# From experimental clues to theoretical modeling: Evolution associated with the membrane-takeover at an early stage of life

## TL;DR

This paper explores how early life might have evolved from simple fatty acid membranes to more complex phospholipid membranes through a process called membrane-takeover.

## Contribution

The study uses computer modeling to demonstrate the plausibility of membrane-takeover and its evolutionary consequences in RNA-based protocells.

## Key findings

- Phospholipid-synthesizing ribozymes may have emerged due to membrane stabilization benefits.
- Reduced membrane permeability led to the evolution of RNA species for material utilization and transport.
- Theoretical modeling supports the experimental idea of early membrane evolution in protocells.

## Abstract

Modern cell membranes are primarily composed of phospholipids, while primitive cell membranes in the beginning of life are believed to have formed from simpler lipids (such as fatty acids) synthesized in the prebiotic environment. An attractive experimental study suggested that the corresponding “membrane-takeover” (as an evolutionary process) is likely to have occurred very early (e.g., in the RNA world) due to some simple physical effects, and might have subsequently triggered some other evolutionary processes. Here, via computer modeling on a system of RNA-based protocells, we convinced the plausibility of such a scenario and elaborated on relevant mechanisms. It is shown that in protocells with a fatty-acid membrane, because of the benefit of phospholipid content (i.e., stabilizing the membrane), a ribozyme favoring the synthesis of phospholipids may emerge; subsequently, due to the reduced membrane permeability on account of the phospholipid content, two other functional RNA species could arise: a ribozyme exploiting more fundamental materials (thus more permeable) for nucleotide synthesis and a species favoring across-membrane transportation. This case exemplifies a combination of experimental and theoretical efforts regarding early evolution, which may shed light on that notoriously complicated problem: the origin of life.

Based on both logical reasoning and relevant evidence, protocells in the origin of life should have had a membrane composed of simple lipids like fatty acids. An interesting lab study suggested that the primitive membrane might have evolved towards a phospholipid membrane like that in modern cells quite early, due to phospholipids’ role of stabilizing the membrane. That is, the protocells with a function of facilitating phospholipid synthesis may have been favored in the competition and thus overwhelmed other protocells. Then, for such protocells, which have a less permeable membrane due to higher phospholipid content, selective pressure may have further driven the emergence of the functions of utilizing more permeable raw materials and improving the across-membrane transportation. Indeed, concerning early evolution occurring during the origin of life, experiments may offer valuable clues, but usually have difficulty in providing insights into the corresponding process. On the other hand, computer modeling (simulation) has become a powerful tool for us to investigate relevant scenarios. Here, by modeling we demonstrated the suggested evolutionary process and investigated relevant mechanisms. The combination of experimental efforts and theoretical modeling efforts like this is expected to be an effective way for us to explore the complicated early evolution.