For decades, biology has told a simple story: all life on Earth descends from a single ancestor, a cell known as LUCA, the last universal common ancestor, which lived roughly four billion years ago. A new study suggests the picture may be more complicated. Scientists probing life's deepest origins have uncovered evidence that the first free-living cells may have emerged not once, but twice. The findings suggest a striking possibility: life may share one ancient genetic code, yet have undergone two separate transitions into free-living existence.
Tracing enzymes back to the first cells
The research was led by biologists at the Institute of Molecular Evolution at Heinrich Heine University Düsseldorf, and the findings were published in the journal Science Advances. According to ScienceDaily, by tracing the origins of enzymes during the earliest split between bacteria and archaea, the researchers found evidence that free-living cells may have originated independently twice.
The team compared genomes across representatives of both domains and found something unexpected. Five reactions offered clear evidence of independent enzyme origins. In each case, bacteria and archaea evolved structurally unrelated proteins that carried out the same essential reaction. Those examples included enzymes involved in alanine production, the shikimate pathway and riboflavin synthesis. The parallel solutions suggest that the two lineages completed their metabolic systems separately after diverging from LUCA.
“"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," said Natalia Mrnjavac, a biologist at the University of Düsseldorf and lead author of the study, in comments distributed via the research institution's press release.”
A world run on metal, not enzymes
Much of modern biochemistry runs on enzymes, proteins that speed up chemical reactions, and on energy-carrying molecules such as ATP. Neither would have existed at life's earliest stage. The study found that the last universal common ancestor of all cells, known as LUCA, appears to have had enzymes for only about half of these reactions, with the other reactions likely carried out by naturally occurring metals in the environment where LUCA lived. That suggested early life depended much more on its surroundings than modern cells do.
Harun Tüysüz, an inorganic chemist and co-author of the study, explained that metals that naturally occur in hydrothermal vents can replace a surprisingly large number of enzymes in metabolism. Manon Schlikker, another member of the Düsseldorf team, said the researchers had identified palladium, a metal prized by industrial chemists as a catalyst, as a plausible early energy source that could have driven reactions before enzymes existed to do the job.
William Martin, the study's senior author, put the implication directly. "The new data leave only one conclusion. The bacteria and archaeal lineages made the transition to the free-living state independently. Only free-living cells are alive." He added: "Let's call it by name: we are looking at one origin of the genetic code, but two origins of life."
A hypothesis, not yet a settled fact
The idea is striking, but it remains a hypothesis rather than a proven history. Scientific American, reporting on the same paper, noted that by comparing genomes representing all groups of bacteria and archaea, Martin and his colleagues found that these two branches of life shared common genes for about half of the enzymes needed for these basic reactions, with the rest unique to either bacteria or archaea, evidence that the two lineages, in Mrnjavac's words, "traversed the path to free-living cells, at least in part, independently." Other science writers covering the paper have also flagged that gaps in archaeal data and laboratory evidence still limit the conclusion.
For readers across Europe, where German university research has long shaped debates on the chemical origins of life, the study adds a fresh twist to a question that has occupied biologists since Darwin: not just where life began, but how many times the step from chemistry into independent, self-sustaining cells actually happened. If confirmed by further genomic and laboratory work, the finding would not overturn the idea of a single common ancestor, but it would suggest that the final step from prebiotic chemistry to biology was crossed more than once.
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