One of the still-unexplained mysteries surrounding the origin of life is how droplets of RNA floating in the primordial soup turned into membrane-encased cells, the building blocks of all life forms. A new study suggests that rainwater may have played a crucial role in forming a protective web around protocells about 3.8 billion years ago.
This step was crucial for the evolution of simple RNA droplets into complex cells from which all life forms eventually emerged.
The study, led by experts from the University of Chicago and the University of Houston, was published today in the journal Scientific advances.
Research focus
The study focuses on “coacervate droplets” – naturally occurring clusters of complex molecules such as proteins, lipids and RNA. These droplets, which resemble drops of oil in water, have long been considered possible precursors of the first protocells.
However, there was a problem: these droplets exchanged molecules too quickly, which would have prevented the differentiation necessary for evolution.
“If molecules are constantly exchanged between droplets or between cells, after a short time all cells look the same and no evolution takes place because you end up with identical clones,” explains lead study author Aman Agrawal.
Rainwater and the origin of life
To address this problem, researchers investigated the role of rainwater in stabilizing these droplets. Szostak, director of the Chicago Center for the Origins of Life at the University of Chicago, emphasized the interdisciplinary nature of this research, which combined insights from molecular engineering and chemical engineering.
“Engineers have been studying the physical chemistry of these types of complexes – and polymer chemistry in general – for a long time. It makes sense that there is expertise in engineering school,” said study co-author and Nobel Prize-winning biologist Jack Szostak.
“When we look at something like the origin of life, it is so complicated and involves so many aspects that we need the involvement of people with relevant experience.”
Which came first: RNA or DNA?
Szostak had previously theorized that RNA may have been the first biological material to evolve because of its ability to both store genetic information and catalyze chemical reactions.
This solved a long-standing problem: DNA encodes information but does not perform functions, while proteins perform functions but do not encode heritable information. RNA, on the other hand, can do both.
“RNA is a molecule that can encode information like DNA, but it also folds like proteins, so it can also perform functions like catalysis,” Agrawal said.
Coacervate drops in rainwater
Although RNA-containing coacervate droplets seemed like a natural next step in the evolution of life, Szostak’s 2014 work showed that RNA within these droplets was exchanged too quickly, preventing the stability necessary for evolution.
The new study shows that when these droplets are transferred to distilled water – similar to rainwater – a “hard skin” forms around them, slowing RNA exchange from minutes to several days, thus enabling mutation and evolution.
“You can make all kinds of droplets from different types of coacervate, but they don’t retain their own identity. They tend to exchange their RNA content too quickly. This has been a problem for a long time,” Szostak said.
“What we show in this new paper is that you can overcome at least part of this problem by transferring these coacervate droplets into distilled water – for example rainwater or fresh water of any kind – and they form a kind of hard skin around the droplets that prevents them from exchanging RNA contents.”
Potential of rainwater to stabilize procells
Agrawal’s initial research, which he began during his PhD at the University of Houston, focused on the behavior of coacervate droplets under an electric field. His work eventually caught the attention of Tirrell, who linked the research to the origin of life.
Tirrell asked where there might have been distilled water 3.8 billion years ago, and the answer was, of course, rainwater. This idea led to a collaboration with Szostak, who recognized the potential of rainwater to stabilize protocells and create the conditions for evolution.
Using RNA samples from Szostak, Agrawal was able to demonstrate that transferring coacervate droplets into distilled water significantly slowed RNA exchange, thus creating the potential for Darwinian evolution within protocell populations.
“If you have protocell populations that are unstable, they exchange their genetic material with each other and become clones. Darwinian evolution is not possible,” Agrawal said.
“But if they stabilize themselves against exchange so that they store their genetic information well enough, at least for several days, for mutations to occur in their genetic sequences, then a population can evolve.”
One step closer to discovering the origin of life
To address concerns about the practical applicability of their results, Agrawal and his team tested the stability of their droplets in real rainwater collected in Houston, as well as in laboratory water that had been modified to mimic the acidity of ancient rainwater.
The results were consistent: the net-like walls formed around the drops, supporting the idea that rainwater may have played a crucial role in the early evolution of life.
“The molecules we used to build these protocells are just models until more suitable molecules are found as replacements,” said Agrawal. “Even if the chemistry would be a little different, the physics would remain the same.”
This study helps researchers better understand the chemical and environmental conditions that enabled the evolution of protocells and brings them one step closer to solving the mystery of the origins of life.
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