A radical new study published in Science Advances suggests that life on Earth may have emerged not once, but twice. Researchers led by William Martin at Heinrich Heine University Düsseldorf focused on the most rudimentary metabolic reactions. They found that the enzymes catalyzing these reactions are not conserved across the evolutionary divide separating bacteria and archaea. "The new data leave only one conclusion," Martin says. "The bacterial and archaeal lineages made the transition to the free-living state independently. Only free-living cells are alive."

The study proposes that the ~400-reaction network converting H2, CO2, and other compounds into amino acids and bases could not have arisen in an instant. The team analyzed protein structures of core metabolic enzymes in bacteria and archaea, ordering them by complexity. They identified four phases in the development of catalysis. Initially, metals in the environment catalyzed reactions. Over time, proto-cells like LUCA developed their own enzymes, gradually reducing dependency on external metals until they became free-living cells.

Importantly, this transition occurred after bacteria and archaea had already diverged. Each lineage independently evolved structurally distinct enzymes for the same essential reactions. "Such parallel inventions could have paved the way to the independent emergence of free-living bacteria and archaea," says Natalia Mrnjavac. The study also found that phosphite and palladium from hydrothermal vents can replace ATP and enzymes in metabolic phosphorylation, offering an alternative to modern biochemistry.

The findings suggest the tree of life may have two separate trunks, diverging at the roots before life was technically alive. Further research is needed to confirm these bold claims.