A couple of possibilities.
They could be very simplified versions of cellular organisms; cellular parasites that simplified to the point of no longer being cells.
They could be the products of cellular organisms. Some cellular organism experienced a mutation that gave it a set of genes that reproduced independently of the organism itself -- sort of like a cancer.
They could be an independent product of the abiogenetic process that also produced cellular life.
Probably other possibilities exist.
Clearly no one really knows; this is a continuing area of research.
"The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one." -- George Bernard Shaw
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