Ivancevic agrees, and wants to sequence BovB in many more species, parasites included. “I feel like the [patterns we have] look really sporadic because we don’t have all the intermediate species,” she says. By looking at more animals, it may become easier to trace the gene’s history.

That was certainly the case when Ivancevic turned her attention to a different retrotransposon called L1. This jumping gene is more directly relevant to humans because it fills up 17 percent of our genome. Most copies are now broken and stationary, but some have kept their ability to move around—and their presence has been linked to diseases like schizophrenia and cancer.

L1 exists in the genomes of almost every mammal, and was presumably around in the DNA of our common ancestor. And while it can jump around any given genome, scientists believed that, unlike BovB, it doesn’t move between species.

Not so, says Ivancevic. She identified at least three possible cases in which L1 seems to have jumped between major animal groups, typically those that live in water. And she showed that it’s completely absent from the DNA of platypuses and echidnas, which means that it must have jumped into mammalian genomes after these oddities had split off from the main dynasty, between 160 and 191 million years ago.

“There’s not yet a smoking gun for this transfer, like the identification of the donor species,” says Edward Chuong, soon to be at the University of Colorado Boulder. Still, “their failure to find any trace of L1 in [platypuses and echidnas] is fairly strong evidence supporting their claim.”

“These [jumping genes] really seem to be good at moving between genomes,” says Alexander Suh, from Uppsala University. But why, he asks, are some genes like BovB more likely to do so than L1, or at least more likely to establish themselves in a new setting? Possibly, Ivancevic says, it’s because BovB is about half the size of L1 and so is easier to move. Maybe it’s because BovB hitches a ride on parasites, and L1 doesn’t.

Whatever the reason, both genes—and others like them—must surely have influenced the evolution of their hosts. Jumping genes were first discovered by the pioneering scientist Barbara McClintock in the 1940s, but it took decades for people to realize how common they are—and how influential. They could potentially cause diseases by jumping into the wrong place and disrupting vital genes. But they could also provide raw material for evolution, by reshaping or rewiring existing genes. In this way, they spurred the evolution of the placenta, and supercharged our immune systems.

“It’s mind-boggling to think about how just a few [jumps] have fundamentally altered the course of our evolution,” says Chuong. Evolutionary biologists like to wonder what would happen if we replayed the tape of life—if we went back to some earlier point in history and let evolution run its course again. Would history repeat itself? Chuong thinks not. These jumps are so unpredictable, but so potentially important when they happen, that it’s hard to imagine the events unfolding twice.

And what would happen if we ran the tape forward? Where will genes like BovB end up next, and how will they shape the destinies of their hosts? “It would be interesting to see what BovB looks like in a few million years, but I’ll probably not get a chance to do that,” says Ivancevic.

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