Scientists Discover Evidence of Two Origins of Life on Earth
A recent study suggests that the first free-living cells may have emerged independently twice, with life sharing one ancient genetic code but undergoing two separate transitions into free-living existence.
The origins of life on Earth remain one of the greatest mysteries of our time. Scientists have long been searching for answers, and a recent study has shed new light on this complex question.
## Reconstructing the Chemistry of Early Life
The study, led by biologists at Heinrich Heine University Düsseldorf, examined the chemical reaction network used by the earliest cells to produce the basic ingredients of life. 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, which included scientists from around the world, focused on the complete group of chemical reactions that cells use to manufacture key biological components from materials available on the early Earth. These 420 chemical reactions form the metabolic network known as metabolism, and the researchers were surprised to find that the enzymes responsible for carrying out those reactions do not show the same degree of conservation between bacteria and archaea.
## From Metal Catalysts to Enzymes
The study's major advances include the reconstruction of four stages in the early evolution of biological catalysis. The process appears to have begun with reactions driven entirely by metals, followed by a stage in which metals and enzymes worked together. Afterward, bacteria and archaea began moving along separate evolutionary paths, with newly evolved enzymes gradually replacing the inorganic catalysts supplied by the environment where metabolism had first developed.
Importantly, the researchers identified examples in which bacteria and archaea appear to have independently developed different enzymes capable of performing the same essential metabolic task. This parallel evolution may have been crucial because it could have allowed the two lineages to become less dependent on the chemistry of hydrothermal vents and eventually survive as independent cells.
## How Early Metabolism May Have Been Powered
Another major question concerns energy. Modern cells rely heavily on ATP to power metabolism, but ATP is itself a complex molecule produced with the help of enzymes. The researchers therefore investigated what could have supplied energy before ATP-based metabolism existed.
They found that phosphite, a form of phosphorus naturally found in hydrothermal vents, can react with organic compounds in the presence of palladium and produce reactions associated with metabolic phosphorylation. This result provides a possible explanation for how some of the earliest metabolic reactions could have obtained the energy needed to proceed before modern biological energy systems evolved.
## Mapping 420 Reactions at the Origin of Metabolism
The research is the first study specifically focused on the full reaction network known as metabolism. The network contains 420 highly connected reactions, and many of the same compounds participate in multiple parts of the system. The researchers developed a method that allowed them to arrange metabolic reactions from the simplest to the most complex, which may reflect, at least in part, the order in which those reactions appeared during the earliest stages of biological evolution.
## One Genetic Code, but Two Origins of Life
The broader implication of the researchers' findings is that bacteria and archaea did not make the transition to independent cellular life together. Instead, each lineage appears to have reached the free-living state separately. According to the researchers, 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.
The study's findings have significant implications for our understanding of the origins of life on Earth. By shedding new light on the chemical reaction network used by the earliest cells, the researchers have provided a new perspective on the complex question of how life began.
