A new simulation of the time before life began has suggested that ancient amino acids may have kick-started evolution before there was even life on Earth.
The study found that ancient amino acids – which are found in every life form from humans to bacteria – shaped the genetic code of ancient microorganisms.
Life thrived on Earth because some amino acids were especially good at helping proteins adopt specific shapes to perform crucial functions, the study's authors said.
Professor Stephen Fried, a Johns Hopkins chemist who co-led the research, said: "Protein folding was basically allowing us to do evolution before there was even life on our planet.
"You could have evolution before you had biology, you could have natural selection for the chemicals that are useful for life even before there was DNA."
The scientists said their conclusion means that if we find life on other planets, it may be more similar to life on Earth than we imagine.
Even though the primordial Earth had hundreds of amino acids, all living things use the same 20 compounds.
Fried said: "You see the same amino acids in every organism, from humans to bacteria to archaea, and that's because all things on Earth are connected through this tree of life that has an origin, an organism that was the ancestor to all living things.
"We're describing the events that shaped why that ancestor got the amino acids that it did."
In the lab, the researchers mimicked primordial protein synthesis of four billion years ago by using an alternative set of amino acids that were highly abundant before life arose on Earth.
They found ancient organic compounds integrated the amino acids best suited for protein folding into their biochemistry.
In its first billion years, Earth's atmosphere consisted of an assortment of gases like ammonia and carbon dioxide that reacted with high levels of ultraviolet radiation to concoct some of the simpler canonical amino acids.
"We're trying to find out what was so special about our canonical amino acids," Fried said. "Were they selected for any particular reason?"
Scientists estimate Earth is 4.6 billion years old, and that DNA, proteins, and other molecules didn't begin to form simple organisms until 3.8 billion years ago.
The new research offers fresh clues into the mystery of what happened during the time in between.
Fried said: "To have evolution in the Darwinian sense, you need to have this whole sophisticated way of turning genetic molecules like DNA and RNA into proteins. But replicating DNA also requires proteins, so we have a chicken-and-egg problem.
"Our research shows that nature could have selected for building blocks with useful properties before Darwinian evolution."
Scientists have spotted amino acids in asteroids far from Earth, suggesting those compounds are ubiquitous in other corners of the universe.
That's why Fried thinks his team's research could also have implications for the possibility of finding life beyond Earth.
He said: "The universe seems to love amino acids. Maybe if we found life on a different planet, it wouldn't be that different."
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