Rainwater may have played a key role in the origin of life

One of the enduring mysteries of life’s origin is how droplets of RNA floating in the primordial soup transformed into membrane-bound cells, which are the building blocks of all life forms. A new study proposes that rainwater might have played a crucial role in forming a protective mesh around protocells some 3.8 billion years ago. 

This step was essential in the evolution from simple RNA droplets to the complex cells that eventually led to all forms of life.

The research, led by experts at the University of Chicago and the University of Houston,  was published today in the journal Science Advances. 

Focus of the research 

The study focuses on “coacervate droplets” – naturally occurring clusters of complex molecules like proteins, lipids, and RNA. These droplets, similar to drops of oil in water, have long been considered potential precursors to 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 continually exchange between droplets or between cells, then all the cells after a short while will look alike, and there will be no evolution because you are ending up with identical clones,” explained study lead author Aman Agrawal.

Rainwater and the origin of life

To tackle this problem, the researchers explored the role of rainwater in stabilizing these droplets. Szostak, director of UChicago’s Chicago Center for the Origins of Life, noted 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 more generally – for a long time. It makes sense that there’s expertise in the engineering school,” said study co-author and and Nobel Prize-winning biologist Jack Szostak. 

“When we’re looking at something like the origin of life, it’s so complicated and there are so many parts that we need people to get involved who have any kind of relevant experience.”

Which came first: RNA or DNA?

Szostak had previously theorized that RNA might have been the first biological material to develop because it could both store genetic information and catalyze chemical reactions. 

This solved a longstanding problem: DNA encodes information but doesn’t perform functions, while proteins perform functions but don’t encode heritable information. RNA, on the other hand, can do both.

“RNA is a molecule which, like DNA, can encode information, but it also folds like proteins so that it can perform functions such as catalysis as well,” Agrawal said.

Coacervate droplets in rainwater

Although RNA-containing coacervate droplets seemed like a natural next step in the evolution of life, Szostak’s 2014 paper showed that RNA within these droplets exchanged too rapidly, preventing the necessary stability for evolution. 

The new study demonstrates that transferring these droplets into distilled water – similar to rainwater – creates a “tough skin” around them, slowing down RNA exchange from minutes to several days, allowing for mutation and evolution.

“You can make all kinds of droplets of different types of coacervates, but they don’t maintain their separate identity. They tend to exchange their RNA content too rapidly. That’s been a long-standing problem,” Szostak said. 

“What we showed in this new paper is that you can overcome at least part of that problem by transferring these coacervate droplets into distilled water – for example, rainwater or freshwater of any type – and they get a sort of tough skin around the droplets that restricts them from exchanging RNA content.”

Potential for rainwater to stabilize protocells

Agrawal’s initial research, which began during his PhD at the University of Houston, focused on studying the behavior of coacervate droplets under an electric field. His work eventually caught the attention of Tirrell, who connected the research to the origins of life. 

Tirrell asked where distilled water could have existed 3.8 billion years ago, to which the answer was of course rainwater. This idea sparked a collaboration with Szostak, who saw the potential for rainwater to stabilize protocells and enable the conditions for evolution.

Working with RNA samples from Szostak, Agrawal demonstrated that transferring coacervate droplets into distilled water significantly slowed RNA exchange, creating the potential for Darwinian evolution within protocell populations.

“If you have protocell populations that are unstable, they will exchange their genetic material with each other and become clones. There is no possibility of Darwinian evolution,” Agrawal said. 

“But if they stabilize against exchange so that they store their genetic information well enough, at least for several days so that the mutations can happen in their genetic sequences, then a population can evolve.”

One step closer to discovering life’s origins 

To address concerns about the real-world applicability of their findings, Agrawal and his team tested the stability of their droplets in actual rainwater collected in Houston, as well as lab water modified to mimic the acidity of ancient rainwater. 

The results were consistent: the meshy walls formed around the droplets, supporting the idea that rainwater could have played a critical role in the early evolution of life.

“The molecules we used to build these protocells are just models until more suitable molecules can be found as substitutes,” Agrawal said. “While the chemistry would be a little bit different, the physics will remain the same.”

This study brings researchers closer to understanding the chemical and environmental conditions that allowed protocells to evolve, moving one step closer to unraveling the mystery of life’s origins.

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