Are viruses alive?

Viruses have genomes of DNA or RNA, they mutate, they are subject to selection, and they have deep evolutionary histories that can be reconstructed.

They have no ribosomes, no metabolism that runs without a host, and no way of dividing. Some carry genes for parts of a cell’s metabolism, such as the photosynthesis proteins of viruses that infect cyanobacteria, but those work only inside the cell they infect. Reproduction is carried out by the host’s machinery, directed by the virus’s instructions.

Viruses discovered since 2003 in amoebae have particles as large as some bacteria and genomes with more than a thousand genes, including genes for parts of the protein-making machinery that viruses were supposed not to have.

That did not answer the question. It made the boundary harder to draw exactly where most definitions had put it.

The 2013 report of pandoraviruses described particles about a micrometre long, visible under a light microscope, with genomes of up to 2.5 million base pairs and more than two thousand predicted genes. A 2018 reannotation by the same group, checked against the viruses’ RNA and proteins, cut the larger genome’s count to about 1,430.

Even after that, about seven in ten of their proteins resemble nothing outside the pandoravirus family, viral or cellular. The first report read that as an origin that cannot be traced to any known cellular lineage. The 2018 study proposed another reading: many of those genes, especially the ones unique to a single strain, may have been made from scratch out of non-coding DNA.

They still have no ribosomes and no way of dividing, so by the usual criteria they are viruses. They are viruses with more genes than some bacteria.

A 2022 study found a voltage across the membrane of one pandoravirus’s particles, and genes weakly resembling most of the enzymes of the cycle cells use to make energy, one of which worked as an enzyme. The team did not detect the particles making ATP.

Three accounts have long competed. Viruses may be reduced cells that lost everything but their genomes; they may be genetic elements that escaped from cells and acquired coats; or they may descend from the world before cells, when replicating molecules preceded cellular organisation. A 2019 proposal mixes the last two: primordial replicators that took the proteins for their coats from the cells they infected.

Each explains part of the evidence. Some viral proteins have no cellular counterparts at all, which is difficult for the escape account, and many groups of viruses share no genes at all, which is difficult for a single origin of any kind.

Viruses are the most numerous biological entities known. In the ocean an estimated 10²³ infections take place every second, a major cause of death for marine life. The cells they burst spill their contents into the water, which changes how carbon moves through it.

That is not a question about definitions. Whatever they are called, they are a major part of how the living world works, and were left out of the accounting for most of the twentieth century.