All life on Earth relies on a standard set of 20 molecules called amino acids to build the proteins that carry out life's essential actions. But did it have to be this way?
All living creatures on this planet use the same 20 amino acids, even though there are hundreds available in nature. Scientists therefore have wondered if life could have arisen based on a different set of amino acids.
And what's more, could life exist elsewhere that utilizes an alternate collection of building blocks?
"Life has been using a standard set of 20 amino acids to build proteins for more than 3 billion years," said Stephen J. Freeland of the NASA Astrobiology Institute at the University of Hawaii. "It's becoming increasingly clear that many other amino acids were plausible candidates, and although there's been speculation and even assumptions about what life was doing, there's been very little in the way of testable hypotheses."
So Freeland and his University of Hawaii colleague Gayle K. Philip devised a test to try to learn if the 20 amino acids Earth's life uses were randomly chosen, or if they were the only possible ones that could have done the job.
Amino acids are molecules built primarily from carbon, hydrogen, oxygen, and nitrogen. They assemble in particular shapes and patterns to form larger molecules called proteins that carry out biological functions.
"Technically there is an infinite variety of amino acids," Freeland told Astrobiology Magazine. "Within that infinity there are lots more than the 20 that were available [when life originated on Earth] as far as we can tell."