The RNA World hypothesis resolves the chicken-and-egg conundrum posed by the structure of growth shared by all living organisms. DNA encodes RNA, which directs the synthesis of proteins. Proteins do the biochemical work of capturing energy. This energy is directed into the synthesis of new copies of DNA, resulting in the growth of new cells and organisms.
Proteins require DNA to store information, but DNA also requires protein to do biochemical work. Neither DNA nor protein can support life alone. It’s a complex system, far too complex to have appeared spontaneously at life’s origin. Something simpler must have preceded it.
Tom Cech and Art Zaug’s discovery that RNA could do biochemical work[1] , in addition to its known ability to store information, was an electrifying revelation. RNA could be both a chicken and an egg. The origin of self-replicating systems became an addressable scientific problem, not an embarrassing mystery.
The catalytic properties of RNA were added to several other clues regarding primitive life:
- RNA is central to the protein synthesis process. Indeed, the ribosome, the site of protein synthesis, is mostly RNA and it is the RNA portion that links amino acids together into proteins[2] .
- RNA-like nucleotides are still present in the enzymes that catalyze many fundamental biochemical reactions[3] , suggesting that protein enzymes gradually took over from RNA enzymes.
- The building blocks of DNA are made from RNA precursors, indicating that RNA preceded DNA.
But attempts to create self-replicating RNA worlds under plausible early-Earth conditions have pretty much failed. The fundamental problem is the RNA is not very stable in water. It requires a constant input of energy in order for synthesis rates to outpace degradation rates.
Nick Lane used this observation (along with many others) to postulate that the RNA World was an intermediate stage of biotic evolution. He proposes that a primitive form of energy metabolism appeared in alkaline hydrothermal vents. RNA precursors captured this energy and began the process of Darwinian evolution that resulted in life as we know it today[4] . I’ve written a bit more about this here.
The RNA World probably was not at the origin of life. But it almost certainly existed, and was a critical step in the jump-starting of Darwinian evolution and thus the appearance of complex life forms. It is possible that life could have evolved without the existence of self-coding and self-replicating molecules, but it may not have proceeded nearly as fast or as far, and could well have failed altogether.
Published on Forbes
Footnotes
[1] Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena.
[2] Structural biology. The ribosome is a ribozyme.