No-one knows, nor is it clear that they share a common origin. There is some reason to believe some viruses predate cellular life but since viruses leave no fossils we have no direct evidence to work with.
quote:Comparative genomics provides no evidence of a monophyletic origin of all viruses. Many virus groups simply share no common genes, effectively, ruling out any conventional notion of common origin. ... For example, most of the viruses of hyperthermophilic crenarchaea have literally no genes in common with any other viruses, whereas RNA viruses share with DNA viruses and plasmids that replicate via the rolling circle mechanism only extremely distant domains in their respective replication proteins.
Which leaves us with genetic evidence and (informed?) speculation.
Absolutely, but let's not confuse coherent hypothesis with directly evidenced theory.
all other organisms tend to increase their functionality/complexity , like secreting enzymes, developing protective coats around them, etc. But in case of viruses, they became more simpler and lost their functions rather than gaining new ones.
Viruses are developing new functions all the time. They're extremely refined replicators with some of the most remarkable adaptations.