” Trends in Biochemical Sciences 23 (12): 491–95.
Pearce, Ben K. D., Ralph E. Pudritz, Dmitry A. Semenov, and Thomas K. Henning. 2017. “Origin of the RNA World: The Fate of Nucleobases in Warm Little Ponds.” Proceedings of the National Academy of Sciences of the United States of America 114 (43): 11327–32.
Rosas, Juan Carlos, and Jun Korenaga. 2021. “Archaean Seafloors Shallowed with Age due to Radiogenic Heating in the Mantle.” Nature Geoscience 14 (1): 51–56.
About the Author:
Alexa Jeanne Loste is a first-year prospective Molecular Biophysics & Biochemistry major in Ezra Stiles College. In addition to writing for YSM, she is a project head for GREEN at Yale, a member of the Environmental Education Collaborative, the STEM Panel Chair for the Conference Committee of the Women’s Leadership Initiative, and a copy desk staffer at the Yale Daily News.
Acknowledgments:
The author would like to thank professor Jun Korenaga for his time and enthusiasm in sharing his research.
Extra Reading:
Rosas, Juan Carlos, and Jun Korenaga. 2021. “Archaean Seafloors Shallowed with Age due to Radiogenic Heating in the Mantle.” Nature Geoscience 14 (1): 51–56.
Pearce, Ben K. D., Ralph E. Pudritz, Dmitry A. Semenov, and Thomas K. Henning. 2017. “Origin of the RNA World: The Fate of Nucleobases in Warm Little Ponds.” Proceedings of the National Academy of Sciences of the United States of America 114 (43): 11327–32.