At the heart of the cell’s operations is the relationship between DNA, RNA, and proteins. Although the complete role of each is more complicated, the traditional reduced “central dogma” of molecular biology puts it this way: DNA makes RNA makes proteins. Proteins do the work in the cell. DNA contains the instructions for building proteins. RNA acts as the messenger, copying DNA instructions which are then “read” by the ribosome as it assembles proteins. Any explanation of how the cell came to be must take into account this three-part system.
The “RNA-world” hypothesis proposes that life evolved first based on RNA, and that DNA and proteins came later. The general picture proposed in this hypothesis is that life began as some form of self-replicating RNA, which later evolved into dependent associations with DNA and proteins to make up cells as we know them today. The advantage of this proposal is that it posits the “core” of the DNA-RNA-protein triad first, allowing the other two to develop “on either side”, so to speak, and it is also supported by the fact that RNA possesses many other, central functions in the cell. However, like all origin of life theories, the RNA world hypothesis is still almost entirely speculative, and is subject to the challenge of explaining just how something that is today an irreducible system could have emerged in a step-wise manner.
Now a new challenge to the RNA world hypothesis has emerged in the analysis of the ribosome. The ribosome is the protein/RNA structure which is responsible for reading RNA messages copied from the DNA and assembling proteins. By examining the evidence for the history of the ribosome, researchers have concluded that the earliest parts of the ribosome suggest that the ancient, early ribosome was never part of an “RNA world”, but rather always existed as part of an RNA-protein relationship:
“This is the crucial piece of the puzzle,” Caetano-Anollés said. “If the evolutionary buildup of ribosomal proteins and RNA and the interactions between them occurred gradually, step-by-step, the origin of the ribosome cannot be the product of an RNA world. Instead, it must be the product of a ribonucleoprotein world, an ancient world that resembles our own. It appears the basic building blocks of the machinery of the cell have always been the same from the beginning of life to the present: evolving and interacting proteins and RNA molecules.”
Or in other words, the continued picture of the origin of life is that it has always been “life as we know it” at the most basic level. Research has yet to uncover any evidence that life was ever simpler at the core cellular level. The gross-scale forms of life have clearly evolved over time. The fundamentals of life, on the other hand, have always been there, as far as evidence shows. This is not to say that there is no relationship between the non-living world and the emergence of life; it simply shows that, for science, those origins remain mysterious and speculative.