Researchers have identified a number of marine worm species that have evolved the ability to completely regrow their heads and brains after having them severed, despite sharing a common ancestor that couldn't. The discovery could help scientists understand the traits that allow animals to dramatically regrow lost limbs.

All animals are able to regenerate themselves to a certain extent. Human beings, for example, are able to regrow skin cells over a cut, or on a larger scale regenerate entire sections of organs, such as the liver. However, our abilities pale in comparison to those of other species, such as the amphibious axolotl, which are capable or growing back lost arms, legs, tails and even sections of damaged spinal cord.

The regenerative abilities of other creatures are nothing short of miraculous. For example, one species of worm, Lineus Sanguineus, can grow an entire body from a sliver of flesh one two-hundred thousandth the size of its original, intact form.

It is broadly accepted that these incredible abilities are passed down from a common ancestor. As species became more biologically complex, the majority of them lost the ability to regrow entire limbs.

A new study has challenged this line of thinking by providing evidence that a number of marine ribbon worm species may have independently developed dramatic regenerative abilities, and that their powers are not entirely the result of a hand-me-down from a common ancestor.

The international team behind the new research collected specimens of 22 ribbon worm species discovered along coastal regions including the US, Argentina, Spain and New Zealand. They tested the regenerative abilities of the worms by cutting them apart from front to back and observed how much of the animal was able to regrow.

The research was supplemented by data from a further 13 species of marine ribbon worms that had been experimented on as part of previous studies.

All of the worms were found to have the ability to regrow the tail section of their bodies, but only eight species had the ability to regenerate their heads. Four of the species identified had not previously been known to boast this ability.

The scientists then used their data to reconstruct the evolutionary regeneration pattern for the entire phylum. They discovered that the worms' ability to regrow their heads and brains had developed relatively 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," said Alexandra Bely, an associate professor of biology at the University of Maryland, and one of the authors of the new paper. "One of these origins is inferred to have occurred just 10 to 15 million years ago."

In evolutionary terms this is very recent. For context, the first regenerative characteristics were thought to have appeared roughly 500 million years ago, prior to the onset of the Cambrian Period.

According to the researchers, their work represents the clearest documentation of animals gaining regenerative abilities. Further examination of the species involved could even reveal some of the traits that must be present to develop dramatic regenerative powers.

The study may also have discovered species of worms that are in the process of developing the ability to regrow their heads and brains. A number of the specimens that weren't able to regrow their heads managed to survive for months after being bisected. It's possible that being capable of surviving such an injury for a prolonged period is the first step in learning to regrow these vital areas.

"These species that have recently evolved head regeneration will make for excellent models for studying the emergence of new regeneration abilities in animals," Bely said. "We can now ask such questions as what changes in molecular processes led to novel head regeneration ability."

The paper has been published in the journal Proceedings of the Royal Society B.

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