Rainwater's role in forming first protocell walls explored by researchers

One of the major unanswered questions about the origin of life is how droplets of RNA floating around the primordial soup turned into the membrane-protected packets of life we call cells.

A new paper by engineers from the University of Houston’s William A. Brookshire Department of Chemical Engineering (UH ChBE) and the University of Chicago’s Pritzker School of Molecular Engineering (UChicago PME), along with biologists from UChicago's Chemistry Department, has proposed a solution.

In the paper published today in Science Advances, UH ChBE’s former graduate student Aman Agrawal (now a postdoctoral researcher at UChicago PME) and his co-authors – including UH ChBE’s Alamgir Karim, UChicago PME Dean Emeritus Matthew Tirrell, and Nobel Prize-winning biologist Jack Szostak – show how rainwater could have helped create a meshy wall around protocells 3.8 billion years ago. This step was critical in transitioning from tiny beads of RNA to every bacterium, plant, animal, and human that ever lived.

“While it is impossible to know the exact conditions on early Earth, our experiments show that this pathway for stabilizing protocells might have been a critical step in enabling evolution in these protocells,” said Karim. Karim is UH Dow Chair and Welch Foundation Professor of chemical and biomolecular engineering and director of both the International Polymer & Soft Matter Center and the Materials Engineering Program at UH.

“This is a game-changing discovery in the context of pre-biotic life,” Karim added.

University of Houston Prof. Alamgir Karim first suggested rain as a possible source of distilled water that would have existed when protocells first formed. The investigator trained under Karim while at UH.

UChicago Pritzker School postdoctoral researcher Aman Agrawal discusses his coacervate droplet research with Nobel Prize laureate Jack Szostak. Agrawal began his research at UH, initially unaware of its possible implications for life’s early formation.

The research looks at “coacervate droplets” – naturally occurring compartments containing complex molecules like proteins, lipids, and RNA. These droplets have long been considered candidates for the first protocells but faced issues due to rapid molecular exchange which hindered differentiation and competition 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,” Agrawal explained.

Szostak noted that understanding such complexes involves interdisciplinary collaboration: “When we're looking at something like the origin of life...we need people to get involved who have any kind of relevant experience.”

In earlier work dating back to 2014, Szostak demonstrated that RNA in coacervate droplets exchanged too rapidly for stable evolution. However, transferring these droplets into distilled water resulted in them forming tough skins that restricted RNA content exchange.

Agrawal started transferring coacervate droplets into distilled water during his PhD research without an initial focus on life's origins but later saw its relevance through collaborative discussions with Tirrell who connected him with Szostak's ongoing work on prebiotic chemistry.

Further tests showed similar results even when using actual rainwater collected during downpours in Houston or lab water modified to mimic rainwater acidity – supporting their hypothesis that ancient rainwater could stabilize protocells leading to evolutionary processes.

“This approach...is possible and can work together to compartmentalize molecules...putting researchers closer than ever to finding...conditions that allow protocells to evolve,” Agrawal stated.