Proteins, essential molecules in biological systems, are formed from amino acid building blocks. Amino acids can exist as mirror-image forms, similar to left and right hands, but life on Earth consistently utilizes the left-handed variety, a phenomenon called homochirality. Scientists have long puzzled over the reasons for this uniformity, as theoretically, life could function with right-handed amino acids just as effectively.
DNA, the repository of life's genetic instructions, relies on RNA to transfer and implement these instructions. This reliance, coupled with DNA's complexity, has led researchers to propose that RNA could have preceded DNA in an "RNA world" phase of early life evolution. RNA's dual capabilities - storing genetic information and facilitating protein synthesis - make it a strong candidate for the precursor to DNA.
However, experiments designed to simulate early-Earth conditions suggest RNA itself did not inherently favor left-handed amino acids. Researchers tested ribozymes, RNA molecules that act like enzymes, to evaluate whether they exhibit any preference for left- or right-handed amino acids. "The experiment demonstrated that ribozymes can favor either left- or right-handed amino acids, indicating that RNA worlds, in general, would not necessarily have a strong bias for the form of amino acids we observe in biology now," said Irene Chen of UCLA's Samueli School of Engineering, the corresponding author of the study.
In their experiments, researchers incubated ribozymes with amino acid precursors under conditions mimicking the early Earth. They examined the relative proportions of left-handed and right-handed phenylalanine produced by 15 different ribozyme combinations. Results showed ribozymes could lean toward either orientation, undermining the idea that RNA chemically predisposed early life toward left-handed amino acids. "The findings suggest that life's eventual homochirality might not be a result of chemical determinism but could have emerged through later evolutionary pressures," explained co-author Alberto Vazquez-Salazar, a postdoctoral scholar in Chen's research group.
The lack of direct evidence from Earth's prebiotic history complicates efforts to pinpoint the origin of homochirality. Early Earth's crust has been reshaped by plate tectonics, erasing ancient records. Asteroids, which may have delivered amino acids to Earth, offer another avenue for research. Meteorites have been shown to contain both left- and right-handed amino acids, fueling debates about their influence on early life.
"Understanding the chemical properties of life helps us know what to look for in our search for life across the solar system," said Jason Dworkin, senior astrobiology scientist at NASA's Goddard Space Flight Center and director of its Astrobiology Analytical Laboratory. Dworkin, also a project scientist for NASA's OSIRIS-REx mission, noted that the team is analyzing asteroid Bennu samples for amino acid chirality and expects to apply similar methods to future Mars samples.
The study underscores that life's molecular handedness might be the result of later evolutionary processes rather than chemical inevitability, leaving a profound question about life's origins still unanswered.
Research Report:Prebiotic chiral transfer from self-aminoacylating ribozymes may favor either handedness
Related Links
Astrobiology Analytical Laboratory at Goddard
Darwin Today At TerraDaily.com
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