Origin of Species before Origin of Life: The Role of Speciation in Chemical Evolution

Abstract

Speciation, an evolutionary process by which new species form, is ultimately responsible for the incredible biodiversity that we observe on Earth every day. Such biodiversity is one of the critical features which contributes to the survivability of biospheres and modern life. While speciation and biodiversity have been amply studied in organismic evolution and modern life, it has not yet been applied to a great extent to understanding the evolutionary dynamics of primitive life. In particular, one unanswered question is at what point in the history of life did speciation as a phenomenon emerge in the first place. Here, we discuss the mechanisms by which speciation could have occurred before the origins of life in the context of chemical evolution. Specifically, we discuss that primitive compartments formed before the emergence of the last universal common ancestor (LUCA) could have provided a mechanism by which primitive chemical systems underwent speciation. In particular, we introduce a variety of primitive compartment structures, and associated functions, that may have plausibly been present on early Earth, followed by examples of both discriminate and indiscriminate speciation affected by primitive modes of compartmentalization. Finally, we discuss modern technologies, in particular, droplet microfluidics, that can be applied to studying speciation phenomena in the laboratory over short timescales. We hope that this discussion highlights the current areas of need in further studies on primitive speciation phenomena while simultaneously proposing directions as important areas of study to the origins of life.

A material‐based panspermia hypothesis: The potential of polymer gels and membraneless droplets

“…[111,112] Such compartments providing encapsulation functionality are absolutely essential to the emergence and evolution of early life, as they provide a number of essential functions to a primitive system such as material exchange, recombination and evolution. [113,114] However, primitive compartments come in many sizes and shapes, including lipid bilayer vesicles, membraneless droplets and even mineral pores, each providing their own unique advantages and disadvantages. For example, lipid bilayer vesicles can provide stable compartmentalization to encapsulated genetic polymers, [115,116] which would have promoted genetic evolution.…”

“…However, depending on the composition, some lipid compartments are unstable in high salinity [117] or extreme pH, [118,119] although this instability can be ameliorated through introduction of divalent cation chelators (like citric acid) [120] or increasing membrane diversity. [121][122][123] Membraneless droplets generated by liquid-liquid phase separation (LLPS), [114,124] such as coacervates [125] or aqueous two-phase systems, [126] have also shown the ability to segregate primitive biomolecules such as RNA or peptides. [127][128][129][130] While such systems can be cyclically assembled and disassembled (e.g., through modulation of environmental conditions such as pH, salt, or temperature [131][132][133] ), depending on the composition, membraneless droplets may have been more "leaky" than vesicles to encapsulated components.…”

“…Evolution of a primitive compartment system with limited function into a complex compartment system, similar to a cell, may have been one mechanism by which the first cells emerged on early Earth. [ 114 ] However, this is not the only mode of primitive compartment evolution, as it may be possible for primitive compartments to evolve more function even if they do not ultimately become more “cell‐like.” [ 137 ] While it is plausible for a polymer gel material compartment to have evolved through sequence replication and evolution of an encapsulated genetic polymer (such as a nucleic acid), [ 164 ] it is also possible for such compartments to evolve in other ways. As such, here, we briefly discuss two modes by which polymer gel material droplets could have evolved: compositionally through evolution of its polymer sequence or structurally through acquisition of more complex structural components.…”

Effect of Multivalency on Phase-Separated Droplets Consisting of Poly(PR) Dipeptide Repeats and RNA at the Solid/Liquid Interface

“…Liquid–liquid phase separation (LLPS) has emerged as a ubiquitous physicochemical process in which solutions of biomacromolecules spontaneously separate into two liquid phases. , It has been considered to play an essential role in many fields, such as the protocell models in the origin of life research, , the formation of membraneless organelles like P granules, nucleoli, , and even the pathology of some specific neurodegenerative diseases like amyotrophic lateral sclerosis (ALS). − In principle, LLPS displays liquid-like properties with highly dynamic behaviors, where component molecules inside the droplets exhibit dynamic exchange with the surrounding environment on time scales of seconds through the interface. − The formation of LLPS is governed by synergistic intermolecular interactions, including electrostatics, cation−π, dipole–dipole, and π–π interactions …”

Increasing complexity of primitive compartments

“…DNA quadruplex structures have also been found to promote phase separation and droplet formation for protein-DNA coacervates [21]. Such structures and other functions may be affected by physical properties such as molecular crowding within the droplets [22][23][24], and as such, further demonstration of life-like functions within or by primitive membraneless droplets, such as catalytic cascades [25], primitive gene expression [26,27], or speciation [28] could lead to more understanding of the suitability of phase-separation as a relevant prebiotic compartmentalization method. To highlight recent work in the area of membraneless droplet assembly and function in our recent symposium, invited symposium speaker Tony Z. Jia (ELSI, Tokyo Institute of Technology and BMSIS) began with a discussion of a novel DNA liquid crystal coacervate droplets [19,20].…”