In a chapter in a new book on symbiosis by Springer Nature, I redefine and defend the essential nature of viruses as "non-cellular, capsid-encoding obligate symbionts". I also propose that they have played, and are still playing, a vital role in the evolution of biodiversity, and massively so the evolution of the placental mammals, and contribute an essential role in the health and functioning of the biosphere. When you put all of that together, it becomes rational to classify viruses as the fourth, and only non-cellular, domain of life.

Publication of the book was delayed by COVID. I outlined some of the evidence for the above in my book Virusphere, which was aimed at a non-expert reader. When the chapter is published, hopefully in August, I shall duly provide the reference.