Scientists Discover a 3.7-Billion-Year-Old Secret of Early Life

Scientists funded by NASA have found that organisms living on Earth more than 3 billion years ago were already using molybdenum, even though the metal was extremely rare in the environment at the time. The research, published in Nature Communications, is the first study to show that ancient life depended on molybdenum this early in Earth's history.

Today, molybdenum sits inside cells as part of essential enzymes that speed up major carbon, nitrogen and sulfur reactions — chemistry that matters not only for individual organisms but for planet-scale cycles such as the nitrogen cycle. Without molybdenum, these reactions could still occur naturally, but far too slowly to support life. "Molybdenum sits at the catalytic center of enzymes that run major carbon, nitrogen, and sulfur reactions," said Betul Kacar, head of the Kacar Lab at the University of Wisconsin-Madison and senior author of the study. "Asking when life began using molybdenum is really asking when some of the most consequential metabolic strategies became possible."

Molybdenum is fairly widespread in the environment today, but Earth's oceans contained only tiny amounts of it billions of years ago; its availability rose around the time microbes began using photosynthesis, a shift that eventually caused a large increase in atmospheric oxygen roughly 2.45 billion years ago, known as the Great Oxidation Event. Because the metal was so scarce on early Earth, astrobiologists had considered whether life first relied on tungsten, which behaves similarly inside cells and is still used by some organisms in extreme environments today, before later shifting to molybdenum as it became more available.

The new research suggests that story is not so simple. By compiling evidence on how much molybdenum was available through time and reconstructing the history of its use across the tree of life, the team found that ancient microbes managed to use molybdenum even when it was rare — and the same pattern applies to tungsten. "Our work shows that both molybdenum- and tungsten-using enzyme systems have Archean roots, which suggests that early life likely worked with both metals rather than following a simple 'tungsten first, molybdenum later' story," Kacar said. Molecular dating places molybdenum use back to roughly 3.7 to 3.1 billion years ago, well before the Great Oxidation Event.

Earlier research from the same NASA-funded consortium, MUSE, had pointed to hydrothermal vents on the seafloor as a source of trace metals, including molybdenum, for early organisms even when Archean seawater held little of the metal overall. "Scarcity did not make molybdenum unimportant; its catalytic advantages may have made it worth evolving ways to acquire and use," Kacar said.