Jigsaw Model of the Origin of Life

John F. McGowan
GFT Group Incorporated
jmcgowan11@earthlink.net

ABSTRACT

It is suggested that life originated in a three-step process referred to as the jigsaw model. RNA, proteins, or similar organic molecules polymerized in a dehydrated carbon-rich environment, on surfaces in a carbon-rich environment, or in another environment where polymerization occurs. These polymers subsequently entered an aqueous environment where they folded into compact structures. It is argued that the folding of randomly generated polymers in water tends to partition the folded polymer into domains with hydrophobic cores and matching shapes to minimize energy. In the aqueous environment, hydrolysis or other reactions fragmented the compact structures into two or more matching molecules, occasionally producing simple living systems. It is argued that the hydrolysis of folded polymers such as RNA or proteins is not random. The hydrophobic cores of the domains are rarely bisected due to the energy requirements in water. Hydrolysis preferentially fragments the folded polymers into pieces with complementary structures and chemical affinities. Thus the probability of synthesizing a system of matched, interacting molecules in prebiotic chemistry is much higher than usually estimated. Environments where this process may occur are identified. The implications of this hypothesis for seeking life or prebiotic chemistry in the Solar System and the laboratory are explored.

Illustrations

Step 1: A primordial soup with amino acids or similar monomers, possibly a deposit of primordial hydrocarbons similar to the chemicals in carbonaceous chondrites, a kind of meteorite.

Step 2: The monomers polymerize into a protein alpha-helix or similar helical structure on the surface of a chiral crystal that selects for the handedness of the amino acids or other monomers.

Step 3: The helix is unstable because it has exposed hydrophobic residues (red spheres). It folds into a disk-shaped structure made of stable folding domains that have hydrophobic cores and matching shapes that fit together like a lock-and-key.

Step 4: Hydrolysis fragments the folded polymer into four domains with hydrophobic cores (red spheres) and matching surfaces (green spheres for non- hydrophobic residues). Each domain is a wedge-shaped four-helix bundle.

Step 5: The domains catalyze the polymerization of the matching faces of the adjacent domains. Each face consists of two alpha helices with exposed hydrophobic residues. After polymerization, each domain becomes a half of the original structure.

Step 6: The halves recombine, burying the hydrophobic residues, releasing energy to perpetuate the process.

Step 7: For illustrative purposes the ends of the structures have been ignored. This is solved, for example, if the original alpha helix folds into a donut-shaped structure (torus) comprised of the disk-shaped structures. The axial faces of the disk- shaped structures fit together like a lock- and-key.

Key points

• The fragmentation by partial hydrolysis of folded RNA, proteins, or protein-like polymers may provide a simple mechanism that occasionally makes autocatalytic sets of RNA, proteins, or other polymers.
• While synthesis of an autocatalytic set is probably rare, it is more likely than the astronomical odds often calculated in origin of life research.
• Molecular Lindenmayer systems (not shown in poster) may provide a simple genetic system used by the autocatalytic sets. Molecular Lindenmayer systems may survive as mobile genetic elements in modern organisms.
• Autocatalytic sets may exist on present day Earth and may cause some poorly understood diseases.

Reference: John F. McGowan, III, "Jigsaw model of the origin of life", in Instruments, Methods, and Missions for Astrobiology IV, Richard B. Hoover, Gilbert V. Levin, Roland R. Paepe, Alexei Yu. Rozanov, Editors, Proceedings of the SPIE Vol. 4495, pp. 199-210 (2002) (http://www.jmcgowan.com/JigsawPreprint.pdf)