Life on Earth Evolved Twice From a Common Genetic Code

Life on Earth may have originated twice, according to a study published in Science Advances. Researchers from Heinrich Heine University Düsseldorf argue that while the genetic code emerged once, the transition to free-living cells happened independently for bacteria and archaea, meaning the Last Universal Common Ancestor (LUCA) was not actually a living organism.

Why LUCA May Not Have Been ‘Alive’

For decades, biologists assumed a single organism sat at the base of all ancestral lines. However, Natalia Mrnjavac, a PhD student at Heinrich Heine University Düsseldorf, questions if LUCA deserves to be called alive. According to the study, LUCA was likely a cluster of RNA, DNA, and enzymes trapped within the rocks of hydrothermal vents, lacking the boundaries required to be a free-living cell.

Professor William Martin, an evolutionary biologist at the same university, states that “only free-living cells are alive.” He argues that we are seeing one origin of the genetic code, but two distinct origins of life. Under this model, bacteria and archaea emerged as “pioneer” cells making their first attempts at life outside the confines of hydrothermal vents roughly 4 billion years ago.

The Metabolic Divide Between Bacteria and Archaea

The research team analyzed 420 chemical reactions used to build the necessities of life from early Earth chemicals. They found that the enzymes catalyzing these reactions are not conserved across the divide between bacteria and archaea. According to Professor Martin, LUCA possessed enzymes for only about half of these metabolic reactions.

The other half of these reactions were catalyzed by metals in the environment. This suggests that the two lineages independently evolved structurally distinct enzymes to replace those inorganic catalysts. ScienceAlert reports that this “radical” finding implies the two branches of life independently figured out the secrets of metabolism to upgrade from non-living matter to living cells.

Comparison of Metabolic Evolution

Palladium and the Energy Problem

Modern metabolism relies on adenosine triphosphate (ATP) for energy. Because ATP is too complex to exist without enzymes, researchers sought an alternative for early life. The study identifies palladium as a potential catalyst that could perform the reactions today handled by ATP and enzymes, specifically converting phosphite to phosphate.

Manon Schlikker, a PhD student involved in the study, describes this finding as a way to make early evolution easier to grasp. While palladium is now rare and expensive, the researchers argue it only needed to provide the initial energy resource to get the process started. As genetic activity increased, more common metals like iron and nickel took over other reactions before eventually being replaced by more targeted enzymes.

This reliance on metals strongly supports the theory that hydrothermal vents, which are rich in minerals, served as the cradles of life rather than lakes or ponds.

Frequently Asked Questions

Was there only one common ancestor?

Yes, the researchers still acknowledge a Last Universal Common Ancestor (LUCA), but they argue it was a collection of genetic code and chemicals rather than a living, free-living cell.

Why are bacteria and archaea different?

The study found that they evolved different enzymes to catalyze the same essential metabolic reactions, suggesting they transitioned to independent life separately.

What role did metals play in early life?

Metals like palladium, iron, and nickel acted as catalysts for chemical reactions before the evolution of proteins (enzymes) that could perform those tasks more specifically.