About four billion years ago, two very different kinds of primitive cells may have made their first attempts at life outside hydrothermal vents: pioneer bacteria and pioneer archaea. A new study published in Science Advances argues that these two branches of life reached the free-living state independently — challenging the assumption that all cellular life descends from a single free-living ancestor.
Led by Natalia Mrnjavac and William Martin at Heinrich Heine University Düsseldorf, an international team reconstructed the complete network of 420 metabolic reactions cells use to build amino acids, RNA bases and vitamins from ingredients available on the early Earth — hydrogen, ammonia and CO2.
The surprise came when they compared the enzymes that catalyze those reactions across the bacterial-archaeal divide. LUCA, the last universal common ancestor, appears to have possessed enzymes for only about half of the network; the other half was driven by metals naturally present in hydrothermal vents. 'The closer we look, the more clearly we can see that early biochemical evolution was a hybrid of enzymatic and metal catalysts,' says co-author Joseph Moran of the University of Ottawa.
The team reconstructed four stages in the evolution of biological catalysis: reactions driven entirely by metals, then a hybrid phase in LUCA, then separate bacterial and archaeal paths in which each lineage independently evolved structurally distinct enzymes for the same essential reactions. 'Such parallel inventions could have paved the way to the independent emergence of free-living bacteria and archaea,' says Mrnjavac.
The study also identified a candidate energy source before ATP existed: phosphite — a phosphorus compound found in hydrothermal vents — reacts with organic compounds in the presence of palladium to drive metabolic phosphorylation reactions overnight in water, replacing both ATP and enzymes.
'The new data leave only one conclusion,' says Martin. 'The bacterial and archaeal lineages made the transition to the free-living state independently. Only free-living cells are alive. We are looking at one origin of the genetic code, but two origins of life.'
