How Life May Have Originated in Phase-separated Polymer Droplets

Solutions of oppositely charged polymers, upon mixing, undergo liquid-liquid phase separation. The phase-separated membraneless droplets formed by such polyelectrolyte blends, called "coacervates," find their place in the biological world not only in numerous processes inside cells but also in model early-earth scenarios for the origin of life. Modern cells with complex architectures, including lipid membranes and associated membrane proteins, must have evolved from an architecturally simple state. Coacervate droplets have recently been proposed as simple prebiotic cells (or protocells) as they provide compartmentalization to separate life from non-living surroundings, sequestration of biomolecules, and easy transportation of materials across membraneless interface. Unfortunately, the uncontrollable transport across droplet interface (t ~ sec), in addition to the droplet's inherent instability towards coalescence (merging, t ~ ms) poses a major challenge in considering these droplets as protocell models. We stabilized these "coacervate colloids" by submerging them in distilled (ion-free) water. Here, we produced droplets that showed no exchange of RNA and were stable against coalescence for more than two years. We hypothesize that the loss of counterions upon transfer to water creates a thin crosslinked layer on the droplet interface that prohibits droplet coalescence. This is a unique, minimalistic "stable" protocell model supporting Darwinian evolution and natural selection.