Life on Earth is thought to have emerged from the primordial ooze some 4 billion years ago. Now, a provocative new study argues that this miraculous transition from chemistry to biology happened not once, but twice.

Bacteria and archaea, the first two branches of the tree of life, each sprung separately, according to the study, published in the journal Science Advances. Their last common ancestor, known as LUCA (for “Last Universal Common Ancestor” of all life), was in fact not fully alive—it depended on metals deposited in hydrothermal vents to run its metabolism.

Over time and with countless chemical combinations, the bacteria and archaea that evolved from LUCA found ways to make their own internal enzymes to catalyze the chemical reactions needed for life, says study senior author William Martin, an evolutionary biologist at Heinrich Heine University in Düsseldorf.

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“We’re getting a view into the phase of evolution where these very primitive proto-organisms were inventing enzymes that enabled them to cut lose from the crust and become a free-living cell,” Martin says.

Genetic data suggests that LUCA existed about 4.2 billion years ago, during the Hadean eon when Earth’s surface was still hot. Only one set of Canadian rocks might date back to this era of Earth history—but their provenance is hotly debated and otherwise, geological evidence is sparse. The oldest uncontested fossilized life ever found are 3.5 billion-year-old microbial mats called stromatolites that must have evolved from simpler ancestors.

Some researchers have argued that LUCA was a complex cell with a primitive immune system to protect against viruses. But the genetic methods they use can be misleading when peering back through over 4 billion years of evolution, Martin says. He and his colleagues employ a metabolic lens, focusing on what genes are needed to convert the components of early Earth such as hydrogen, ammonia, carbon dioxide, water, hydrogen sulfide and phosphate into the 20 amino acids, 8 nucleotide bases and 20 cofactors that keep cells humming. “It’s only 420 chemical reactions,” Martin says.

By comparing genomes representing all groups of bacteria and archaea, Martin and his colleagues found that these two branches of life shared common genes for about half of the enzymes needed for these basic reactions. The rest were unique to either bacteria or to archaea, suggesting that these two forms of life “traversed the path to free-living cells at least in part independently,” says Natalia Mrnjavac, a graduate student in Martin’s lab and the paper’s first author.

In other words, bacteria and archaea didn’t split off on their own paths from LUCA; instead, LUCA would have been a cluster of RNA, DNA and enzymes stuck inside the rock in hydrothermal vents, dependent on the metals from these vents to make amino acids. Genes from various clusters could percolate through rocks, leading to a sort of evolutionary experiment that might eventually have led to life as the clusters replaced metal-catalyzed processes with their own enzymatic ones.

Martin’s lab has shown that metals can replace biological enzymes in many basic metabolic processes, bolstering the case. In the new study, the researchers found that palladium can catalyze a crucial reaction from phosphite (abundant at hydrothermal vents) to phosphate (crucial to cellular processes) that releases energy and could have powered LUCA.

“The idea that environment and geochemistry can provide what biology has not yet invented is very plausible,” says Betül Kaçar, the director of a NASA astrobiology center and an assistant professor at the University of Wisconsin-Madison who was not involved in the study. But modern life’s last common ancestor is not the same thing as the origin of life, she cautions. How chemistry and geology passed the baton to biology remains unknown. Further experiments will be needed to test the ideas raised by this study.

“Biologists have a tall order ahead of them,” she says. “There’s a lot we need to be doing in order to understand the nature of the last common ancestor and the role of bacteria and archaea.”