New Delhi: Life on Earth may have taken two separate paths to become free-living, with the ancestors of bacteria and archaea independently developing the ability to survive outside the chemical environment of hydrothermal vents, states a study.
The study, titled “Intermediate stages in the origin of metabolism at a phosphorylating hydrothermal vent”, was led by researchers at Heinrich Heine University Düsseldorf and was published in Science Advances on 5 August.
The findings suggest that while all life may share an ancient genetic code, bacteria and archaea may have evolved separately into independent cells. Researchers examined genes, protein structures and chemical reactions involved in some of the earliest stages of life. They built a network of around 420 reactions by which cells make key compounds such as amino acids and nucleotides.
The researchers found that the enzymes (proteins that speed up chemical reactions in cells) involved in these processes are not equally conserved between bacteria and archaea. This implies that the last universal common ancestor (LUCA), the hypothetical ancestor shared by all living cells, relied in part on metals in its environment to perform chemical reactions that modern cells perform using enzymes.
The study suggests early metabolism developed in stages. At first, chemical reactions may have been driven mainly by metals present around hydrothermal vents. LUCA later appears to have used a combination of metals and enzymes. Bacteria and archaea then evolved separately, developing their own enzymes and becoming less dependent on the surrounding environment.
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‘Two origins of life’
Natalia Mrnjavac, a biologist at Heinrich Heine University Düsseldorf and lead author of the study, said the researchers found cases where the ancestors of bacteria and archaea independently developed structurally different enzymes to perform the same essential metabolic reaction.
“Such parallel inventions could have paved the way to the independent emergence of free-living bacteria and archaea,” Mrnjavac said.
The team also explored how the early metabolism could have acquired energy before the advent of adenosine triphosphate, or ATP, that now sits at the heart of modern cellular processes.
Researchers discovered that phosphite, a form of phosphorus found in hydrothermal environments, can react with organic compounds in the presence of palladium to create reactions that transfer energy within cells. This implies that some of the earliest chemical reactions may have been driven by naturally occurring chemicals and metals before the introduction of modern energy systems.
Using a mathematical method, the researchers also arranged the 420 reactions from simpler to more complex. This could help scientists understand the order in which parts of early metabolism developed.
William F Martin, a biologist at Heinrich Heine University Düsseldorf and senior author of the study, said the researchers found that LUCA had enzymes for only about half of the reactions in metabolism, while metals in its environment may have catalysed the rest.
“We are looking at one origin of the genetic code, but two origins of life,” Martin said.
The researchers do not mean that two completely unrelated forms of life appeared independently on Earth. Rather, their results suggest that the ancestors of bacteria and archaea may have transitioned from being dependent on hydrothermal-vent chemistry to being free-living cells themselves.
(Edited by Saptak Datta)