Scientists Find Clues to Two Independent Origins of Life

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 life. The findings, published in Science Advances, offer new insights into how primitive cells produced essential biological substances and generated energy.

The researchers suggest that about 4 billion years ago, two distinct types of primitive cells may have emerged as life began moving 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 transitioning from dependence on their surroundings to becoming more self-sufficient.

The researchers focused on about 420 chemical reactions that cells use to produce essential compounds such as amino acids, RNA building blocks and vitamins. These reactions use substances believed to have been available on the early Earth, including hydrogen, ammonia and carbon dioxide.

Together, these reactions form a basic metabolic network. The researchers found that although the reactions are extremely ancient and widely shared across living organisms, the enzymes that carry 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 for only about half of these reactions.

The remaining reactions may have been facilitated by naturally occurring metals in the environment where LUCA lived, the researchers said.

This suggests that early life may have been much more dependent on its 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 found examples in which bacteria and archaea appear to have independently developed different enzymes to perform the same essential metabolic functions.

These parallel developments may have helped the two groups emerge as separate forms of free-living life, Mrnjavac said.

The findings do not necessarily mean that bacteria and archaea had completely unrelated beginnings. All modern life shares a common genetic code and deep evolutionary connections. Instead, the study suggests that the earliest transition to independent, free-living cells may have occurred through more than one evolutionary pathway.

The research provides new clues to how simple chemical systems on the early Earth may have developed into the first independent forms of life.