Well, that does make sense

Dogs and humans share DNA, thanks to viruses: Research suggests genetic material was exchanged over millennia

Humans and dogs may have exchanged genetic material over the millennia via viruses, scientists conjecture.

Retroviruses — the most infamous example of which is likely HIV, the virus that causes AIDS — have the ability to incorporate their genetic material into that of their hosts. In this manner, these hitchhikers can reproduce when their hosts do.

All mammals and most vertebrates, or creatures with backbones, apparently possess these "endogenous retroviruses" in their genomes. In fact, nearly 1 percent of the human genome consists of these unwelcome guests. Mice and opossums are even more greatly compromised, with these viruses making up about 2 percent of their genomes.