By simulating ancient Earth conditions in the laboratory, a team of scientists led by Charles University in the Czech Republic and Johns Hopkins University in the U.S. has found that without a set of specific amino acids, ancient proteins would not have evolved into life as we know it today. These findings, detailing how amino acids shaped the genetic code of ancient microorganisms, shed new light on the origins of life on Earth.
“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,” said study co-author Stephen Fried, a chemist at Johns Hopkins. “We’re describing the events that shaped why that ancestor got the amino acids that it did.”
The scientists mimicked primordial protein synthesis of four billion years ago in a laboratory setting by using an alternative set of amino acids which were highly abundant before life emerged on our planet. They discovered that ancient organic compounds integrated the amino acids best suited for protein folding into their biochemistry, suggesting that life emerged and thrived on Earth not only because some amino acids were available in ancient environments, but mainly because some of them were particularly efficient in helping proteins adopt specific shapes to perform crucial functions.
“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,” Fried explained.
Although the primordial Earth had hundreds of amino acids, all living beings use just 20 of these compounds – what scientists refer to as “canonical” amino acids. In its first billion years, our planet’s atmosphere consisted of a mixture of gases such as ammonia and CO2 which, by reacting with high levels of UV radiation, gave rise to some of these acids, while others arrived from meteorites. How the rest of them emerged remains an open question which the researchers are currently investigating. “We’re trying to find out what was so special about our canonical amino acids,” Fried said. “Were they selected for any particular reason?”
“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,” he added.
Finally, the fact that many of these canonical amino acids arrived from space suggests that such compounds may be abundant in other parts of the universe, possibly giving rise to life there too. “The universe seems to love amino acids. Maybe if we found life on a different planet, it wouldn’t be that different,” Fried concluded.
The study is published in the Journal of the American Chemical Society.
By Andrei Ionescu, Earth.com Staff Writer
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