A major source of the genetic variability in species is due to viruses. Some viruses can insert their genetic information into the DNA of their host cells, a process known as endogenization. If this insertion occurs in a germ cell (giving rise to eggs or sperm) then all subsequent generations will contain that virus.
Think this is a rare event? About 8% of the human genome appears to have originated from viruses. Many of these viral insertions must have predated our separation from the other apes, as modern chimpanzees have viruses in the exact same positions in their genomes.
Until now, scientists had thought that the only group of viruses that could integrate itself in this manner were retroviruses. These viruses have an RNA genome. They carry encode an enzyme called reverse transcriptase that transcribes that RNA into DNA, which is then incorporated into the host genome.
Now Keizo Tomonaga and his team at Osaka University in Japan have discovered that bornavirus (BDV) are also capable of integrating themselves. Although BDV contains an RNA genome, it is not normally reverse transcribed into DNA. Nevertheless, endogenous Borna-like N (EBLN) elements were found in a variety of mammals, including humans. In addition, the researchers were able to show that BDV is still capable of inserting itself into the genome of human cultured cells.
For those who are interested, this topic has been covered in more depth by the blog Not Exactly Rocket Science.