Speciation, an evolutionary process by which new species form, is ultimately responsible for the vast biodiversity that is observed on Earth. Many aspects of speciation and even the definition of species are highly debated.  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, ELSI scientists Tony Z. Jia, Melina Caudan, and Irena Mamajanov discuss the possibility of speciation prior to the origin of life. The scientists describe several possible prebiotic speciation mechanisms emphasizing compartment-prompted speciation and modern technologies that can be applied to studying speciation phenomena in the laboratory over short timescales. The discussion highlights the current areas of need in further studies on primitive speciation phenomena while simultaneously proposing directions as essential areas of study to the origins of life.

Modern living organisms tend to live in colonies rather than in isolation in their natural environment. These colonies are, in turn, rarely comprised of a single type of organisms but an array of various species. The importance of a multitude of species, or biodiversity, has long been recognized in agriculture and functional ecology. In "On the Origin of Species" ^[1], Darwin recorded an observation that a plot of land sown with several distinct grass species was more productive than a similar grass-planted plot with a single species. Subsequent studies suggested that biodiversity ensures life's survivability in a catastrophic event and protects a community against parasite invasion. Recent studies estimate a staggering total of 8.7 million species, give or take 1.3 million, and further assess that 86% and 91% of all terrestrial and marine species, respectively, are yet to be discovered and cataloged ^[2].

Yet, one tends to think that life originated from a single system producing the first cell later diversified and complexified. The single-origin would have unlikely produce a stable living organism colony; single species would have been capable of coping with the early Earth's turbulent environment. In their new publication entitled, "Origin of Species Before Origin of Life: The Role of Speciation in Chemical Evolution," ELSI scientists Tony Z. Jia, Melina Caudan, and Irena Mamajanov explore the process of speciation before the onset of life.

In biology, there is no unequivocal definition of species. The species problem is the set of questions guiding the definition of species. Such a definition is referred to as a species concept; there are at least 26 recognized species concepts ^[3]. Species is usually defined as a taxonomic unit to fit the desired framework. In the prebiotic chemistry context, the authors suggested prebiotic species to be defined as delimited chemical systems of similar makeup and properties. Prebiotic species might or might not have physical borders but must be identifiable as separate entities. The species would be composed of commensurate assortments of chemical compounds and would possess similar reactivity and physicochemical characteristics, such as solubility and colligative properties. 

The prebiotic chemistry literature features multiple examples speciating chemical systems. For example, Patel et al. demonstrated ^[4] the synthesis of RNA, lipid, and peptide precursors in a cyanosulfidic protometabolic reaction network modulated by the dynamic co-flow schedules separate streams bearing different components. Adam et al. described radiolytic chemical reactions resulting in the differential synthesis of prebiotic chemicals affected significantly by differences in physical environments ^[5]. Vincent et al. showed that chemical ecosystems with variable dilution and recursion rates (simulating differences in primitive geological sites) resulted in differential emergence of mutually catalytic systems ^[6]. Oparin discussed the formation of complex coacervate assemblies subject to natural selection through environmental perturbations ^[7]. 

Perhaps the easiest to imagine prebiotic mechanism of speciation is one prompted by compartments with a clear identifiable boundary. In the origin of life research, most studied compartments are vesicles and coacervates; the concept also includes systems like oil droplets, micelles, and even mineral surfaces. Imagine a prebiotic soup with multiple types of simple molecules dissolved in it, picture a couple of compartments of any kind and start sorting the molecules within them. There are few ways of sorting; compartments can be selective and accept only certain types of molecules. Following the sorting step, one will end up with discriminately formed prebiotic species that would support different chemical processes and evolve into systems with vastly different compositions.

Alternatively, stuff the compartments with a random fraction of the prebiotic soup. The various compartments will still affect the chemistry within them and still develop into systems with vastly different compositions. These sorting methods broadly describe the discriminate and indiscriminate compartment-triggered prebiotic speciation mechanisms. 

The studies of prebiotic speciation can not rely on observational studies, such as paleontology or metagenomic studies of environmental niches. Since the prebiotic Earth can not be sampled and the timescales of prebiotic processes vastly exceed the average duration of doctoral or postdoctoral programs, there is a need to conceptualize any prebiotic chemistry experiment. Droplet microfluidics platforms, while unable to precisely mimic prebiotic Earth, offer advantages in studying compartmentalization-driven speciation. The technique generates in situ droplet compartments that can be either combinatorially composed or impregnated with different loads, resulting in multiple microreactors allowing researchers to probe a myriad of chemical, biochemical, or biological system variants simultaneously. Related techniques had already been utilized to study the chemical evolution of droplet motility ^[8,9].

The presence of multiple prebiotic species would allow the utilization of various resources, potentially lead to increased stability of each species through compatible interactions, and offer a greater chance of survival in the aftermath of catastrophic events. Prebiotic speciation is, therefore, a topic that warrants further in-depth research.

References:

[1] Darwin, C.On the Origin of Species, 5th ed.; D. Appleton and Company: New York, NY, USA, 1871.

[2] Mora, C.; Tittensor, D.P.; Adl, S.; Simpson, A.G.B.; Worm, B. How Many Species are There on Earth and in the Ocean?PLoS Biol.2011,9, e1001127.

[3] Wilkins, J.S. Philosophically speaking, how many species concepts are there?Zootaxa2011,2765, 58–60.

[4] Patel, B.H.; Percivalle, C.; Ritson, D.J.; Duffy, C.D.; Sutherland, J.D. Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism.Nat. Chem.2015,7, 301–307.

[5] Adam, Z.R.; Fahrenbach, A.C.; Kacar, B.; Aono, M. Prebiotic Geochemical Automata at the Intersection of Radiolytic Chemistry, Physical Complexity, and Systems Biology.Complexity2018,2018.

[6] Vincent, L.; Berg, M.; Krismer, M.; Saghafi, S.T.; Cosby, J.; Sankari, T.; Vetsigian, K.; Cleaves, H.J.; Baum, D.A. Chemical Ecosystem Selection on Mineral Surfaces Reveals Long-Term Dynamics Consistent with the Spontaneous Emergence of Mutual Catalysis.Life2019,9, 80.

[7] Oparin, A.I.; Braunshtein, A.E.; Pasynskii, A.G.; Pavlovskaya, T.E.The Origin of Life on the Earth; Elsevier: Amsterdam, The Netherlands, 1959; ISBN 9781483197371.

[8] Gutierrez, J.M.P.; Hinkley, T.; Taylor, J.W.; Yanev, K.; Cronin, L. Evolution of oil droplets in a chemorobotic platform.Nat. Commun.2014,5, 380.

[9] Hanczyc, M.M. Metabolism and motility in prebiotic structures.Philos. Trans. R. Soc. Lond. B Biol. Sci.2011,366, 2885–2893.