Israeli computer simulation sheds light on life’s possible origins

A new simulation by researchers at Israel’s Weizmann Institute shows how life could have originally evolved from simple fatty molecules known as micelles into self-reproducing structures capable of evolving to more complex forms. 

The work, led by molecular geneticist Doron Lancet, is a continuation of attempts to show that early, proto-life was made up of organic compounds simpler than the more complex building blocks, such as RNA and DNA, that we know today. 

In a paper published April 19 in the online journal ChemRxiv, they show that through self-reinforcing catalytic processes, non-living macromolecular assemblies of micelles could develop basic characteristics of life (including growth and reproduction) and then gradually evolve to greater complexity. 

What are micelles?

Micelles are assemblies of fatty molecules, known as phospholipids, dissolved in water or some other liquid. They are usually spherical in shape. They can exist in a non-living form, or within living organisms. 

The usual form of fat in solution is the lipid layer––oil in water is an example, or the lipid bilayer that forms the membrane surrounding the cell nucleus. When the temperature and lipid concentration pass a certain threshold, however, the lipids can form spontaneously into spherically shaped micelles. 

Thermodynamic studies show that the lipid layer phase is favored at lower temperatures and concentrations. As these parameters increase, however, the solution gains entropy by forming the lipids into an approximately spherical shape. 

Developing a model

The lipid/micelle model is a first approximation of conditions necessary for life. Rather than trying to imagine a de novo formation of complex molecules such as RNA at an early stage, the authors follow a second school of thought, which proposes a bottom-up strategy. 

“At the heart of this scenario is the assumption that the assemblies would build up mutually catalytic networks, whose function rests on simple organo-catalysts," they write. "Under certain quantitatively defined kinetic constraints, similar to those that govern present-day metabolism, such multi-molecular networks would reproduce and evolve.” 

To study such an evolution, the scientists have produced a computer model known as GARD (Graded Autocatalysis Replication Domain) that uses techniques of systems chemistry applied to the study of catalytic networks. They view their model as a generalization of the CAS (collectively autocatalytic set) model developed earlier by theoretical biologist Stuart Kauffman, a pioneer in the field. 

Protobiotic attractors

The model allows researchers to characterize the dynamic behavior of a collection of molecules known as a “catalytic network” by inputting the kinetic parameters of every participating molecule type. The GARD model goes beyond many other models in considering such features as molecular concentrations and catalytic constants. 

One of the surprising results of the studies was that the modeled assemblages represented “dynamic attractors.” An attractor is a set of mathematical states that a system tends to evolve into beginning from various starting conditions. 

In the simulations, the modelers “seed a micellar assembly with a random composition and follow the growth-split dynamics driven by the entry and exit of lipid monomers.”

Describing the remarkable results, the authors write, “Upon reaching a prescribed maximal size, the assembly fissions, and one randomly selected progeny initiates the next growth generation. Typically, upon examining subsequent growth-split generations, the composition of a fully-grown assembly bears only small resemblance to that of the previous generation. 

"However, in some cases the system reaches a privileged compositional state, termed composome, in which a high compositional similarity is seen for three or more generations,” they said.

These are the sorts of conditions that are thought to be necessary in the early protobiotic state. 

“Crucially, the lipid micelle embodiment of GARD conforms to many of the requirements for early life: being compartmentalized, out-of-equilibrium, capable of self reproduction and adaptive to external changes,” the authors conclude. 

“Rewardingly, further scrutiny shows that all self-reproducing states are also dynamic attractors of the catalytic network," they added. "This suggests a greatly enhanced propensity for the spontaneous emergence of reproduction and primal evolution, vastly augmenting the likelihood of protolife appearance.”

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Kahana A, Segev L, Lancet D. Protobiotic network reproducers are compositional attractors: enhanced probability for life’s origin. ChemRxiv. Cambridge: Cambridge Open Engage; 2022; This content is a preprint and has not been peer-reviewed.

https://chemrxiv.org/engage/chemrxiv/article-details/625c0ea711b146242525ad37