evolution - How did viruses come to be?

I found a book chapter for you here

Quick summary:
3 hypotheses to Origin of viruses

1. From pre-cellular world (virus first hypothesis)


2. From reductive evolution of parasites (reduction hypothesis)


3. From fragments of cellular genetic material (escape hypothesis)

Drawbacks:

1. virus require cells (to infect) so how can they come first


2. virus do not look like known reduced parasites from Bacteria/Eukarya/Archaea


3. unlikely that genetic fragments form complex viral structures for viral function

Because of these drawbacks, the problem of virus origin was for a
long time considered untractable and not worth serious consideration

The rest of the chapter looks more in-depth into the 3 hypotheses