Scientists have made a major breakthrough in the mystery of how life first emerged on Earth by demonstrating how two essential biological ingredients could have spontaneously joined together on our planet some four billion years ago.
All life on Earth contains ribonucleic acid (RNA), a special molecule that helps build proteins from simpler amino acids. To kickstart this fundamental biological process, RNA and amino acids had to become attached at some point. But this key step, known as RNA aminoacylation, has never been experimentally observed in early Earth-like conditions despite the best efforts of many researchers over the decades.
Now, a team has achieved this milestone in the quest to unravel life’s origins. As they report in a study published on Wednesday in Nature, the researchers were able to link amino acids to RNA in water at a neutral pH with the aid of energetic chemical compounds called thioesters. The work revealed that two contrasting origin stories for life on Earth, known as “RNA world” and “thioester world,” may both be right.
“It unites two theories for the origin of life, which are totally separate,” said Matthew Powner, a professor of organic chemistry at University College London and an author of the study, in a call with 404 Media. “These were opposed theories—either you have thioesters or you have RNA.”
“What we found, which is kind of cool, is that if you put them both together, they're more than the sum of their parts,” he continued. “Both aspects—RNA world and thioester world—might be right and they’re not mutually exclusive. They can both work together to provide different aspects of things that are essential to building a cell.”
In the RNA world theory, which dates back to the 1960s, self-replicating RNA molecules served as the initial catalysts for life. The thioester world theory, which gained traction in the 1990s, posits that life first emerged from metabolic processes spurred on by energetic thioesters. Now, Powner said, the team has found a “missing link” between the two.
Powner and his colleagues didn’t initially set out to merge the two ideas. The breakthrough came almost as a surprise after the team synthesized pantetheine, a component of thioesters, in simulated conditions resembling early Earth. The team discovered that if amino acids are linked to pantetheine, they naturally attach themselves to RNA at molecular sites that are consistent with what is seen in living things. This act of RNA aminoacylation could eventually enable the complex protein synthesis all organisms now depend on to live.
Pantetheine “is totally universal,” Powner explained. “Every organism on Earth, every genome sequence, needs this molecule for some reason or other. You can't take it out of life and fully understand life.”
“That whole program of looking at pantetheine, and then finding this remarkable chemistry that pantetheine does, was all originally designed to just be a side study,” he added. “It was serendipity in the sense that we didn't expect it, but in a scientific way that we knew it would probably be interesting and we'd probably find uses for it. It’s just the uses we found were not necessarily the ones we expected.”
The researchers suggest that early instances of RNA aminoacylation on Earth would most likely have occurred in lakes and other small bodies of water, where nutrients could accumulate in concentrations that could up the odds of amino acids attaching to RNA.
“It's very difficult to envisage any origins of life chemistry in something as large as an ocean body because it's just too dilute for chemistry,” Powner said. For that reason, they suggest future studies of so-called “soda lakes” in polar environments that are rich in nutrients, like phosphate, and could serve as models for the first nurseries of life on Earth.
The finding could even have implications for extraterrestrial life. If life on Earth first emerged due, in part, to this newly identified process, it’s possible that similar prebiotic reactions can be set in motion elsewhere in the universe. Complex molecules like pantetheine and RNA have never been found off-Earth (yet), but amino acids are present in many extraterrestrial environments. This suggests that the ingredients of life are abundant in the universe, even if the conditions required to spark it are far more rare.
While the study sheds new light on the origin of life, there are plenty of other steps that must be reconstructed to understand how inorganic matter somehow found a way to self-replicate and start evolving, moving around, and in our case as humans, conducting experiments to figure out how it all got started.
“We get so focused on the details of what we're trying to do that we don't often step back and think, ‘Oh, wow, this is really important and existential for us,’” Powner concluded.