[ceejayoz]

Insertions can occur naturally.

https://en.wikipedia.org/wiki/Insertional_mutagenesis

> Because many viruses integrate their own genomes into the genomes of their host cells in order to replicate, mutagenesis caused by viral infections is a fairly common occurrence. Not all integrating viruses cause insertional mutagenesis, however.

This paper does seem to allege that these particular sets of insertions aren't via natural causes, though.