Life on Earth may not have begun once, but twice. New evidence from an international team led by biologists at Heinrich Heine University Düsseldorf in Germany suggests that the first free-living cells emerged independently in two separate events, even though all life shares a common genetic code.
The findings, published in Science Advances, challenge the long-held assumption that the transition from chemistry to biology happened only once. Instead, the research points to a striking possibility: bacteria and archaea, the two oldest branches of life, may have each made the leap from hydrothermal vent chemistry to free-living existence on their own.
Two kinds of pioneer cells, one ancient code
About 4 billion years ago, Earth was a hostile place. The earliest cells were not yet free-living; they depended on the chemical energy of hydrothermal vents. But at some point, two distinct forms of primitive cellular life began to emerge.
Natalia Mrnjavac, a biologist at the University of Düsseldorf and lead author of the study, describes the scene: two very different kinds of cells, pioneer bacteria and pioneer archaea, making their first attempts at life outside the confines of a hydrothermal vent.
The researchers reached this conclusion by tracing the origins of enzymes during the earliest split between bacteria and archaea. They examined genomes, protein structures, and chemical reactions to reconstruct the metabolic network that the earliest cells used to produce amino acids, RNA bases, and vitamins from simple materials like hydrogen gas, ammonia, and CO2.
The surprise hidden in ancient metabolism
This metabolic network consists of 420 chemical reactions that are extremely ancient, conserved across all life to a degree comparable with the genetic code itself. But here is what surprised the researchers: the enzymes that carry out those reactions are not conserved between bacteria and archaea.
William Martin, senior author of the study, says the last universal ancestor of all cells, known as LUCA, possessed enzymes for only about half of the reactions of metabolism. The other half appear to have evolved separately in the two lineages.
This suggests that while the chemical reactions themselves were present from the very beginning, the biological catalysts that drive them were assembled independently in bacteria and archaea. In other words, the two domains of life may have inherited the same chemistry from a common ancestor, but each developed its own way to use it once free-living cells emerged.
The researchers say these comparisons are giving unprecedented insights into the phase of evolution when enzyme-catalyzed metabolism was arising from spontaneous reactions catalyzed by metals in the Earth's crust.
A new view of life's deepest roots
The implications are profound. If free-living cells emerged twice, then the origin of life on Earth was not a single, fragile event but a more robust process that could happen in parallel. This also raises questions about how common life might be elsewhere in the universe, since the transition to free-living cells may not be as improbable as once thought.
For now, the evidence points to a shared chemical foundation but a divergent biological path. Life on Earth may be united by its genetic code, but its journey to becoming free-living may have been taken twice, by two different kinds of pioneers.
