There's New Evidence for an Alternate Theory of the Origin of Life
How the building blocks of life helped build each other.
The first biological molecules may have emerged in a steaming hot spring like this one in Yellowstone National Park. Image: Wikimedia
Four billion years ago, primordial Earth was a hellish wasteland, and life as we know it didn't exist. But scattered across a radiation soaked, asteroid-pummeled planet, steaming cauldrons of cosmic chemicals were busy cooking up life's building blocks.
The emergence of complex biological molecules, long before the first cells, is one of the most mysterious chapters in the origins of life story. One popular theory suggests that RNA—the more active, single stranded version of DNA—came on the scene first, making copies of itself and building the first proteins. But new research, which appears this week in the Proceedings of the National Academies of Sciences, refutes the idea that RNA alone jumpstarted life. Rather, RNA and peptides, the amino acid chains that build proteins, probably co-created each other.
"The collaboration between RNA and peptides was likely necessary for the spontaneous emergence of complexity," said biochemist Charles Carter of the University of North Carolina School of Medicine in a statement. "In our view, it was a peptide-RNA world, not an RNA-only world."
Carter's theory flies in the face of decades of "RNA world" research and, if correct, could have profound implications for the emergence of life on other worlds.
"The RNA world hypothesis is a shortcut to life that says, hey, you need catalysis and you need information transmission, so why not have RNA do all the work?" Carter told me when I spoke with him over the phone. "But there's virtually no evidence for this."
Carter argues that the popular RNA world hypothesis does not have enough evidence.
Three molecules—DNA, RNA, and proteins—underpin nearly everything about our biology. In a nutshell, DNA contains life's blueprints, RNA translates those blueprints into proteins, and proteins do everything else, from building and repairing cells to driving metabolic reactions. The million dollar question at the heart of origins of life research is how these large and complex biomolecules emerged from very simple precursors.
We've got a good handle on step one. In 1952, the famous Miller-Urey experiment demonstrated that some of life's most fundamental building blocks—the amino acids and nucleic acids that chain together to form proteins or DNA—can spontaneously assemble from nonliving chemicals, via a process called abiogenesis. By recreating primordial Earth's conditions in a bottle, Stanley Miller and Harold Urey showed that abiogenesis could probably explain the emergence of life's most elemental units here on Earth. We've since learned that clouds of interstellar dust and other space environments are constantly churning out these tiny biological lego bricks, as well.
But the next step, of actually linking those building blocks into chains, and folding and twisting those chains into three-dimensional molecules, is much harder to imagine. Some scientists believe that RNA was the key player, a concept known as the RNA world hypothesis.
Like DNA, RNA stores and transmits information. Like proteins, it can catalyze certain biochemical reactions. Given that RNA seems to straddle the line between information storage and biological activity, one can envision a pre-cellular world where RNA molecules acted as independent, self-replicating biological machines.
Carter, who has been a strong critic of the RNA world hypothesis for decades, explained that the theory, for all its elegance, is premised on a major assumption: That on primordial Earth, RNA could have actually copied itself. While RNA can indeed drive some biological reactions, he argues that, as far as we know, self-replication isn't one of them. (Self-replicating RNA has been artificially created, however.)
"There are no RNA molecules that have been discovered that are related to the synthesis of other RNA molecules," Carter told me. "All RNA is made by proteins, and there's no [evolutionary] evidence that RNA was ever made by RNA. This makes the RNA world hypothesis the intellectual equivalent of creation science."
Carter first proposed a radical alternative nearly 40 years ago: that RNA and peptides might have co-assembled, pulling each other out of the primordial ooze. Through a series of biochemical analyses, he showed that the range of chemical environments RNA and peptides can survive in is very well aligned. Now, two companion studies led by Carter and fellow origins of life researcher Richard Wolfenden offer the first major updates to this theory.
"I believe that when we discover life elsewhere in the universe, that it will be based on the same molecules as life on Earth"
The first study, led by Wolfenden, studies the physical properties of amino acids across a wide range of temperatures. This work demonstrates that proteins could have folded into their proper, functional 3D structures in the very hot environments where life first emerged. The second study, led by Carter, offers new insights into how the proteins and RNA molecules that translate the genetic code recognize and interact with each other.
Taken together, these two works support the notion that the very first peptides were RNA factories, and the very first RNA molecules built peptides.
"Our work shows that the close linkage between the physical properties of amino acids, the genetic code, and protein folding was likely essential from the beginning, long before large, sophisticated molecules arrived on the scene," Carter said. "This close interaction was likely the key factor in the evolution from building blocks to organisms."
The origins of life model that Carter and his colleagues have built may offer a template for how life could emerge on other worlds. Carter, for one, believes that the co-evolution of RNA and proteins underpins the emergence of life anywhere.
"I think an awful lot about what life would be like on other planets," Carter told me. "I believe that when we discover life elsewhere in the universe—and I believe we will—that it will be based on the same molecules as life on Earth, and that it'll evolve through a series of stages that begin with very low-specificity, high-diversity products. Gradually, that system builds the ability to learn and become more complicated."
So, as the search for life on other worlds kicks into high gear, it's worth remembering our own humble origins. We may encounter any number of rocky, Earth-like worlds that, on the surface, appear to be as lifeless as Mars. But just maybe, somewhere out there lies a pool of cosmic chemicals in which simple molecules are banding together, helping each other grow, diversify and evolve.
In four billion years, that planet could be the next Earth.