Life on Earth May Have Begun Twice, New Study Suggests

A groundbreaking study published in Science Advances proposes that life on Earth may not have emerged from non-living matter just once, but twice. Researchers suggest that the two primary domains of life, bacteria and archaea, independently developed the essential metabolic processes that distinguish living from non-living matter. This radical hypothesis challenges the long-held view of a single origin for all life.

The study focused on the foundational chemical reactions believed to be crucial for life's emergence. "The surprise is that the enzymes that catalyze those reactions are not conserved across the evolutionary divide that separates bacteria and archaea," explained William Martin, a biologist at Heinrich Heine University Düsseldorf in Germany. "The new data leave only one conclusion. The bacterial and archaeal lineages made the transition to the free-living state independently. Only free-living cells are alive. Let's call it by name: we are looking at one origin of the genetic code, but two origins of life."

Defining life itself is a complex task, but it is generally understood as matter that responds to its environment, consumes energy, grows, and reproduces. Even viruses, which exhibit some of these traits, are not typically classified as alive due to their lack of independent metabolism. Metabolism, the intricate network of chemical reactions that organisms use to sustain themselves, is a universal characteristic of life. A key question in understanding life's origin is how the very first metabolic enzymes—proteins that act as catalysts—could have formed without pre-existing enzymes.

Scientists theorize that early life likely arose in highly reactive environments, such as hydrothermal vents, where naturally occurring metals could have served as the initial catalysts for metabolism. These metallic catalysts would have facilitated the conversion of simple inorganic compounds like hydrogen gas, ammonia, and carbon dioxide into more complex, useful molecules necessary for life's precursors. "Almost everything about the origin of metabolism is debated, including the roles of energy, genetics, autocatalysis, phosphate, cofactors, cyanide, CO2, and water," Martin and his colleagues noted in their paper. "Yet on one aspect all will agree: the ~400-reaction network that converts H2, CO2, NH3, H2S and phosphate into amino acids, bases and cofactors cannot have arisen in an instant. Its emergence from spontaneous environmental reactions had to traverse intermediate states of assembly, which have previously been elusive."

Evolution of Catalysis and Early Metabolism

The research team meticulously analyzed the protein structures of core metabolic enzymes found in bacterial and archaeal genomes. They developed a specialized algorithm to reconstruct the evolutionary order of these enzymes, thereby establishing a rough timeline for metabolic development. "We found that the last universal ancestor of all cells, LUCA, possessed enzymes for only about half of the reactions of metabolism," stated Martin. "The other half was catalyzed by metals in the environment where LUCA arose."

The study outlines four proposed phases in the evolution of catalysis. The initial phase relied exclusively on environmental metals. As useful molecules accumulated, proto-cells—entities that were not yet fully alive—began to develop their own enzymes to replicate some of the metal-catalyzed functions. This process continued, with later proto-cells evolving increasingly sophisticated enzymes, reducing their reliance on external metallic catalysts until they achieved independence. Crucially, the researchers contend that this transition to 'free-living cells' occurred after the divergence of bacteria and archaea, implying separate evolutionary paths for each.

Natalia Mrnjavac, a biologist also at Heinrich Heine University Düsseldorf, highlighted the significance of parallel evolution. "We can see cases where the ancestors of bacteria and archaea independently evolved structurally distinct enzymes to catalyze the same essential metabolic reaction," she said. "Such parallel inventions could have paved the way to the independent emergence of free-living bacteria and archaea."

This research may also shed light on another puzzle: the origin of ATP, the universal energy currency of life today. ATP is synthesized by enzymes and was not readily available in prebiotic conditions. The study identified a plausible alternative energy source. "When we react phosphite, a form of phosphorus that naturally occurs in hydrothermal vents, with organic compounds, we get metabolic phosphorylation reactions overnight in water," reported Manon Schlikker, a molecular evolutionary scientist at Heinrich Heine University Düsseldorf. "Phosphite and palladium replace ATP and enzymes; it's amazing, and it makes early evolution a lot easier to grasp."

The precise chemical environment where these reactions occurred remains a subject of debate, with possibilities ranging from water to viscous organic mixtures. If these findings are validated, the traditional depiction of the tree of life, with LUCA as a single trunk, may require revision. The possibility of two independent origins suggests that the foundational 'roots' of life might have diverged much earlier, before life as we understand it even began.