How did life on earth begin?
No one can say for sure. But for centuries, scientists have developed and tested hypotheses. Some theories suggest that lightening catalyzed chemical reactions in the earth’s early atmosphere, creating amino acids and sugars that gave birth to life. Others postulate that energy and mineral rich deep sea vents might have nurtured the building blocks necessary for life. Another suggestion, first proposed in the 1960s, is that ribonucleic acid (RNA) is responsible for life on earth. This line of thinking has a name, the ‘RNA world hypothesis,’ and it has steadily gained traction within the scientific community over the last few decades.
Support for the RNA world hypothesis began to mount in 1982 when a particular type of RNA molecule called a ribozyme was discovered. As Charles Q. Choi at LiveScience notes about the hypothesis, “Nowdays DNA needs protieins in order to form, and proteins require DNA to form, so how could these have formed without each other? The answer may be RNA, which can store information like DNA, serves as an enzyme like proteins, and helps create both DNA and proteins1.” A particular type of ribozyme called the Group I introns (RNA molecules with the capacity to synthesize proteins and, importantly, to splice RNA molecules) may have had an essential role in how the earliest strands complex of RNA formed.
Here at PSU, Ph.D. student Tharuka Jayathilaka, working under the direction of Professor Niles Lehman, Department of Chemistry, in the Center for Life in Extreme Environments, investigates these particular ribozymes, using their capacity to splice RNA into smaller fragments in order to get at an answer to the question: if DNA and enzymatic proteins evolved from an RNA world, then how did large RNA molecules form without DNA; under what conditions; and from what humble beginnings?
“What we’re trying to do in the lab,” Jayathilaka said, “is find a simple form of an RNA molecule that might have been present when life began.”
In the lab, Dr. Lehman and his research team have been able to use Group I introns to splice RNA molecules into four fragments of 39-63 nucleotides long in order to observe them reassemble in a prebiotic soup into strands of around 200 nucleotides long. As a part of her Ph.D. work, Jayathilaka has found a way to splice the RNA molecule into five smaller fragments of around 30 nucleotides—a laboratory innovation, the results of which she presented at the university-wide student research symposium and plans to publish. With smaller fragments of RNA, Jayathilaka can get closer to determining what the minimum size a fragment needs to be if it is to join others in the formation of a longer, more complex molecule.
“The reason for this study,” Jayathilaka said, “is that we believe that at the beginning of life on earth, essential resources were in very low concentrations. And so we thought that under prebiotic conditions, shorter molecules that require fewer resources might have a better chance at coming together and forming longer molecules. That’s what we’re looking for: how short can we make the fragments while retaining their enzymatic properties as well as their ability to pass genetic information on.”
There are ongoing debates as to when life took hold on earth, though many agree it happened right at the beginning some 3.85 to 3.6 billion years ago. While science will never know with certainty the earliest events in the history of life, the curiosity and dedication of scientists like Tharuka Jayathilaka can help identify a potential starting point, a place where the RNA world, the possible precursor to the DNA-driven world we live in now, began.
Author: Shaun McGillis