Todd Whitcombe
Science and You
Last week, Dr. Stephen Rader presented an interesting talk at Caf Scientifique on the "RNA World." This is not a new planet or a code name for some science fiction-based video game. Rather, it is a postulate about how biological organisms could have evolved on Earth.
To explain, consider some of the major turning points in the evolution of life on this planet. For example, about 200,000 years ago, modern humans first appeared in the geological record. We are descended from a common ancestor with other modern primates with the split occurring about five million years ago. Although we are only one of the many species inhabiting this planet, we tend to view these as important points in the story of life.
But if you go back further in time, various forms of life have come and gone.
About 65 million years ago, dinosaurs went extinct. Around 250 million years ago, 83 per cent of all species of life on this planet were wiped out at the end of the Permian era.
The late Ordovician extinction, some 455 million years ago, saw 57 per cent of life eradicated. All of which are major turning points in the story of life.
Another major turning point in the geological record tells us that some 1,200 million years ago, single celled organisms discovered a neat trick. They began to aggregate into multi-cellular colonies and organisms. The photosynthetic Volvox is a modern day remnant of these earlier times.
Before that time, somewhere around 2,000 million years ago, the first eukaryotes appeared. These are organisms with a nucleus that keeps DNA separate from the rest of the cellular apparatus. It was a major event in the history of life as it was also at this time that a variety of other organelles appear to have arisen, a separate and fascinating tale.
But prior to the rise of the eukaryotes, the world was dominated by bacteria and archaea. They reigned supreme for around 1,800 million years. Prior to 3,800 million years ago, there is little evidence of life as we know or understand it.
Yet, somewhere in the first 500 million or so years, life did get started. The "primordial soup" gave rise to the complex molecules that are the basis of all life on Earth. To understand what might have happened in that time, we need to consider what defines "life," which is a bit tricky and open to a certain amount of interpretation.
However, I don't think that anyone would dispute some form of minimum criteria centred on two basic processes. The first is that living organisms have the ability to grow and, by necessity, this means that they must consume the chemicals around them to make the more complex chemical compounds they use. The second is that living organisms must be able to reproduce.
Both of these actions require some form of molecular machinery in order to proceed.
In modern cells, we know that this machinery consists of three distinct parts. DNA carries the instructions in the form of genes which code for the synthesis of proteins. It is the "blue print." RNA takes the instructions from the DNA and builds the cell's protein molecules. It is the "contractor." Proteins are the components that make up all of the material in a cell and, here's the twist, they are responsible for making DNA and RNA. They are the "product" and also the "machines."
The question then arises, if you need to have DNA to make RNA, and RNA to make proteins, and proteins to make DNA, which came first? Isn't this the "chicken and egg" problem?
The answer has been revealed over the past 20 years. Biochemists have discovered that RNA - ribonucleic acid - has some pretty amazing properties. It can play all three roles.
RNA is capable of building proteins all by itself. It is also capable of reproducing itself. Or, put another way, RNA can do the jobs that DNA and protein do now, just not as well. But RNA appears to be entirely self-sufficient as a molecular species and capable of starting life all by itself.
Hence, "RNA World." At the very beginning of time, it appears that RNA may have been the simple building block from which all other life began. All of the major turning points along the way may have been just stepping stones in the evolution of RNA. It is capable of all the tasks necessary but early life certainly wouldn't be "life as we know it."
As to "where did RNA come from?", that isn't really that hard to explain. It is composed of three components - a sugar, ribose, which can be generated from formaldehyde, a nucleic base which is synthesized from hydrogen cyanide, and phosphate, which is abundant in rocks.
According to the RNA World hypothesis, nothing much more is needed to give rise to life on Earth.
