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?

One of the long-held theories is that viruses could be cellular living beings reduced to the bare minimum, which have dispensed with everything unnecessary, in order to become parasites of cellular beings. If this were the case, viruses would also descend from LUCA.

But since many biologists seem determined that viruses are not living beings, they should not be descendants of LUCA. So could there be another relationship between them?

In the decade from 2010 to 2019, some huge viruses have been discovered, with genomes larger than those of the smallest bacteria (mycoplasmas). Among them are Megavirus chilensis (discovered in 2011), whose genome is 1.2 megabases; the Pandoravirus genus (2013), one of whose species (P. salinus) reaches 2.5 megabases and one micron in diameter, making it as large as many bacteria; and two new, even larger giants, discovered in 2019 but not yet confirmed, which parasitize marine worms called chaetognaths. These newly discovered giant viruses appear to have ribosomes, so they would be able to perform by themselves some of the functions for which other viruses have to resort to a host. Some of the genes of giant viruses were previously considered exclusive to cellular beings, because they have to do with metabolism and the translation of genes into proteins. If all this is confirmed, these viruses would be fully-fledged living beings, even according to Maynard Smith's definition. These discoveries have led to the formation of a new group of viruses, the nucleocytoplasmic large DNA viruses, or Nucleocytoviricota, which are divided into two large subgroups, both considered giant viruses: the Megaviricetes and the Pokkesviricetes. The human smallpox virus is classified in the latter group.

There are currently two proposals on how to link the tree of giant viruses to the tree of life. In the first, the two trees would be parallel and would have separated from the beginning (before LUCA), as in the attached figure.

In the second proposal, cellular living beings would be descendants of giant viruses, instead of their ancestors, according to the attached figure. If this were the case, viruses would have to be considered as living beings and we would have to invent a new LUCA, older than previously thought.

The same post in Spanish

Thematic thread on Primitive Life: Preceding Next

Manuel Alfonseca