Their research shows that deionized water's interaction with coacervate protocells could've formed a "meshy skin layer" that kept these cells from near-immediate fusion with one another.
The question of "how life began on Earth" has been an obsession of scientists and theologians alike for millennia. After more than a decade of research, scientists feel pretty confident that Earth's first life forms were protocells, which emerged from organic molecules found on Earth. The prefix "proto" is doing a lot of heavy lifting in that word -- if modern cells found throughout our bodies are the ultra-sleek Bugattis of cellular evolution, these protocells were like rusted-out Model Ts.
Our modern cells are enclosed in membranes that control what moves in and out, but early protocells contained no such structure. So, how exactly did they keep their ancient RNA from essentially leaking out?
Turns out, rain is the answer. At least, that's the argument of the University of Chicago's Aman Agrawal, who recently wrote an article in The Conversation detailing his research into the quest to answer this membrane conundrum. Published in August in the journal Science Advances, Agrawal and his colleagues came to a startling conclusion that maybe the answer came from the skies -- literally.
"My colleagues...and I explored a fascinating possibility that rainwater played a crucial role in stabilizing early cells," Agrawal wrote for The Conversation, "paving the way for life's complexity."
There are two primary types of protocells -- vesicles and coacervates -- but this new research focuses on the latter. Agrawal described coacervates as "droplets formed from an accumulation of organic material," and said that they can be best visualized as a "drop of cooking oil suspended in water." These droplets would've been made of two fundamental components: matrix material (which would have formed the overall structure) and genetic material (like RNA).
First discovered in 1929, coacervates were considered the first protocells, as their primitive compartmentalization was essential for creating early metabolic process. But without a membrane, Agrawal said, the RNA within these droplets would rapidly fuse together and ultimately destroy any chance at genetic variability. Of course, the genetic variability found on the planet today means something intervened that allowed these protocells to avoid immediate fusion.
"[A 2017 study showed] that rapid fusion and exchange of materials can lead to uncontrolled mixing of RNA, making it difficult for stable and distinct genetic sequences to evolve," Agrawal wrote. "This limitation suggested that coacervates might not be able to maintain the compartmentalization necessary for early life."
In 2022, Agrawal conducted a study that found that deionized water (free of both ions and minerals) could stabilize these protocells and help them avoid incessant fusion. The water's interaction with oppositely charged polymers on the protocell's periphery formed a "meshy skin layer," which provided a kind of primitive membrane that possibly helped cells maintain their structure.
"We placed two separate protocell populations, treated with deionized water, in test tubes. One of these populations contained RNA," Agrawal wrote. "When the two populations were mixed, RNA remained confined in their respective protocells for days. In contrast, when we mixed protocells that weren't treated with deionized water, RNA diffused from one protocell to the other within seconds."
Because rainwater is a major source of ion-free water, the researchers argue that this natural source might've stabilized protocells on a prebiotic Earth -- essentially kickstarting life. The paper itself doesn't quite address the lofty philosophical question of why life exists on Earth, but it offers compelling evidence as to how the very first protocells could've evolved beyond their membrane-less existence.