How do you know this? Were you there at the time? It is an assumption based on the observation of DNA sequences that resemble genes but appear (at present) to be nonfunctional, and which are assumed to be the byproduct of random and useless but not fatal mutations, and which is built entirely on the bare assumption of complete knowledge of the organism and its history, something we do not have because we were not there and could not observe it.
Cordially,
ROFL!
Sorry, you just entered the self-parody domain. My work here is done!