Two origins of life: Free-living cells may have emerged twice as bacteria and archaea diverged

Two origins of life: Free-living cells may have emerged twice as bacteria and archaea diverged Scientists have uncovered compelling evidence suggesting that free-living cells may have originated independently twice, once leading to the emergence of bacteria and once to archaea, as these two domains of life diverged billions of years ago. A study published in Science Advances by researchers from the Institute of Molecular Evolution at Heinrich Heine University Düsseldorf, along with collaborators from multiple institutions, presents this groundbreaking finding based on genome analyses, protein structure studies, and reconstructions of ancient metabolic pathways. According to the research, the earliest forms of life likely emerged in environments rich in hydrothermal activity, such as deep-sea vents. These conditions provided a mix of inorganic catalysts, including metals, which played a crucial role in facilitating the chemical reactions necessary for life. The study suggests that the initial stages of cellular metabolism relied heavily on these environmental catalysts rather than enzymes, which came later in evolutionary history. Natalia Mrnjavac, a biologist at the University of Düsseldorf and lead author of the study, described the process as akin to watching the first cells emerge from the primordial soup. “We would see two very different kinds of cells emerging, pioneer bacteria and pioneer archaea, making their first attempts at life outside the confines of a hydrothermal vent,” she explained. The findings challenge the long-held assumption that life originated through a single evolutionary pathway and instead propose a scenario where two separate origins gave rise to the two primary branches of cellular life. The research team examined the chemical reactions that enabled the synthesis of fundamental biological molecules, such as amino acids, RNA bases, and vitamins, from simple precursors like hydrogen gas, ammonia, and carbon dioxide. They identified a total of 420 enzymatic reactions that are now considered universal to all living organisms. However, the study revealed that the enzymes responsible for these reactions are not shared between bacteria and archaea. Instead, the last universal common ancestor (LUCA) of all known life forms possessed enzymes for only about half of these reactions, while the remaining were catalyzed by metals in the environment. William Martin, a biologist from the University of Düsseldorf and senior author of the study, emphasized the significance of this discovery. “The surprise is that the enzymes that catalyze those reactions are not conserved across the evolutionary divide that separates bacteria and archaea,” he stated. “We found that LUCA possessed enzymes for only about half of the reactions of metabolism. The other half was catalyzed by metals in the environment where LUCA arose.” Harun Tüysüz, an inorganic chemist from the Max-Planck-Institut für Kohlenforschung and the IMDEA Materials Institute in Madrid, added that the metals commonly found in hydrothermal vents could effectively replace a substantial portion of the enzymes required for metabolism. “Metals that naturally occur in hydrothermal vents can replace a surprisingly large number of enzymes in metabolism,” he noted. Joseph Moran, a researcher from the University of Ottawa specializing in the use of metals to catalyze metabolic reactions, further elaborated on the implications of the study. “The closer we look, the more clearly we can see that early biochemical evolution was a hybrid of enzymatic and metal catalysts,” he said. This hybrid system allowed early life forms to adapt and evolve, eventually leading to the development of fully functional enzymes capable of carrying out complex metabolic tasks. The study’s findings suggest that the transition from non-enzymatic, metal-catalyzed processes to enzyme-based metabolism occurred in stages. Researchers reconstructed four key phases of early catalysis: an initial period dominated by metal-only catalysts, followed by a transitional phase involving both metals and enzymes, and finally the diversification of enzymatic systems within the bacterial and archaeal lineages. As these lineages evolved, new enzymes replaced the inorganic catalysts originally provided by the environment. This research provides a deeper understanding of how life began and how the fundamental processes of metabolism evolved over time. It highlights the complexity of early biochemical systems and underscores the importance of environmental factors in shaping the course of life’s development.