Today, life on Earth relies immensely on an external power source—namely, solar radiation—to provide the energy needed to forge bonds between atoms and assemble the complex organic compounds necessary for life. Since photosynthesis didn’t evolve until relatively late in the planet’s history, scientists have long debated what source of energy the first organisms utilized, throwing everything from meteorites to lightning strikes into the proverbial ring.
But the metabolism of the planet’s first organisms may not have required an external source of energy. Under the conditions present in a hydrothermal vent, a core set of metabolic reactions unfolds spontaneously in line with the laws of thermodynamics, according to calculations published December 13 in Frontiers of Microbiology.
“The present data uncover a hitherto unique thermodynamic link between core biochemistry as a whole and the conditions of a geochemical environment known to have existed on the early Earth,” writes biochemist John Allen, who has previously collaborated with the study’s lead author William Martin but was not involved in this paper, in an email to The Scientist. He adds that the paper convincingly demonstrates that the chemical reactions plausibly performed by the earliest lifeforms near a hydrothermal vent release energy, and thus “will spontaneously move in the direction of synthesizing metabolic precursors.”
Hydrothermal vents, hot springs near fault lines on the ocean floor, were discovered 40 years ago, and soon after became recognized as environments conducive to the origin of life, as they are warm, rich with minerals, and contain physical and chemical gradients that facilitate reactions. In 2020, Martin and his group showed experimentally that, if kept in water in the presence of the nickel iron alloy awaruite overnight at 100°C, hydrogen gas (which is abundant in hydrothermal vents) will combine with carbon dioxide to form formate, acetate and pyruvate—the latter of which is one of the most central compounds of metabolism. Awaruite catalyzes the formation of pyruvate from hydrogen and carbon dioxide by acting as a reducing agent, allowing it to stand in for the suite of enzymes that usually construct the molecule stepwise in organisms.
The new work stems from Martin’s curiosity as to “how big this problem of the origin of metabolism is. . . . How many reactions do we need to make all the basic components of life, from the elements on early Earth?”
Martin and his team had previously identified a core set of more than 400 metabolic reactions which would likely have been present in the Last Universal Common Ancestor (LUCA), the organism that gave rise to all extant life on Earth, and are needed to generate amino acids, nucleotides, and other essential cellular molecules. They next asked whether these reactions would have required an external energy source to proceed.
To find out, the team calculated the Gibbs free energy for each reaction—a thermodynamic measure of the energy needed for or released from a chemical reaction, assuming the surrounding temperature and pressure are kept stable and there are no external inputs. It turns out that under the conditions present in hydrothermal vents, “almost all of the reactions in the metabolism of the last universal common ancestor are downhill,” says Martin—97 percent, to be exact. “That means there’s a natural tendency of metabolism to unfold all by itself, under the right conditions from the elements,” he continues. “You don’t have to add any energy. The energy for life is within life itself.”
For Allen, the findings are remarkable. “It means that you don’t have to come up with ideas like lightning strikes. . . .The essential components of modern metabolism will assemble themselves from gases, primarily hydrogen and carbon dioxide, and in the atmosphere in the vicinity of hydrothermal vents.”
The hydrogen is key, Martin notes. “If we take hydrogen out, nothing works. If we add the amounts of hydrogen present today in modern hydrothermal vents, everything works,” he says. In addition, the minerals within the vents’ walls could have acted as catalysts for early metabolic reactions, as the awaruite did in previous experiments. “These first conditions want to unfold and go forward, but they won’t unless the right catalysts are provided,” he explains—such as awaruite
When the team mapped out how the reactions might interact, the results suggested that precursors for amino acids can be produced, as well as citric acid cycle intermediates and the first steps toward building other cellular components, including enzymatic cofactors and sugars. These reaction products could organize into self-catalyzing networks with the help of catalysts from the environment, says Martin, allowing a metabolic system to assemble, with all core reactions pushed along by hydrogen.
Eloi Camprubi-Casas, a biochemist at the Earth-Life Science Institute in Tokyo who was not involved in the work, tells The Scientist in an email that he regards this study as a “needed article in the origins field,” adding that the findings suggest the few energy-requiring reactions identified by the team could have obtained the energy they need from the products of others, such as acetyl phosphates. In that way, these spontaneously synthesized molecules “could have acted as ATP primordial analogues.”
Camprubi-Casas says he would have been interested in the energetics of forming nucleic acid strands in addition to the reactions studied, though he acknowledges that would have increased the amount of work “10-fold.” Camprubi-Casas also says the study could have more thoroughly considered the possibility that products of some reactions could inhibit other reactions or compete for catalysts, “making both pathways incompatible at least in principle.”
David Deamer, a biochemist at UC Santa Cruz who recently proposed a volcanic hot spring hypothesis for the origin of life but was not involved in the study, writes in an email to The Scientist that the “paper is impressive.” He adds, “Particularly significant is that their argument is based firmly on thermodynamic principles that revealed how molecular hydrogen in solution could act as a reducing agent to drive multiple reactions related to metabolism.” What is not yet addressed, he points out, is how the reactions were encapsulated in some form of microscopic compartment so that the developing networks could be maintained.