The majority of our retrotransposons can no longer jump. They’re genetic fossils, which have mutated so much that their days of wanderlust are behind them. But one group of sequences—the L1 or LINE-1 elements—includes a small number that are still on the move. They’re still copying and pasting themselves, still creating variation between people, still causing disease.

It seems that our genome takes a particularly conservative attitude to L1 elements. By comparing our cells to those of our closest relatives, chimpanzees and bonobos, Carol Marchetto and Inigo Narvaiza from the Salk Institute for Biological studies have shown that we keep these slippery bits of DNA under a particularly tight leash. By contrast, the other two apes allow L1s to move with much greater abandon—a trait that might help to explain why their genetic diversity is far greater than ours.

Marchetto and Narvaiza began by reprogramming skin cells from four humans, two chimps and two bonobos into a state where they’re almost like stem cells. Rather than being stuck down the skin path, these stemmy cells (or iPSCs) can produce all the various types of cell in their host bodies.