In our initial Origins essay looking at the origin of life on Earth, Carl Zimmer discussed research on how the key genetic molecule RNA may have arisen from an abiotic broth. Part of the discussion centered on the RNA work of John Sutherland of the University of Manchester in the U.K., some of which is being published today in Nature. Here's the relevant excerpt from our essay:
Step 1: Make RNA
An RNA molecule is a chain of linked nucleotides. Each nucleotide in turn consists of three parts: a base (which functions as a "letter" in a gene's recipe), a sugar molecule, and a cluster of phosphorus and oxygen atoms, which link one sugar to the next. For years, researchers have tried in vain to synthesize RNA by producing sugars and bases, joining them together, and then adding phosphates. "It just doesn't work," says Sutherland.
This failure has led scientists to consider two other hypotheses about how RNA came to be. Cleaves and others think RNA-based life may have evolved from organisms that used a different genetic material—one no longer found in nature. Chemists have been able to use other compounds to build backbones for nucleotides (Science, 17 November 2000, p. 1306). They're now investigating whether these humanmade genetic molecules, called PNA and TNA, could have emerged on their own on the early Earth more easily than RNA. According to this hypothesis, RNA evolved later and replaced the earlier molecule.
But it could also be that RNA wasn't put together the way scientists have thought. "If you want to get from Boston to New York, there is an obvious way to go. But if you can't get there that way, there are other ways you could go," says Sutherland. He and his colleagues have been trying to build RNA from simple organic compounds, such as formaldehyde, that existed on Earth before life began. They find they make better progress toward producing RNA if they combine the components of sugars and the components of bases together instead of separately making complete sugars and bases first. Over the past few years, they have documented almost an entire route from prebiotic molecules to RNA and are preparing to publish even more details of their success. Discovering these new reactions makes Sutherland suspect it wouldn't have been that hard for RNA to emerge directly from an organic soup. "We've got the molecules in our sights," he says.
Sutherland can't say for sure where these reactions took place on the early Earth, but he notes that they work well at the temperatures and pH levels found in ponds. If those ponds dried up temporarily, they would concentrate the nucleotides, making conditions for life even more favorable.
Were these Darwin's warm little ponds? "It might just be that he wasn't too far off," says Sutherland.