Giant Viruses

John Maynard Smith

In 1966, H.J. Muller defined living beings as follows: Any being capable of multiplying with inheritance and variation. With this definition, which prioritizes reproduction and evolution as the definition of life, viruses should be considered as living beings. Other biologists, such as John Maynard Smith, thought that this criterion was too broad. It would mean that nucleic acids are alive, since they are capable of reproducing with inheritance and variation. That is why they propose adding another criterion: A living being is capable of reproducing and metabolizing. This would exclude nucleic acids, and therefore viruses, which are nucleic acids enclosed in a protein capsule, and viroids, which are isolated nucleic acids.

The tree of life, the family tree of all species of living beings, seems to indicate that all beings formed by one or more cells descend from a single individual, the first living being, which is called LUCA (Last Universal Common Ancestor). As I said in another post, some think that perhaps LUCA was not a single individual, but a network of individuals. But where do viruses come from?

Is there a tree of life?

Tree of life

Many biologists would not hesitate to answer this question in the affirmative. However, in these lines I am going to raise the possibility that the answer should be negative. Let us see why.

First of all, what is a tree? When we use this term, talking about the tree of life, we don’t mean a biological tree, i.e. a plant with a woody trunk that grows to a height of several (or many) meters. We mean a mathematical tree. But what is that?

The three trunks of the tree of life

The tree of life

As time goes by and more and more genomes of living beings of very different types are sequenced, we are learning a lot about the tree of life. This is a summary of what we know:

• From the fact that all current living things use the same genetic code (with very minor variations) it follows that all the living beings we know, current or extinct (including viruses), descend from a single ancestor, unknown, of course, because there is no trace of in the fossil record, and if we found it, we would not recognize it. This hypothetical common ancestor has received the curious name LUCA, the acronym of Last Universal Common Ancestor. The first living creature should be placed at the very origin of the tree of life (in the root). Many biologist also think that this common ancestor appeared over 3000 million years ago, near the hydrothermal vents found on the mid-ocean ridges that separate the plates of the earth’s crust, where the magma in the mantle tends to rise to the surface.