A team of researchers at the University of Tokyo have discovered a set of RNA molecules that are capable of replication and diversification. This potentially allows the molecules to undergo Darwinian evolution, pointing the way to a possible first step to life on Earth. As lead author Assistant Professor Ryo Mizuuchi said, “The results could be a clue to solving the ultimate question that human beings have been asking for thousands of years — what are the origins of life?”
Life is so complex that it’s nearly impossible to define. Indeed, scientists have developed over 200 independent definitions of the concept. Some things are clearly alive (you and me, for example) and others are not (like the screen you’re reading this on). But many microscopic entities like viruses and prions straddle the boundary between life and not-life.
We need simple and broad definitions of life to understand its origins, because at one time in the past our planet was very much not alive, and then it was. So one useful definition that tries to break life down into as simple of terms as possible includes three components. One, life is capable of storing information. Two, life must self-replicate. And lastly, life must be able to catalyze other reactions.
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Putting these three together means that life is a set of molecules that is capable of self-reproduction and is subject to Darwinian evolution. By storing information, interacting with its environment, and making copies of itself, life can do all the interesting things that life likes to do, and evolve into more efficient processes that continue doing those things.
It’s an RNA, RNA, RNA World
Speaking very broadly, modern life uses three sets of molecules to accomplish the business of being alive. DNA stores the information and generates RNA. The RNA takes those instructions to build proteins, which go on to catalyze other reactions, including the reproduction of DNA. Obviously this system didn’t exist in the early Earth, because it’s so complex and inter-dependent.
Many scientists believe that life started as just RNA, because RNA is capable of self-reproduction, storing information, and catalyzing other reactions. It’s not nearly as efficient or stable at any of these as the modern DNA-RNA-protein trifecta, but as long as Darwinian evolution can apply to RNA molecules, then it can steadily advance to the more efficient, robust, and complex form of life that we see today.
However, scientists have had difficulty finding which RNA sequences might survive in the harsh conditions of the early Earth. But the University of Tokyo researchers have found something interesting, according to a recent press release.
“We found that the single RNA species evolved into a complex replication system: a replicator network comprising five types of RNAs with diverse interactions, supporting the plausibility of a long-envisioned evolutionary transition scenario,” said Mizuuchi.
This was the first time that a laboratory experiment found a set of RNA molecules that could do all the basic functions required for life.
“Honestly, we initially doubted that such diverse RNAs could evolve and coexist,” commented Mizuuchi. “In evolutionary biology, the ‘competitive exclusion principle’ states that more than one species cannot coexist if they are competing for the same resources. This means that the molecules must establish a way to use different resources one after another for sustained diversification. They are just molecules, so we wondered if it were possible for nonliving chemical species to spontaneously develop such innovation.”
There’s a lot more work to be done to go from this lab-based RNA setup to a candidate for the origins of life. According to Mizuuchi, “The simplicity of our molecular replication system, compared with biological organisms, allows us to examine evolutionary phenomena with unprecedented resolution. The evolution of complexity seen in our experiment is just the beginning. Many more events should occur towards the emergence of living systems.”