To find out, they've been looking to space. And on Tuesday, researchers announced at the annual meeting of the American Astronomical Society that they'd found their first ever interstellar chiral molecule. The results will be published this week in the journal Science.
Chiral molecules have almost all the same physical properties as their mirror images, but living things pick one version of a molecule to use in all of their biological processes. All living things exclusively use the right-handed form of the sugar ribose (the backbone of DNA), for example, and grapes only synthesize the left-handed form of the molecule tartaric acid. All amino acids – which make up proteins – skew left.
There's a biological advantage to picking sides, because molecules can build into more complex structures if they all match in handedness. The double-helix structure of DNA wouldn't have been able to form if a mishmash of right and left-handed molecules tried to come to the party. But that doesn't explain how Earth got its particular assortment of chiral molecules, and why the biases we see in living things emerged.
"You could just as easily imagine us building things out of right-handed molecules," said Brandon Carroll, a chemistry graduate student at the California Institute of Technology and co-first author of the new study. "So asking how and why we settled on what we did is one of the biggest unanswered questions in biology."
But the answer might be over our heads – literally.
Along with fellow first author Brett McGuire, a chemist and Jansky Postdoctoral Fellow with the National Radio Astronomy Observatory, Carroll detected propylene oxide (CH3CHOCH2) in a massive, star-forming cloud of gas and dust called Sagittarius B2, which was observed for years under the Green Bank Telescope Prebiotic Interstellar Molecular Survey (PRIMOS).
The cloud, the largest of its kind in the galaxy, is over 150 light years across and contains most of the molecules that have been found in space. In future studies, Carroll, McGuire and their colleagues hope to determine whether the propylene oxide in the cloud tends to be more left- or right- handed, which could help support or debunk one obvious explanation for the bias on Earth.
"One plausible theory is that a small excess of left-handed molecules was produced in interstellar space before our solar system was formed, and biochemical processes on the young Earth amplified that initial interstellar excess," said Ben McCall, an astrophysicist from the University of Illinois who wasn't involved in the new study. Chiral molecules have already been detected in meteorites, leading many scientists to suggest that chirality has origins far older than our planet or solar system. "This theory can only be tested by observing chiral molecules in space, and this work sets the stage for such tests," McCall said.
If Earth inherited its lefty bias from some kind of phenomenon in the molecular clouds that give birth to our galaxy's stars, it would follow that we'd see more left-handed propylene oxide than right-handed.
"We can use this molecule as a kind of tracer to look back in time to how this selection might have happened," McGuire told The Post.
Detecting the molecule required a marriage of lab science on Earth and radio telescope observations, he explained. Every molecule has a unique set of frequencies, wavelengths of radio light that it either absorbs or reflects. When a radio telescope looks at a light source shining behind a molecule, it can detect the molecule's presence based on which wavelengths of light that molecule has chewed up and spit out. Scientists test tumbling molecules on Earth to figure out their exact signature, called the spectrum, so that they can recognize those fingerprints in telescope data.
"There's no other molecule that could give that exact spectrum," McGuire said. It must be propylene oxide, and that means it has the ability to form on either side of a mirror-edge schism.
Propylene oxide itself isn't a molecule of particular significance for life as we know it. "But you really only have to understand one chiral molecule to understand them all," Carroll said. And observing bigger, more complex molecules is much more difficult to do. But after the researchers confirm whether Sagittarius B2's propylene oxide has a chirality bias, tracking down the molecules that helped build life on Earth could be an exciting next step.
"Ideally we should strive to detect other chiral species, especially those of interest in biology or found in meteorites," said NASA Goddard astrophysicist Stefanie Milam, who wasn't involved in the study. "This is a very exciting discovery for astrobiology."
Apart from the staggering existential implications of understanding our own origins, the study of chiral molecules could help scientists figure out how much luck was required for life's intricate double-helix to form. When it comes to the hunt for life on other planets, the question of how special we are is paramount. If the bias of one hand over another is commonplace for chiral molecules, other planets seeded by the same molecular clouds might have a shot at building complex structures like DNA.