Unidentified ribbon worm out on the reef at night, Phylum: rhynchocoela, Pierbaai, Curacao, Netherlands Antilles, (Photo by Wild Horizons/UIG via Getty Images)
Regeneration describes the spellbinding spectacle of an animal’s ability to regrow a lost or injured body part in that the structures and functions are restored, which differs from when scars form during wound healing. Animals vary widely in their ability to regenerate, suggesting that regenerative ability is a prehistoric trait that has a rich evolutionary history.
However, our grasp of the evolution of regeneration is limited because it has only been studied in too few species that are too distantly related to be compared. Understanding how regeneration is gained during evolution across related animal groups could provide insight into how body parts can be activated to regenerate when damaged.
A recent study surveying closely related marine ribbon worm species found that the ability to regenerate an entire head, including a brain, evolved fairly recently in some seaworm species. This research shows that these seaworms are useful for studying the evolution of regeneration and pinpointing how animals, and maybe humans, can gain regenerative abilities.
Regeneration for the Dar-win
Regeneration of specific body structures, such as heads, tails, limbs, and appendages (e.g. the antlers of a deer) as well as whole body regeneration from a single, small fragment are both found scattered across organisms made of more than one cell.
In humans, some tissues, such as skin, and large organs including the liver regrow quite readily, while others have been thought to have little or no power to regenerate, such as the heart.
Simpler multicellular animals like those without two-sided symmetries—such as sponges, comb jellies, corals, sea anemones, and jellyfish—generally have high regenerative ability. This suggests that some of the earliest animals had high regenerative ability.
Animals that have a head and a tail as well as a back and a belly—and, therefore, they also have a left side and a right side—have extremely inconsistent regenerative abilities. This indicates that the pattern of evolution for regeneration within these animals is complex.
For example, for crustaceans (e.g. lobsters, crabs, and shrimp) and insects, regenerative abilities are generally very restricted, with limb regeneration being the main exception. But, for most animals with two-sided symmetries, regenerative ability ranges widely.
A worm’s eye view of regeneration
Estimating where and when changes in regenerative abilities have happened across animal groups is a critical step towards understanding how regeneration evolves. However, evidence for clear increases in regenerative abilities in animals is very limited. So far, no studies across relatively close groups have yet uncovered clear gains of regeneration.
For instance, although regeneration of limbs in salamanders and tails in lizards may show gains of regenerative ability, these events happened so long ago that methods comparing these animals to their closest relatives are unlikely to reveal important insights into their main causes.
Regrowing a whole animal from a worm-steak
Ribbon worms are tube-like, primarily marine predatory worms. These animals were thought to have high regenerative abilities, but this label is based almost entirely on the amazing ability of one seaworm species: Lineus sanguineus.
This species unquestionably is one of the champions of regeneration, with some of the highest regenerative abilities known among animals. Lineus sanguineus worms can be repeatedly cut into 200,000 pieces and they will spawn 200,000 full-size worms.
Dr. Bely explains, “If you take one of these worms and you take a slice out of it to like a little filet section out of the middle of it—you have essentially a worm steak that’s like a disc. If you took one of these small slices, one tiny sliver and then quartered it, even just one-quarter of that slice would be able to regenerate an entirely new individual.”
“When you think about it from the perspective of wounds, it’s extra-remarkable because the vast majority of the surface of that little piece of tissue is a wound surface without a normal covering,” Dr. Bely remarks. “So, the animal is able to re-establish an entire animal from a very small section that has a lot of wound area.”
Wormholes: regenerating across space and time
Yet, whether regenerative abilities are typical for this group of animals is unclear. So, Eduardo E. Zattara and Jon L. Norenburg set out on a worldwide trip, traveling to different places in Europe, South America, and the coasts of the United States collecting samples of seaworms.
At each place, they collected different species of seaworms, cut off heads or tails, and assessed whether those animals could regenerate these ends or not. Also, when back at the lab, they used DNA sequencing tools to map out the evolutionary links between their samples of seaworms.
“What was really surprising was, that given how fantastic this one, a phenomenal regenerator is, many of its relatives are not good at regenerating,” Dr. Bely says. “The majority of the species that we tested for their regenerative abilities were able to reform a new tail end, but the vast majority were not able to reform the head end.”
But, the big unexpected finding was that the ability to regenerate a head arose basically from scratch across several species of the seaworms they collected.
“These are the first animals where we have really clear evidence that the ability to regenerate a head has evolved from ancestors that were not able to regenerate a head,” says Dr. Bely. “The fact that we’re finding species that can regenerate a head and have close relatives—at least they’re the closest relatives that we assessed—were not able to regenerate a head, tells us that that ability to regenerate a head is an evolutionary novelty.”
In over our heads?
A movie moment etched in my mind is during Men in Black when Jeebs, an alien pawn shop owner played by Tony Shalhoub, shockingly regrows his head in an instant after having it blasted off by agent Kay’s, Tommy Lee Jones’ character, raygun. But will we ever come close to being able to achieve a similar feat—regenerating a whole human head?
To be honest, it seems unlikely—I don’t think humans can live very long, or at all, without a head. However, this study using this group of animals shows us is that there may be just a few molecules that could lead to very big changes in regenerative ability.
Dr. Bely speculates, “If, within just a couple of million years, an adult worm can go from not being able to regenerate a head to fully being able to regenerate a head with a brain, mouth, eyes, sensory structures, and more, then that understanding can give us optimism for being able to continue to look for ways to enhance human regenerative abilities.”
With heart disease, caused by scarring, still the leading cause of death worldwide and other prevalent wound-related health conditions like paralysis, there is a huge need for regenerative therapies.
In the future, maybe when people have heart attacks or experience spinal cord injury, there’ll be a shot or a pill to mend hearts and regrow nerves. This future world would no longer require transplant waiting lists and wheelchairs—failing hearts would mend and severed nerves would be regrown.
For now, we still can dream about regeneration, through the likes of healing spells in games and movies, such as Dungeons and Dragons or the Harry Potter series.