Billions of years ago, some unknown location on the sterile, primordial Earth became a cauldron of complex organic molecules from which the first cells emerged. Origin-of-life researchers have proposed countless imaginative ideas about how that occurred and where the necessary raw ingredients came from. Some of the most difficult to account for are proteins, the critical backbones of cellular chemistry, because in nature today they are made exclusively by living cells. How did the first protein form without life to make it?
Scientists have mostly looked for clues on Earth. Yet a new discovery suggests that the answer could be found beyond the sky, inside dark interstellar clouds.
Last month in Nature Astronomy, a group of astrobiologists showed that peptides, the molecular subunits of proteins, can spontaneously form on solid, frozen particles of cosmic dust drifting through the universe. Those peptides could in theory have traveled inside comets and meteorites to the young Earth — and to other worlds — to become some of the starting materials for life.
The simplicity and favorable thermodynamics of this new space-based mechanism for forming peptides make it a more promising alternative to the known purely chemical processes that could have occurred on a lifeless Earth, according to Serge Krasnokutski, the lead author on the new paper and a researcher at the Max Planck Institute for Astronomy and the Friedrich Schiller University in Germany. And that simplicity “suggests that proteins were among the first molecules involved in the evolutionary process leading to life,” he said.
Whether those peptides could have survived their arduous trek from space and contributed meaningfully to the origin of life is very much an open question. Paul Falkowski, a professor at the School of Environmental and Biological Sciences at Rutgers University, said that the chemistry demonstrated in the new paper is “very cool” but “does not yet bridge the phenomenal gap between proto-prebiotic chemistry and the first evidence of life . ” He added, “There’s a spark that’s still missing.”
Still, the finding by Krasnokutski and his colleagues shows that peptides might be a much more readily available resource throughout the universe than scientists believed, a possibility that could also have consequences for the prospects for life elsewhere.
Cosmic Dust in a Vacuum
Cells make the production of proteins look easy. They manufacture both peptides and proteins extravagantly, empowered by environments rich in useful molecules like amino acids and their own stockpiles of genetic instructions and catalytic enzymes (which are themselves typically proteins).
But before cells existed, there was no easy way to do it on Earth, Krasnokutski said. Without any of the enzymes that biochemistry provides, the production of peptides is an inefficient two-step process that involves first making amino acids and then removing water as the amino acids link up into chains in a process called polymerization. Both steps have a high energy barrier, so they occur only if large amounts of energy are available to help kick-start the reaction.
Because of these requirements, most theories about the origin of proteins have either centered on scenarios in extreme environments, such as near hydrothermal vents on the ocean floor, or assumed the presence of molecules like RNA with catalytic properties that could lower the energy barrier enough to push the reactions forward. (The most popular origin-of-life theory proposes that RNA preceded all other molecules, including proteins.) And even under those circumstances, Krasnokutski says that “special conditions” would be needed to concentrate the amino acids enough for polymerization. Though there have been many proposals, it is not clear how and where those conditions could have arisen on the primordial Earth.