Ed Yong in The Atlantic:
Imagine if a word in a book—say, bubble—had the ability to magically copy itself, and paste those copies elsewhere in the text. Eventually, you might bubble end up bubble bubble with bubble bubble bubble sentences bubble bubble bubble bubble like these.
This is exactly what happens in our genomes. There are genes known as retrotransposons that can copy themselves and paste the duplicates in other parts of our DNA, creating large tracts of repetitive gobbledygook. Around half of the human genome consists of these jumping genes. And a quarter of a cow’s DNA consists of one particular jumping gene, known as BovB. It, and its descendants, have bloated out the cow genome with thousands of copies of themselves.
This jumping gene seems to have entered the cow genome from the unlikeliest of sources: snakes and lizards.
Retrotransposons typically jump around within a single genome, but sometimes they can travel further afield. Through means that scientists still don’t fully understand, they can leave the DNA of one species and enter that of another. And so it is with BovB. No one knows the animal in which it originated. But from that mystery source, it has jumped into the DNA of snakes and cows, elephants and butterflies, ants and rhinos.