Metabolic processes that underpin life on Earth have arisen spontaneously outside of cells. The serendipitous finding that metabolism – the cascade of reactions in all cells that provides them with the raw materials they need to survive – can happen in such simple conditions provides fresh insights into how the first life formed. It also suggests that the complex processes needed for life may have surprisingly humble origins
[pullquote]”People have said that these pathways look so complex they couldn’t form by environmental chemistry alone,” says Markus Ralser at the University of Cambridge who supervised the research.[/pullquote]
But his findings suggest that many of these reactions could have occurred spontaneously in Earth’s early oceans, catalysed by metal ions rather than the enzymes that drive them in cells today.
The origin of metabolism is a major gap in our understanding of the emergence of life. “If you look at many different organisms from around the world, this network of reactions always looks very similar, suggesting that it must have come into place very early on in evolution, but no one knew precisely when or how,” says Ralser.
One theory is that RNA was the first building block of life because it helps to produce the enzymes that could catalyse complex sequences of reactions. Another possibility is that metabolism came first; perhaps even generating the molecules needed to make RNA, and that cells later incorporated these processes – but there was little evidence to support this.
“This is the first experiment showing that it is possible to create metabolic networks in the absence of RNA,” Ralser says.
Remarkably, the discovery was an accident, stumbled on during routine quality control testing of the medium used to culture cells at Ralser’s laboratory. As a shortcut, one of his students decided to run unused media through a mass spectrometer, which spotted a signal for pyruvate – an end product of a metabolic pathway called glycolysis.
To test whether the same processes could have helped spark life on Earth, they approached colleagues in the Earth sciences department who had been working on reconstructing the chemistry of the Archean Ocean, which covered the planet almost 4 billion years ago. This was an oxygen-free world, predating photosynthesis, when the waters were rich in iron, as well as other metals and phosphate. All these substances could potentially facilitate chemical reactions like the ones seen in modern cells.