The spontaneous coalescence of the molecules that led to life on primordial Earth, some 4 billion years ago, may have finally been observed in a laboratory.
Replicating the likely conditions of our newborn planet, chemists have joined together RNA and amino acids – the crucial first step that would eventually lead to the proliferation of living organisms that crawl all over Earth today.
The experimental work could yield important clues about the origins of one of the most important biological relationships: the one between nucleic acids and proteins.
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“Life today uses an immensely complex molecular machine, the ribosome, to synthesize proteins. This machine requires chemical instructions written in messenger RNA, which carries a gene’s sequence from a cell’s DNA to the ribosome. The ribosome then, like a factory assembly line, reads this RNA and links together amino acids, one by one, to create a protein,” explains chemist Matthew Powner of University College London.
“We have achieved the first part of that complex process, using very simple chemistry in water at neutral pH to link amino acids to RNA. The chemistry is spontaneous, selective, and could have occurred on the early Earth.”
Although we know that life must have wriggled its way out of Earth’s primordial ooze – after all, here we are – scientists are not as sure about how it happened. One growing school of thought invests in RNA as a self-replicating nucleic acid, which, thanks to its knack for also performing mechanical work, can catalyze other chemical reactions. This is known as the RNA world hypothesis.
Proteins cannot self-replicate; the instructions for their exact sequencing of amino acids are encoded in sequences of nucleic acid, such as RNA.
So while proteins play a necessary role in many biological processes, molecules of nucleic acid provide a crucial template for their production. Still, this means that the two molecular components would have needed to find a way to join together in the soggy, steamy conditions of early Earth.
“Life relies on the ability to synthesize proteins – they are life’s key functional molecules. Understanding the origin of protein synthesis is fundamental to understanding where life came from,” Powner says.
“Our study is a big step towards this goal, showing how RNA might have first come to control protein synthesis.”
Many attempts have been made to replicate the natural coalescence of amino acids and RNA. This process requires a high-energy mediator, and past studies have found that some highly reactive molecules are not fit for this purpose, since they tend to break down in water, leading the amino acids to react with each other rather than the RNA.
Led by chemist Jyoti Singh of University College London, the research team took their cues instead from biology. As a mediator, they tried a thioester, a high-energy, highly reactive compound that contains carbon, oxygen, hydrogen, and sulfur – four of the six elements that are thought to be vital to life.
Thioesters are known to play a key intermediary role in some biological processes, and are thought to have been abundant in the ‘primordial organic soup’. Some scientists believe their proliferation preceded the RNA world, known as the thioester world hypothesis.
In their simulated organic soup, the researchers found that thioester provided the necessary external energy to allow the amino acid to bind to the RNA – a pretty significant breakthrough that neatly unifies the two hypotheses.
“Our study unites two prominent origin of life theories – the ‘RNA world’, where self-replicating RNA is proposed to be fundamental, and the ‘thioester world’, in which thioesters are seen as the energy source for the earliest forms of life,” Powner says.
To be clear, we’re still quite far from having a detailed, comprehensive understanding of the origins of life. The new research shows that it’s possible these components can come together with a high-energy mediator; the next step is to see if RNA will preferentially bind to the specific amino acids that would facilitate the emergence of genetic code.
“Imagine the day that chemists might take simple, small molecules, consisting of carbon, nitrogen, hydrogen, oxygen, and sulphur atoms, and from these Lego pieces form molecules capable of self-replication. This would be a monumental step towards solving the question of life’s origin,” Singh says.
“Our study brings us closer to that goal by demonstrating how two primordial chemical Lego pieces (activated amino acids and RNA) could have built peptides, short chains of amino acids that are essential to life.”
The research has been published in Nature.