The DNA that makes up the human genome is the blueprint for the biochemical processes that constitute the embryonic development of humans. That's all that is really needed: a recipe for the step-by-step development of a baby. The DNA is a long chain of instructions for the creation of proteins that, in the right order, conduct the chemical processes.
And that human genome contains a lot of DNA that came from viruses.
There's debate about whether viruses are 'living' - but at one level, that's just semantics. They are organisms that cannot survive outside a host, and consist of little beyond a strand of DNA (or RNA) and the mechanism to breach a host cell and use the host to help replicate the genetic material.
Retroviruses are so called because their genome is RNA, and they use a reverse transcription process to make DNA equivalents of that RNA. That DNA can end up being integrated into the host cell's DNA (with the help of an integrase enzyme).
That in itself would have no evolutionary significance, except that it's possible for that DNA integation to happen in any germline cells - that is, one of a successive set of cells that would ultimately create the sperm and egg cells. If so integrated, the inserted DNA would then become inherited.
This is the endogenous retrovirus, ERV, and according to a recent New Scientist article, 8% of the human genome clearly comes from that source, and over 50% probably does.