The search for life beyond Earth has gained a new point of reference. A study by an international team of biologists suggests that the first free-living cellular lineages on our planet may have arisen not once, but twice. If life began along two separate pathways even on Earth, then its emergence on other worlds may be more likely than previously thought.
Origin of Independent Cells
Scientists from the Institute of Molecular Evolution at Heinrich Heine University Düsseldorf analyzed genomes, protein structures, and chemical reactions to reconstruct the earliest stages of metabolic development. The results were published in Science Advances.
The main focus of the study was a network of 420 chemical reactions through which cells produce amino acids, RNA bases, and vitamins from hydrogen, ammonia, and carbon dioxide. These reactions are extremely ancient and have been preserved in life in much the same way as the genetic code.
A Surprise Involving Enzymes
The researchers found that the last universal common ancestor of all cells, known as LUCA, had enzymes for only half of the metabolic reactions, ScienceDaily reports. The rest were catalyzed by metals in the environment where LUCA arose. This means that the earliest metabolism depended on the environment far more strongly than modern metabolism does.
“Metals that are present in hydrothermal vents can replace a surprisingly large number of enzymes in metabolism,” explains study co-author Harun Tüysüz of the Max Planck Institute for Coal Research.
When bacteria and archaea went their separate ways, each lineage began replacing inorganic catalysts with its own enzymes. The scientists identified at least five cases in which both groups independently evolved structurally different enzymes for the same metabolic reaction.
An Energy Source Before ATP
A separate question concerned energy. Modern cells depend on ATP, but this complex molecule could not have been freely available on the early Earth. The team found an answer in phosphite, a form of phosphorus that occurs naturally in hydrothermal systems.
Under laboratory conditions, phosphite in water reacted with organic compounds in the presence of palladium and enabled phosphorylation reactions. The experiments showed that phosphite and palladium can substitute for ATP and enzymes. This may explain where the earliest metabolic reactions obtained energy before the emergence of modern biological systems.
Life on Other Worlds
The team’s conclusion was formulated by senior study author William Martin of Heinrich Heine University Düsseldorf. The bacterial and archaeal lineages made the transition to a free-living state independently. “Let’s call it what it is: we see one origin of the genetic code, but two origins of life,” he says.
For astrobiology, this means that hydrothermal systems are promising places to search for extraterrestrial life. Similar conditions may exist on icy moons such as Enceladus and Europa, where oceans are in contact with rock. If metabolism on Earth gained independence from the environment twice, then the likelihood of this process being repeated on other worlds may be higher than previous models predicted. This broadens the range of biomarkers that should be considered when observing exoplanets.
