Scientists have found evidence suggesting that the first free-living cells on Earth may have emerged independently in two different forms — the ancestors of bacteria and archaea.
An international team led by biologists at Heinrich Heine University Düsseldorf (HHU) examined the chemical reactions and enzymes involved in the earliest stages of life. Their findings, published in Science Advances, offer new clues about how the first cells produced essential substances and obtained energy.
The researchers believe that around 4 billion years ago, two distinct types of primitive cells may have developed as life began moving beyond hydrothermal vents.
“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 compared genomes, protein structures and chemical reactions to explore microbial evolution, including the period before cells became fully independent from their environment.
Their study focused on about 420 chemical reactions that cells use to make essential components such as amino acids, RNA building blocks and vitamins. These reactions use substances that were available on the early Earth, including hydrogen, ammonia and carbon dioxide.
Together, the reactions make up the basic metabolic network. The researchers found that while these reactions are extremely ancient and shared widely across life, the enzymes that perform them 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 cells, known as LUCA, appears to have had enzymes for only about half of these reactions.
The other reactions were likely carried out by naturally occurring metals in the environment where LUCA lived, the researchers said.
This suggests that early life depended much more on its surroundings than modern cells do.
“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-based and metal-based chemical reactions.
The researchers also found cases in which bacteria and archaea appear to have developed different enzymes independently to perform the same essential metabolic functions.
Mrnjavac said these parallel developments may have helped bacteria and archaea emerge as separate forms of free-living life.
The findings suggest that although all life today shares a common genetic code and deep evolutionary connections, the earliest free-living cells may have had more than one independent origin.
Source: Science Daily