Hydrogen cyanide is best known as a poison, historically deployed as a murder weapon in gas chambers and by federal executioners. But much further back in Earth’s past, it may have been a precursor to life. The molecule is a key part of the widely accepted “RNA World” hypothesis for the origin of all living things, which characterizes it as one of the early chemicals that spontaneously collided with others to form the nucleic acid RNA. From there, DNA evolved, together with proteins and the other building blocks of life.
Scientists have established that hydrogen cyanide existed on the early Earth, but others have questioned how, exactly, it spontaneously reacted with other chemicals to form more complicated compounds, given the young planet’s forbidding conditions. In a paper published Wednesday in ACS Central Science, a team of researchers from India simulate the violent state of the Earth as it was some 4 billion years ago, showing how some of the fundamental building blocks of RNA can be built using just water, hydrogen cyanide, and heat.
The Urey-Miller Experiment of 1953
The paper, first-authored by Tamal Das, Ph.D., of India’s National Chemical Laboratory, begins with a reflection on a historic 1953 experiment that showed how amino acids, the precursors to proteins, could have formed from just a few key molecules, some water, and jolts of electricity. The Urey-Miller experiment mixed methane, ammonia, hydrogen, and water — the chemicals that made up the early atmosphere — and ran sparks of electricity through it to mimic lightning.
The idea was to simulate what the primordial Earth was like 3.5 to 4.0 billion years ago, down to the compounds already in its oceans and the harsh weather. Combining those ingredients with electricity created hydrogen cyanide, formaldehyde, and amino acids, the foundational elements that make up proteins.
“Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids,” said Stanley L. Miller, Ph.D., one of the chemists behind the experiment, in a 1996 interview.
But What Was Early Earth Really Like?
Some scientists, however, have raised questions about whether Miller and his doctoral advisor Harold Urey, Ph.D., were right about the lightning part of their primordial equation. In the new paper, Das and his team make the case that the atmosphere of the early Earth was too hazy for lightning to make it down to the seas and catalyze reactions in the water, so there must have been a simpler way for life’s building blocks to form.
The “hazy atmosphere of the Hadean Earth would have made it difficult for high-energy photons to reach the Earth’s surface,” they write. In other words, the lightning wouldn’t have been able to catalyze reactions between simple chemicals down at sea level, the way they did in the Miller-Urey experiment.
If that’s the case, then how did RNA’s precursors form? Like sugar needs heat to turn it into caramel, something must have helped speed those chemical reactions along — it just might not have been lightning.
Using a powerful simulation tool called an ab initio nanoreactor (AINR), the team reimagined the Miller-Urey experiment to include only hydrogen cyanide, water, and the searing natural heat of the ocean, which was between 80 °C and 100 °C at the time. At least according to the calculations of the AINR (which is designed to discover which molecules can form and how, given the starting materials), those three things are all that are really necessary to produce some important precursors of RNA, DNA, and proteins.
Namely, the simulation produced cyanamide, glycoladehyde, oxazole derivative, and the amino acid glycine. Even better, they were all made in “one pot,” or during a single simulation. That makes it all the more feasible that this simple chain of events might actually have happened at the beginning of life.
“This insight is valuable because it helps to explain how the reactions could have taken place in the absence of photochemical” — that means lightning-based — “activity on the surface of Earth’s oceans,” the authors write.
“The current work thus indicates that HCN [hydrogen cyanide] and H2O could have been the Adam and Eve of chemical evolution — the source of the precursor molecules that formed the basis of life on Earth.”
Many Possible Scenarios
The whole field of abiogenesis is dedicated to imagining what might have happened to kickstart the evolution of living things out of inorganic chemicals in the early days of our planet. The explanation outlined in this paper is just one of very many showing how some of the building blocks of life could have randomly emerged from a primordial soup of inorganic chemicals. (And this isn’t the only hypothesis for how life formed; there are other chemical explanations, panspermia, and of course intelligent design.)
In July, scientists at University College London showed how peptides might have formed without having to wait for the primordial soup to spontaneously bubble up amino acids, traditionally thought of as a necessary precursor to peptides and proteins. Like the authors of the new paper, they showed that the birth of these fundamental chemicals may have been simpler than we thought.
Finding increasingly simple explanations is good in this field because if we’re trying to understand how life sprouted from a soup of chemicals in a sea and then spread throughout the Earth, it wouldn’t make sense for the recipe to be very complicated.
It may also not make sense for there to have been only a single recipe. As the ab initio nanoreactor and other simulations have shown, there are lots of ways to precipitate chemicals from soup. While we don’t know for sure yet, it’s possible and even likely that the reactions that led to life, — whatever they were — had to be repeated over and over again in warm seas, lakes, and ponds all over the tumultuous early Earth to lead to the diversity of life we know today.
The seminal Urey−Miller experiments showed that molecules crucial to life such as HCN could have formed in the reducing atmosphere of the Hadean Earth and then dissolved in the oceans. Subsequent proponents of the “RNA World” hypothesis have shown aqueous HCN to be the starting point for the formation of the precursors of RNA and proteins. However, the conditions of early Earth suggest that aqueous HCN would have had to react under a significant number of constraints. Therefore, given the limiting conditions, could RNA and protein precursors still have formed from aqueous HCN? If so, what mechanistic routes would have been followed? The current computational study, with the aid of the ab initio nanoreactor (AINR), a powerful new tool in computational chemistry, addresses these crucial questions. Gratifyingly, not only do the results from the AINR approach show that aqueous HCN could indeed have been the source of RNA and protein precursors, but they also indicate that just the interaction of HCN with water would have sufficed to begin a series of reactions leading to the precursors. The current work therefore provides important missing links in the story of prebiotic chemistry and charts the road from aqueous HCN to the precursors of RNA and proteins.