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 and the , and biologists from the have proposed a solution.
In the paper , UH ChBE鈥檚 former graduate student (now a postdoctoral researcher at UChicago PME) and his co-authors 鈥� including , and 鈥� show how rainwater could have helped create a meshy wall around protocells 3.8 billion years ago, a critical step in the transition from tiny beads of RNA to every bacterium, plant, animal, and human that ever lived.
鈥淲hile 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,鈥� 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.
鈥淭his is a game-changing discovery in the context of pre-biotic life,鈥� Karim said.
The research looks at 鈥渃oacervate droplets鈥� 鈥� naturally occurring compartments of complex molecules like proteins, lipids, and RNA. The droplets, which behave like drops of cooking oil in water, have long been eyed as a candidate for the first protocells. But there was a problem. It wasn鈥檛 that these droplets couldn鈥檛 exchange molecules between each other, a key step in evolution, the problem was that they did it too well, and too fast.
Any droplet containing a new, potentially useful pre-life mutation of RNA would exchange this RNA with the other RNA droplets within minutes, meaning they would quickly all be the same. There would be no differentiation and no competition 鈥� meaning no evolution.
Graphic enlargement of rainwater that could have helped create a meshy wall around protocells 3.8 billion years ago, a critical step in the transition from tiny beads of RNA to every bacterium, plant, animal, and human that ever lived. (UChicago Pritzker School of Molecular Engineering / Peter Allen, Second Bay Studios)
And that means no life.
鈥淚f 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,鈥� Agrawal said.
Engineering a solution
Life is by nature interdisciplinary, so Szostak, the director of UChicago鈥檚 , said it was natural to collaborate with both , UChicago鈥檚 interdisciplinary school of molecular engineering, and
鈥淓ngineers have been studying the physical chemistry of these types of complexes 鈥� and polymer chemistry more generally 鈥� for a long time. It makes sense that there's expertise in the engineering school,鈥� Szostak said. 鈥淲hen 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.鈥�
, Szostak started looking at RNA as the first biological material to develop. It solved a problem that had long stymied researchers looking at DNA or proteins as the earliest molecules of life.
鈥淚t's like a chicken-egg problem. What came first?鈥� Agrawal said. 鈥淒NA is the molecule which encodes information, but it cannot do any function. Proteins are the molecules which perform functions, but they don't encode any heritable information.鈥�
Researchers like Szostak theorized that RNA came first, 鈥渢aking care of everything鈥� in Agrawal鈥檚 words, with proteins and DNA slowly evolving from it.
鈥淩NA 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,鈥� Agrawal said.
RNA was a likely candidate for the first biological material. Coacervate droplets
were likely candidates for the first protocells. Coacervate droplets containing early
forms of RNA seemed a natural next step.
That is until Szostak poured cold water on this theory, publishing a paper in 2014 showing that RNA in coacervate droplets exchanged too rapidly.
鈥淵ou 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鈥檚 been a long-standing problem,鈥� Szostak said. 鈥淲hat 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 鈥� for example, rainwater or freshwater of any type 鈥� and they get a sort of tough skin around the droplets that restricts them from exchanging RNA content.鈥�
鈥楢 spontaneous combustion of ideas鈥�
Agrawal started transferring coacervate droplets into distilled water at the 兔子先生, studying their behavior under an electric field. At this point, the research had nothing to do with the origin of life, just studying the fascinating material from an engineering perspective.
鈥淓ngineers, particularly Chemical and Materials, have good knowledge of how to manipulate material properties such as interfacial tension, role of charged polymers, salt, pH control, etc.,鈥� said 兔子先生 , Agrawal鈥檚 former thesis advisor and a senior co-author of the new paper. 鈥淭hese are all key aspects of the world popularly known as 鈥榗omplex fluids鈥� - think shampoo and liquid soap.鈥�
Agrawal wanted to study other fundamental properties of coacervates during his PhD. It wasn鈥檛 Karim鈥檚 area of study, but Karim had worked decades earlier at the University of Minnesota under one of the world鈥檚 top experts 鈥� Tirrell, who later became founding dean of the UChicago Pritzker School of Molecular Engineering.
During a lunch with Agrawal and Karim, Tirrell brought up how the research into the effects of distilled water on coacervate droplets might relate to the origin of life on Earth. Tirrell asked where distilled water would have existed 3.8 billion years ago.
鈥淚 spontaneously said 鈥榬ainwater!鈥� His eyes lit up and he was very excited at the suggestion,鈥� Karim said. 鈥淪o, you can say it was a spontaneous combustion of ideas or ideation!鈥�
Tirrell brought Agrawal鈥檚 distilled water research to Szostak, who had to lead what was then called the Origins of Life Initiative. He posed the same question he had asked Karim.
鈥淚 said to him, 鈥榃here do you think distilled water could come from in a prebiotic world?鈥欌�� Tirrell recalled. 鈥淎nd Jack said exactly what I hoped he would say, which was rain.鈥�
Working with RNA samples from Szostak, Agrawal found that transferring coacervate droplets into distilled water increased the time scale of RNA exchange 鈥� from mere minutes to several days. This was long enough for mutation, competition, and evolution.
鈥淚f 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,鈥� Agrawal said. 鈥淏ut 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.鈥�
Rain, checked
Initially, Agrawal experimented with deionized water, which is purified under lab conditions. 鈥淭his prompted the reviewers of the journal who then asked what would happen if the prebiotic rainwater was very acidic,鈥� he said.
is free from all contaminants, has no salt, and lives with a neutral pH perfectly balanced between base and acid. In short, it鈥檚 about as far from real-world conditions as a material can get. They needed to work with a material more like actual rain.
What鈥檚 more like rain than rain?
鈥淲e simply collected water from rain in Houston and tested the stability of our droplets in it, just to make sure what we are reporting is accurate,鈥� Agrawal said. Agrawal and fellow UH graduate student Anusha Vonteddu grabbed a couple of beakers from Karim鈥檚 lab to collect some rainwater just outside the Agrawal Engineering Research Building during a downpour.
鈥淎grawal and Vonteddu, with their rain samples in beakers, set out to prove our major hypothesis that rainwater could have stabilized the protocells on early Earth,鈥� said Karim.
In tests with the actual rainwater and with lab water modified to mimic the acidity
of rainwater, they found the same results. The meshy walls formed, creating the conditions
that could have led to life.
鈥淭his is a game-changing discovery in the context of pre-biotic life,鈥� Karim said.
The chemical composition of the rain falling over Houston in the 2020s is not the rain that would have fallen 750 million years after the Earth formed, and the same can be said for the model protocell system Agrawal tested. The new paper proves that this approach of building a meshy wall around protocells is possible and can work together to compartmentalize the molecules of life, putting researchers closer than ever to finding the right set of chemical and environmental conditions that allow protocells to evolve.
鈥淭he molecules we used to build these protocells are just models until more suitable molecules can be found as substitutes,鈥� Agrawal said. 鈥淲hile the chemistry would be a little bit different, the physics will remain the same.鈥�
Written at and edited by the 兔子先生 with permission.