section_path: Producing suitable vesicles
section_name: Producing suitable vesicles
paragraph_text: Producing suitable vesicles
  
  The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation, thus initiating the evolution of life. The lipid world theory postulates that the first self-replicating object was lipid -like. Phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. These bodies may expand by insertion of additional lipids, and may spontaneously split into two offspring of similar size and composition. Lipid bodies may have provided sheltering envelopes for information storage, allowing the evolution and preservation of polymers like RNA that store information. Only one or two types of amphiphiles have been studied which might have led to the development of vesicles. There is an enormous number of possible arrangements of lipid bilayer membranes, and those with the best reproductive characteristics would have converged toward a hypercycle reaction, a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles, which would have allowed natural selection.
  
  A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life. A functional protocell has (as of 2014) not yet been achieved in a laboratory setting. Self-assembled vesicles are essential components of primitive cells. The theory of classical irreversible thermodynamics treats self-assembly under a generalized chemical potential within the framework of dissipative systems. The second law of thermodynamics requires that overall entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate ordered life processes from chaotic non-living matter.
  
  Irene Chen and Jack W. Szostak suggest that elementary protocells can give rise to cellular behaviors including primitive forms of differential reproduction, competition, and energy storage. Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today. Such micro-encapsulation would allow for metabolism within the membrane and the exchange of small molecules, while retaining large biomolecules inside. Such a membrane is needed for a cell to create its own electrochemical gradient to store energy by pumping ions across the membrane. Fatty acid vesicles in conditions relevant to alkaline hydrothermal vents can be stabilized by isoprenoids which are synthesized by the formose reaction; the advantages and disadvantages of isoprenoids incorporated within the lipid bilayer in different microenvironments might have led to the divergence of the membranes of archaea and bacteria.
  
  Laboratory experiments have shown that vesicles can undergo an evolutionary process under pressure cycling conditions. Simulating the systemic environment in tectonic fault zones within the Earth's crust, pressure cycling leads to the periodic formation of vesicles. Under the same conditions, random peptide chains are being formed, which are being continuously selected for their ability to integrate into the vesicle membrane. A further selection of the vesicles for their stability potentially leads to the development of functional peptide structures, associated with an increase in the survival rate of the vesicles.