A Sequential Scenario for the Origin of Biological Chirality

Area of Science:

• Origin of Life Studies
• Biochemistry
• Astrobiology

Background:

• Biological systems exhibit homochirality, a phenomenon where molecules exist predominantly in one of two mirror-image forms (enantiomers).
• The origin of this biological chirality remains a significant unanswered question in science.
• Previous models have proposed various mechanisms for chiral selection, but a comprehensive sequential process is lacking.

Purpose of the Study:

• To propose a sequential model for the origin of biological chirality.
• To elucidate the stages of symmetry breaking, chiral amplification, and chiral expansion.
• To integrate abiotic factors with early biological evolution.

Main Methods:

• Utilized a model involving ultraviolet circularly polarized light (UV-CPL) for symmetry breaking, as proposed by Deutsch (1991).
• Incorporated a proposed chiral amplification mechanism involving synchronized tidal and photolysis cycles (3.8-4.2 billion years ago).
• Hypothesized a heterogeneous abiotic symmetry breaking and subsequent expansion of chirality through coevolution of the genetic code and amino acid relationships.

Main Results:

• Proposed a three-stage model: symmetry breaking, chiral amplification, and chiral expansion.
• Identified a unique Earth-based astronomic asymmetry (tidal and photolysis cycles) as a key driver for chiral amplification.
• Suggested initial reliance on a few L-amino acids, with subsequent expansion to all 20 via genetic code coevolution and physiological relationships.
• Attributed D-chirality in pentoses to D-pentose/L-AA relationships and structural evolution.

Conclusions:

• The origin of biological chirality is a sequential process involving distinct prebiotic and proto-biological stages.
• Earth's early unique environmental conditions, specifically synchronized astronomic cycles, played a crucial role in amplifying chirality.
• Chirality expanded beyond initial monomers through integrated biological and chemical evolutionary processes.