A Major Update To The RNA World Hypothesis

A computer model of the ribosome complex. Older components
are shown in red while the younger are in blue

DNA is the molecule which encodes all the necessary information for a cell to generate the proteins it needs in order to grow, repair and stay alive. Yet its complexity has thrown up issues as to how it could have first appeared in the Earth's primordial soup. Biological molecules are delicate. DNA is even more so. It can be broken down by high temperatures, highly acidic or alkaline environments, radiation and the reactive substances around it.

In 1986, a paper published in the scientific journal 'Nature' suggested that the simpler genetic molecule RNA may have evolved first and then developed into DNA. This became known as the RNA world hypothesis. This described increasingly complex interactions between the building blocks of life at a molecular level, leading to the first ever cells. This theory was popular as it simplified the way that genetic molecules may have evolved from simple beginnings to today's level of complexity.

Since then other theories supporting this have arisen including a precursor to the RNA world itself known as the TNA world which uses threose nucleic acid, an even simpler molecule, as the earliest form of genetic information. However a recent study by scientists from the University of Illinois has shown that the RNA world could not have existed unless all the necessary components were present.

Ribose nucleic acid is the basic encoding molecule. It is part of a vast molecular complex containing multiple strands of RNA, interacting with up to 80 different proteins, in a structure known as a ribosome. It is similar to the way office workers intake, process and output information quickly and efficiently. If one worker is not present the system becomes flawed just as if one protein is present, the ribosome can fail.

The current theory states that the peptidyl transferase centre (PTC for short and the part of the ribosome which is responsible for the actual synthesis of proteins) is the most ancient part of the entire structure and appeared first in the primordial soup.  Analysis of the structures in archaea, the most simple and ancient forms of life on the planet, allowed the researchers to build up a dated family tree of the ribosome's protein components.

They found that many of the proteins used by the RNA were actually present long before the actually genetic molecule itself and that the PTC evolved soon after the assembly of the true most ancient components.' This is the crucial piece of the puzzle,' said Gustavo Caetano-Anollés, leader of the study. 'If the evolutionary build-up 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.'

The study has shown that there would have been yet another step in the route from simple chemical compounds to DNA itself. It is possible that the TNA world hypothesis may also have had a nucleoprotein stage, further simplifying the pathway and making clearer how DNA may have evolved. The fact that the components of Earth life have not changed since they first evolved suggests that it only evolved once and did not branch out using different chemicals as the lynchpins to their biology.