Scientists have carefully simulated conditions on Earth in the earliest part of its history, some 4.6 billion years ago, hoping to unlock a greater understanding of how amino acids brought the first ingredients for life into being.
Together, amino acids form proteins that play many vital roles in organisms. This new study was designed to help establish why a specific group of 20 'canonical' amino acids is used again and again to build proteins when there are so many more of these amino acids to pick from.
It's thought that these 20 amino acids are made up of 10 'early' ones picked from the atmosphere and meteorite fragments of early Earth, and 10 'later' ones added on top – but what the selection process for the latter 10 involved isn't clear.
"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," says chemist Stephen Fried, from Johns Hopkins University in Maryland. "We're describing the events that shaped why that ancestor got the amino acids that it did."
Through a reconstruction of primordial protein synthesis, the researchers showed that ancient organic compounds would have favored the amino acids best at folding proteins, tailoring them for specific functions.
In other words, a process of evolution or natural selection was underway even at this stage: it wasn't the amino acids that were most readily available that were picked, but the amino acids most suited to a particular job.
If other amino acids had been selected as part of the core group billions of years ago, the scientists determined that the very building blocks of life would not have been as efficient at doing that life-building.
"Protein folding was basically allowing us to do evolution before there was even life on our planet," says Fried. "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."
Molecules, including proteins, are thought to have started putting together simple organisms some 3.8 billion years ago, so there's a stretch of earlier Earth history that scientists have been very keen to look into.
The team behind the study suggests that the 10 'later' amino acids, in particular, were selected for their protein-folding capabilities, enabling the replication of DNA and the production of proteins that sparked life into being.
This research can teach us more about the potential for microorganisms on other planets and our own: The same amino acids that came to Earth via meteorites can also be found in many other places in the Universe.
"The Universe seems to love amino acids," says Fried. "Maybe if we found life on a different planet, it wouldn't be that different."
The research has been published in the Journal of the American Chemical Society.