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?

What is life?

Saccharomyces cerevisiae

After a century discussing about the origin of life, we are not closer to knowing what did happen. In the mid-twentieth century, when Stanley Lloyd Miller performed the famous experiment where he applied energy to a mixture of methane, hydrogen, ammonia and water, and obtained amino acids, scientists announced the imminent manufacture of artificial life in the laboratory. Such estimates are often too optimistic. In this case they were.

The first question to be solved here is what is meant by being alive. If we consider the problem carefully, we’ll find beings that are clearly alive and others that definitely are not. Plants, animals and ourselves are alive. Stones, distilled water, carbon dioxide, are not. In these cases deciding is no trouble. When Antony van Leeuwenhoek discovered microorganisms (yeast, infusorians, bacteria, spermatozoa and red blood cells) nobody doubted that they are alive. But things are not always so simple.