A NASA-supported study has provided fresh insights into one of biology’s enduring mysteries: why life on Earth exclusively uses left-handed amino acids in proteins, Space Daily reports.
Published in Nature Communications, the research reveals that RNA, a molecule central to theories of early life, does not inherently favor left-handed amino acids, suggesting that life’s molecular handedness, or homochirality, may have emerged later in evolution rather than as a result of chemical inevitability.
Amino acids, the building blocks of proteins, can exist in two mirror-image forms—left-handed and right-handed. However, life on Earth consistently utilizes only the left-handed variety. This phenomenon has puzzled scientists for decades, as there is no obvious reason why life couldn’t function with right-handed amino acids instead.
Proteins rely on amino acids, while DNA, the genetic blueprint of life, depends on RNA to execute its instructions. Due to its simpler structure and dual roles in storing genetic information and facilitating protein synthesis, RNA is thought to have preceded DNA during early life on Earth. This “RNA world” hypothesis positions RNA as a key molecule in understanding life’s origins.
The study, led by researchers from UCLA and NASA, tested whether ribozymes—RNA molecules with enzymatic properties—exhibited any bias toward left- or right-handed amino acids. Using 15 different ribozyme combinations, the researchers incubated RNA with amino acid precursors under conditions mimicking the early Earth. The results showed that ribozymes could catalyze reactions producing either left- or right-handed amino acids, depending on the experimental setup.
These findings challenge the idea that RNA chemically determined the preference for left-handed amino acids. Instead, they suggest that life’s exclusive use of left-handed amino acids might have arisen due to later evolutionary pressures rather than intrinsic chemical factors.
“The results indicate that the homochirality we see in biology now may not have been predetermined by chemistry alone,” said Irene Chen, the study’s corresponding author.
Co-author Alberto Vazquez-Salazar added that the bias could have been shaped by environmental or selective pressures in the early stages of evolution.
Pinpointing the origin of homochirality remains difficult due to the lack of direct evidence from Earth’s prebiotic history, much of which has been erased by geological processes. Some researchers look to meteorites for answers, as these space rocks have been found to contain both left- and right-handed amino acids. It remains unclear whether such extraterrestrial contributions influenced early Earth’s molecular asymmetry.
“Understanding the chemical properties of life is crucial for identifying signs of life elsewhere in the universe,” said Jason Dworkin, a senior astrobiologist at NASA’s Goddard Space Flight Center.
Dworkin’s team is analyzing samples from asteroid Bennu, brought back by NASA’s OSIRIS-REx mission, for amino acid chirality. Similar techniques may soon be applied to samples from Mars.
While the study underscores the complexity of life’s molecular handedness, it also highlights that the preference for left-handed amino acids could be a product of evolutionary processes rather than prebiotic chemistry. This finding opens new questions about how and when homochirality arose in life’s history.
