An international team of researchers has found that at least four species of marine ribbon worms are capable of regrowing their head after being decapitated.

Wait. What?!

Seriously. The study now published in the Proceedings of the Royal Society B found that of the 35 species of marine ribbon worms (Tubulanus sexlineatus – we’re just going to let you sit with that scientific name) surveyed, at least four were able to regenerate their entire heads – brains included.

To some degree, all animals have the ability to regenerate. In fact, your skin will regenerate itself about once a month just for the sake of good hygiene, and even more in the event of injury. Though it’s not all that common, the ability to regenerate amputated limbs has been observed throughout the animal kingdom, from salamanders to spiders and marine organisms like sea stars. This trait was assumed to have been an ancient one that some species were able to maintain throughout the course of their evolution. Wrong. As it turns out, these ribbon worms have independently evolved to regrow their heads – and all the bits that they encase. The authors say their new research shows that animals can gain regenerative abilities and could help us understand how such traits evolve.

“This means that when we compare animal groups we cannot assume that similarities in their ability to regenerate are old and reflect shared ancestry,” said study author Alexandra Bely in a statement. “We need to be more careful when comparing regeneration findings across different groups of animals.”

To study the emergence of new regeneration abilities in worms belonging to the phylum Nemertea, the team collected ribbon worms along the coasts of the US, Argentina, Spain, and New Zealand between 2012 and 2014. After performing operations on 22 of the species, they found that all were able to completely regenerate their backends. However, only eight were able to regrow their heads – four of which scientists had previously known about. A 2013 study found that beheaded flatworms are capable of not only regenerating their head and brain after decapitation but retained some of their memories through the trauma. 

But what’s perhaps more interesting is the number of ribbon worms that weren’t able to regrow their heads. As the authors note, a study from the 1930s found that one species of worm was able to regrow its entire body and head from just one two-hundred-thousandths, or 0.0005 percent, of its former self. (For reference, that’s like regrowing a 150-pound person from just one-sixteenth of a teaspoon.) Previous work led researchers to believe that modern worms had lost their ability to regenerate, but the research suggests the trait was probably evolved fairly recently.

“The ancestor of this group of worms is inferred to have been unable to regenerate a head, but four separate groups subsequently evolved the ability to do so,” Bely said. “One of these origins is inferred to have occurred just 10 to 15 million years ago.”

The team says their research could help understand how regeneration comes into being.