Evolution News - Closing Creationism's God-Shaped Gap

Models for potential precursors of cells endure simulated early-Earth conditions | Eberly College of Science

Another shovel-full of science has just been thrown into Creationism's favourite God-shaped gap - abiogenesis - by a bunch of scientists from Penn State University Eberley College of Science.

The abiogenesis question is the increasingly-used last resort of Creationists who can no longer mount a serious challenge to the science of evolution. Never slow to use a dishonest false dichotomy fallacy, they challenge scientists to explain how the first 'life' assembled without the intervention of a magic creator, and to replicate it in a laboratory - as though the ultimate proof of a scientific theory is to replicate it in a laboratory, regardless of the fact that the theory might involve a whole planet or conditions only found near hydrothermal vents in the ocean abyss, and millions of years!

But that God-shaped gap just got smaller, as it has been doing now for several decades, as chemists, physicists and geologists discover more about the conditions on the early Earth when living systems first arose, and about the chemical and physical processes that were possible, even likely under those conditions.

This piece of research, for example, shows how easy it would have been to make membraneless compartments (in which the necessary machinery for RNA replication could have arisen) in conditions where cycles of dehydration and rehydration occur - conditions which would probably have been common on early Earth.

Membraneless compartments, called complex coacervates, which form micrometer-sized droplets (center), are widely studied as models of protocells, a potential step in the evolution of life on Earth. New research shows that the droplets behave as predicted by an experimentally derived phase diagram (left) in response to a proposed early-Earth environmental process, the wet-dry cycle as could be seen as small ponds or puddles evaporate and reform. The preference for RNA molecules (fluorescently labelled red in the right panel) to accumulate inside the droplets decreases as the solution dries.

Credit: Hadi Fares, Penn State

From the Pen State Eberley College press release by Sam Sholtis:

Membraneless compartments—models for a potential step in the early evolution of cells—have been shown to persist or form, disappear, and reform in predictable ways through multiple cycles of dehydration and rehydration. Such wet-dry cycles were likely common conditions during the early development of life on Earth and could be a driving force for reactions important for the evolution of life.

Understanding how the compartments—known as complex coacervates—respond to wet-dry cycling also informs current applications of the droplets, which are found in many household items, such as adhesives, cosmetics, fragrances, and food, and could be used in drug delivery systems. A paper describing the research, led by Penn State scientists, appears October 27, 2020 in the journal Nature Communications.

“Wet-dry cycling has gotten attention recently in attempts to produce molecules that could be the precursors to life, things like the building blocks of RNA, DNA, and proteins,” said Hadi Fares, a NASA Postdoctoral Program Fellow at Penn State and the first author of the paper. “We are looking into a possible step further in the evolution of life. If these building blocks form compartments—the precursors of cells—what happens if they undergo the same type of wet-dry cycling?”

The researchers make membraneless compartments, which form through liquid-liquid phase separation in a manner akin to oil droplets forming as a salad dressing separates, by controlling the concentrations of reagents in a solution. When the conditions—pH, temperature, salt and polymer concentrations—are right, droplets form that contain higher concentrations of the polymers than the surrounding solution. Like oil drops in water, there is no physical barrier or membrane that separates the droplets from their surroundings.

Dehydrating the solution, like what could happen during dry periods on a pre-life Earth where small ponds or puddles might regularly dry up, changes all of these factors. The researchers, therefore, wanted to know what would happen to the membraneless compartments in their experimental system if they recreated these wet-dry cycles.

“We first mapped out how the compartments form when we alter the concentrations of the polymers and the salt,” said Fares. “This ‘phase diagram’ is experimentally determined and represents the physical chemistry of the system. So, we know whether or not droplets will form for different concentrations of polymers and salt. We can then start with a solution with concentrations at any point on this phase diagram and see what happens when we dehydrate the sample.”

If the researchers start with a solution with concentrations that favor the formation of droplets, dehydration can change the concentrations such that the droplets disappear. The droplets then reappear when the sample is rehydrated. They can also start with a solution in which no droplets form and dehydration could bring the concentrations into the range that droplets begin to form. The behavior of the droplets during dehydration and rehydration match the predictions based on the experimentally derived phase diagram and they continue to do so through several iterations of the wet-dry cycle.

So, experimentation showed that the coacervates could reform when a dehydrated solution was rehydrated. The next thing to do what see what this meant in terms of preservation of coacervate contents such as RNA.

Next, the researchers addressed the ability of droplets to incorporate RNA molecules inside of the membraneless compartments. The “RNA world” hypothesis suggests that RNA may have played an important role in the early evolution of life on Earth and previous experimental work has shown that RNA in these solutions becomes concentrated inside of the droplets.

As mixtures of membraneless compartments, called complex coacervates, are dried, the concentrations of all components increase as the total volume decreases. The preference of an added RNA molecule to locate within the coacervate droplets decreases with drying while its mobility increases. These results emphasize the importance of carefully considering the environment in studies of membraneless coacervate compartments as models of protocells in the early evolution of life on Earth.

Credit: Hadi Fares, Penn State

“As we dry droplets that contain RNA, the overall concentration of RNA in the solution increases but the concentration of RNA inside the droplets remains fairly stable,” said Fares. “The preference of RNA molecules to be inside the droplets seems to decrease. We believe that this is because as they dry the composition inside the droplets is changing to look more like the composition outside the droplets.”

The research team also looked at the ability of RNA to move into and within the droplets during dehydration. As they dry the sample the movement of RNA into and out of the droplets increases massively, but movement within the droplets increases only modestly. This difference in RNA mobility could have implications for the exchange of RNA among droplets during dehydration, which could in turn be functionally important in protocells.

“What we are showing is that as the membraneless compartments dry, they are able to preserve, at least to some extent, their internal environment,” said Fares. “Importantly, the behavior of the coacervates, or protocells, whether they persist or disappear and reappear through the wet-dry cycle, is predicable from the physical chemistry of the system. We can therefore use this model system to think about the chemistry that might have been important for the early evolution of life.”

This showed that any evolutionary changes made, especially during the dehydration stage, when the contents of the coacervates become increasingly concentrated, were preserved, allowing evolutionary changes to accumulate over time.

The team's results were published open access a couple of days ago in the journal, Nature Communications:

Abstract

Wet-dry cycling on the early Earth is thought to have facilitated production of molecular building blocks of life, but its impact on self-assembly and compartmentalization remains largely unexplored. Here, we investigate dehydration/rehydration of complex coacervates, which are membraneless compartments formed by phase separation of polyelectrolyte solutions. Solution compositions are identified for which tenfold water loss results in maintenance, disappearance, or appearance of coacervate droplets. Systems maintaining coacervates throughout the dehydration process are further evaluated to understand how their compartmentalization properties change with drying. Although added total RNA concentrations increase tenfold, RNA concentration within coacervates remains steady. Exterior RNA concentrations rise, and exchange rates for encapsulated versus free RNAs increase with dehydration. We explain these results in light of the phase diagram, with dehydration-driven ionic strength increase being particularly important in determining coacervate properties. This work shows that wet-dry cycling can alter the phase behavior and protocell-relevant functions of complex coacervates.

[...]

Discussion

The experiments in this paper point to an intriguing richness in phase behavior and physical properties of PDADMA/PAA complex coacervates as they dry and rehydrate, and to how these changes can be understood using the phase diagram. The experiments with mimics successfully linked the wet-dry cycling process to movements on the phase diagram. They also provided structural insights on the expanding polymeric network as this distinctive linear compositional change occurred, and predicted the effects of varying the direction of these movements. Based on the generality of the ionic strength and (de)hydration effects underlying these behaviors, we anticipate that similar wet-dry cycling response may be possible for other complex coacervate systems.

[...]

It is thought-provoking to consider that the composition (as well as macromolecules and salt types) and drying method of various ponds on an early Earth could have led to a large number of dehydration processes, providing access to compositionally and functionally different coacervate populations, even if the available components were limited. Considering several recent reports that discuss the potential of coacervates to modulate the reactivity of ribozymes and other enzymes^20,21,53, the changes observed here are expected to result in a wide array of reactivity profiles within the concentrated phase... [My emphasis]

Fares, Hadi M.; Marras, Alexander E.; Ting, Jeffrey M.; Tirrell, Matthew V.; Keating, Christine D.
Impact of wet-dry cycling on the phase behavior and compartmentalization properties of complex coacervates
Nature Communications (2020) 11(1), 5423 . doi: 10.1038/s41467-020-19184-z

Copyright: © 2020 The authors. Published by Springer Nature
Open access
Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0)

It's on days like this that normal people must be glad they don't suffer from the juvenile thinking defect that causes creationism, so findings like this have to be ignore for fear of upsetting the imaginary, invisible, magic mind-reading sky boogie their mummy and daddy frightened them with.

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