Together, amino acids form proteins that play many vital roles in organisms. This new study is designed to help establish why a specific group of 20 ‘canonical’ amino acids is used over and over again to build proteins when there are so many more of these amino acids to choose from.
It is thought that these 20 amino acids are made up of 10 ‘early’ amino acids plucked from the atmosphere and meteorite fragments from the early Earth, and 10 ‘later’ ones added on top – but what the selection process for the latter 10 involved is not clear.
“You see the same amino acids in every organism, from humans to bacteria to archaea, and that’s because everything on Earth is connected through this tree of life that has an origin, an organism that was the ancestor of all living things,” says chemist Stephen Fried, of Johns Hopkins University in Maryland. “We describe the events that shaped why that ancestor got the amino acids that it got.”
Through a reconstruction of primordial protein synthesis, the researchers showed that ancient organic compounds would have favored the amino acids best for folding proteins and tailoring them for specific functions.
In other words, a process of evolution or natural selection was underway even at this stage: it was not the amino acids that were most readily available that were selected, but the amino acids that were best suited to a particular job.
If other amino acids had been selected as part of the core group billions of years ago, the researchers determined that the building blocks of life themselves would not have been as efficient at making that life structure.
“Protein folding basically allowed us to do evolution before there was even life on our planet,” Fried says. “You could have evolution before you had biology, you could have natural selection for the chemicals useful for life even before there was DNA.”
Molecules, including proteins, are thought to have begun to assemble simple organisms around 3.8 billion years ago, so there is a part of Earth’s past history that scientists have been keen to look into.
The team behind the study suggests that the 10 ‘later’ amino acids in particular were selected for their protein-folding ability, which enabled the replication of DNA and the production of proteins that started life.
This research can teach us more about the potential of 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 another planet, it wouldn’t be so different.”
The research has been published in Journal of the American Chemical Society.