Genomes are huge. My 3-year-old daughter and I had a discussion about this. We came to the conclusion that it would take approximately 165 years to "stamp out" the human genome with her new stamping pad set. She was impressed!
All of the creatures you see with the unaided eye have undergone the process of development. Adults come from eggs, and eggs come from adults. It is necessarily circular, as cells come from cells. The egg is nothing more than a cell. In fact, the largest cell on the planet is an egg--an ostrich egg.
Co-option of retrovirus transposable elements into immune GRNs http://dx.doi.org/10.1126/science.aad5497
Impact of endogenous retrovirus propagation on the evolution of gene regulatory networks controlling immune function.