The first single-celled organisms capable of living independently on Earth did not arise just once, but evolved on two separate occasions, according to an international team of researchers led by scientists at Heinrich Heine University Düsseldorf in Germany.
The study, published in the peer-reviewed journal Science Advances, shows that bacteria and archaea, the two oldest major groups of single-celled microorganisms on the planet, independently developed the ability to survive outside the deep-sea hydrothermal systems where their shared ancestors first evolved.
Although both evolutionary lineages originated from a single common ancestor, the researchers found that each group followed its own distinct chemical path to achieve free-living cell status.
To reach their conclusions, the research team went beyond comparing genetic sequences. They analyzed complete genomes, protein structures, and 420 chemical reactions that comprise cellular metabolism, which is the network of processes that enables living cells to manufacture essential compounds such as amino acids, vitamins, and the chemical building blocks of RNA.
Their analysis revealed that the Last Universal Common Ancestor, known as LUCA, which is considered the hypothetical progenitor of all modern living organisms, possessed enzymes for only about half of those 420 metabolic reactions.
The remaining half of the metabolic reactions depended entirely on naturally occurring metals present in the hydrothermal environment where LUCA lived. Lead study author William Martin said the surprising finding was that the enzymes catalyzing those specific reactions were not preserved across the evolutionary divide that separates bacteria from archaea.
Martin explained that LUCA possessed enzymes for only around half of its metabolic reactions, while the other half relied on environmental metals at the site of its origin. The researchers noted that this heavy environmental reliance proves LUCA was not yet a fully self-sufficient cell, but rather an organism tightly linked to the specific chemical conditions found in hydrothermal systems.
Evolutionary phases of cellular catalysts
The investigation also demonstrated that metals played a far more crucial role during the initial stages of biological evolution than scientists previously understood.
The scientists identified four distinct phases in the evolution of biological catalysis, which is the process that speeds up chemical reactions inside cells. In the first phase, only environmental metals catalyzed chemical reactions. This was followed by a hybrid second phase where environmental metals and early biological enzymes coexisted.
In the third and fourth phases, the evolutionary ancestors of bacteria and archaea branched onto separate pathways. Each group independently created new enzymes capable of gradually replacing the inorganic metal catalysts.
Lead author and biologist Natalia Mrnjavac noted that researchers can observe clear instances where the ancestors of bacteria and archaea independently developed enzymes with completely different structural forms to catalyze the exact same essential metabolic reaction.
Mrnjavac stated that these parallel chemical inventions likely paved the way for bacteria and archaea to emerge independently as free-living organisms capable of inhabiting broader environments.
Alternative energy source before ATP
A further key discovery from the study centers on the energy source that powered the earliest metabolic reactions before modern cellular mechanisms existed.
Modern living cells rely primarily on adenosine triphosphate, or ATP, to store and transfer chemical energy within the cell. However, ATP itself requires complex enzymes to form, meaning it was unlikely to be available during the earliest stages of life on Earth.
During laboratory experiments, the researchers demonstrated that phosphite, an inorganic form of phosphorus naturally present in hydrothermal systems, reacted together with the metal element palladium to produce metabolic phosphorylation reactions overnight in water without requiring ATP or enzymes.
Researcher Manon Schlikker stated that the team identified a previously unknown energy source at the very origin of cellular metabolism.
Schlikker explained that reacting phosphite with organic compounds produced metabolic phosphorylation reactions overnight in water, with phosphite and palladium effectively replacing ATP and biological enzymes. She added that this unexpected substitution makes the early stages of evolution much easier to comprehend.
Based on these findings, the study authors concluded that while all cellular life shares a single original genetic code, the first cells capable of independent existence evolved separately in the distinct lineages that led to modern bacteria and archaea.
