Chapter 7: Protocells, Membranes, and Early Metabolism

The foundation of genetic information transfer in organisms today is explained by the Central Dogma of Molecular Biology: the duplication of DNA via replication, the transfer of coded messages to RNA through transcription, and the subsequent synthesis of proteins from the RNA message via translation. This complex system relies on several functional classes of RNA, including structural/storage RNA, messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). However, evidence overwhelmingly supports the RNA World hypothesis, suggesting that RNA originated before DNA or protein, functioning initially as both the hereditary molecule and a biocatalyst. The discovery of ribozymes (RNA molecules with enzyme-like catalytic capabilities) provides key support for this model, demonstrating how self-replication could have occurred in the prebiotic world. Parallel to molecular evolution, the transition from linear molecules to life forms required organization into defined physical structures; before 3.5 billion years ago, structural molecules spontaneously formed membranes, vesicles, and ultimately protocells. These membrane-bounded systems, exemplified by experimental models such as Oparin’s coacervates and Fox’s microspheres, exhibited selective permeability and internal organization necessary to maintain concentration gradients. Crucially, the origin of life was inextricably linked to the evolution of selection, which arose when populations of differing individuals competed for limited resources. Selection acted on molecules and protocells, favoring those that could best maintain their organization and perpetuate themselves, bridging the gap between non-reproductive chemical evolution and biological natural selection. The chapter also examines entities that blur the line between life and non-life, such as host-dependent viruses (composed of DNA or RNA in a protein coat) and prions (infectious particles made entirely of misshapen protein that act as units of inheritance without nucleic acids).