Since recording that video the Galway team have been working to understand how Hydractinia rebuilds its severed body and hope to publish their findings shortly in a scientific journal. While they’re keeping schtum about the details, the paper will focus on how the creature marshalls its stem cells to regrow its head – for example, how stem cells know the head’s missing – and where exactly the embryonic stem cells come from.

Studying Hydractinia has also led Frank and colleagues to ask a bigger question: why can only a few animals regenerate while most can’t? A salamander can regrow a lost tail but closely related frogs can’t regrow a lost limb. And if a tiny marine creature can regrow its own head, why can’t humans even regrow their adult teeth? After all, says Frank, it’s not as if human and Hydractinia stem cell systems are so very different.

Ancient ancestor

Key stem cell processes are ancient and common to many animal species. For instance, the complex “Wnt” signalling system, which controls stem cells in developing embryos and, when uncontrolled, causes cancer, is very similar in all animals, including Hydractinia and people. It’s one of a handful of complex stem cell systems, each involving hundreds of elements, which have remained the same since Hydractinia branched off the evolutionary tree that eventually led to us around 600 million years ago.

Over the past decade or so, researchers have started to believe that stem cells first evolved in a creature even more ancient than Hydractinia, whose soft body has long since dissolved in ancient seabeds. In this as-yet-unknown creature, the power of regeneration may have first evolved, says Frank, endowing all later animals with a basic toolkit for regrowing lost body parts – one which mainly lies dormant in present-day life.