THE search for life in space just got a little sweeter. Sunlight hitting ice grains in the early solar system may have formed sugar molecules on their surfaces. Those sugars include ribose, the backbone of RNA.
All known life makes some use of RNA as a genetic material, and ribose, the “R” in RNA, holds the compounds that encode genetic messages. But it was unclear how ribose could be made without living organisms.
Other components of cells, such as amino acids, have been showing up for years in lab simulations of the early Earth and in meteorite samples. So have molecules that resemble cell membranes. Had all these coexisted with ribose, it could have set the stage for life to evolve.
But sugars like ribose often bind in a way that makes them hard to come by individually. “Sugars like to react with each other,” says Cornelia Meinert at the University of Nice Sophia Antipolis in France. “In the end, everything is brown like caramel.”
“It could all have been made in space, then dropped on Earth just in time for the genesis of life“
Meinert’s team has now made ribose by shining ultraviolet light on a frozen blend of water, methanol and ammonia (Science, doi.org/bd5p). This mixture represents the solar system in its infancy, before tiny grains of dust and ice collapsed into planets.
“It’s another example of how the universe seems to be hardwired to produce a lot of the kinds of compounds you would like to be around if you want to get life going,” says Scott Sandford of the NASA Ames Research Center in California. Sandford says his own team is reporting similar results in a forthcoming paper.
Whether sugars are made on real interstellar ice grains is still an open question. Checking the surfaces of comets or meteorites may help resolve it, because these grains are preserved if they settle gently on small bodies far from the sun. Radio telescopes and the European Space Agency’s Rosetta mission have picked up simple sugars on comets before, but they may struggle to find something complex like ribose, Meinert says.
Finding complex sugars on comets would tell us that amino acids, membrane molecules and ribose could all have been made in space, then dropped on Earth in time for the genesis of life.
And if ribose is relatively easy to come by, it would boost the “RNA world” theory: the idea that DNA evolved from an earlier ecosystem populated by organisms made of RNA. How RNA-based life itself arose isn’t clear, though. “Just because now you have all the molecules doesn’t mean you have life,” Meinert says.
Still, it doesn’t hurt that so many ingredients for life seem to have been present in the early solar system, and so presumably at the birth of other planetary systems. “If you think of all these little molecules we’re making as Lego blocks, and life as a kind of very complex, organised Lego castle, the fact that Lego blocks are falling out of the sky can’t be a bad thing,” Sandford says.
This article appeared in print under the headline “Sugar-coated clue to life’s origins”