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Scientists have found evidence suggesting that the earliest free-living cells on Earth may have emerged independently in two distinct forms, giving rise to the ancestors of bacteria and archaea.
An international team led by biologists at Heinrich Heine University Düsseldorf examined the chemical reactions and enzymes involved in the earliest stages of cellular life. Their findings, published in Science Advances, provide new clues about how the first cells produced essential biological components and obtained energy.
The researchers suggest that around 4 billion years ago, two distinct types of primitive cells may have emerged as life began developing beyond hydrothermal vent environments.
“We would see two very different kinds of cells emerging, pioneer bacteria and pioneer archaea,” said Natalia Mrnjavac, a biologist at the University of Düsseldorf and lead author of the study.
The team analysed genomes, protein structures and chemical reactions to investigate microbial evolution, including the period when early cells were becoming less dependent on their surroundings.
Researchers examined around 420 chemical reactions that modern cells use to produce essential substances such as amino acids, RNA building blocks and vitamins. These reactions rely on materials believed to have been available on the early Earth, including hydrogen, ammonia and carbon dioxide.
Together, the reactions form a basic metabolic network. While the researchers found that these reactions are extremely ancient and widespread across life, the enzymes responsible for carrying them out are not equally conserved between bacteria and archaea.
William Martin, a biologist at HHU and senior author of the study, said the last universal common ancestor of all modern cells, known as LUCA, appears to have possessed enzymes capable of carrying out only about half of these reactions.
The remaining reactions may have been facilitated by naturally occurring metals in the environment where LUCA lived, according to the researchers.
The finding suggests that the earliest forms of life were far more dependent on their surroundings than modern cells.
“Metals that naturally occur in hydrothermal vents can replace a surprisingly large number of enzymes in metabolism,” said Harun Tüysüz, an inorganic chemist and co-author of the study.
Joseph Moran of the University of Ottawa said early biochemical evolution appears to have involved a combination of enzyme-driven and metal-driven chemical reactions.
The researchers also identified instances in which bacteria and archaea appear to have independently developed different enzymes to perform the same essential metabolic functions.
Mrnjavac said these parallel developments may have contributed to the emergence of bacteria and archaea as distinct forms of free-living life.
The findings do not overturn the idea that all modern life shares a deep common ancestry. Instead, they suggest that the transition to free-living cells may have occurred more than once, with bacteria and archaea potentially developing independently from primitive cellular systems.
The study offers a new perspective on one of science’s biggest questions: how the first independent forms of life emerged from the chemistry of the early Earth.