News — On Halloween, the night when the line between the living and the dead grows blurred, the streets fill with creatures that refuse to die: zombies, vampires, ghosts.
But all year round, nature is teeming with real-life examples of astonishing survival and even eternal youth: the jellyfish that resets its life cycle, the salamander that spends its entire life in a larval state, the newt that regenerates parts of its eyes. Some organisms bend the rules of life, reminding us that scientific truth can be stranger than supernatural legend.
These fascinating life forms speak to a timeless human obsession: escaping our ultimate fate. Could we humans tap into our own Halloween-worthy biology to outwit death?
, a professor of zoological medicine in the Faculty of Veterinary Medicine at Université de Montréal, helps us separate myth from reality in the world of species with seemingly supernatural powers.
It’s all about simplicity
The jellyfish Turritopsis dohrnii can revert to the polyp stage, its “baby” form, over and over again, allowing it to rejuvenate and live forever. The axolotl, a type of salamander, retains its juvenile traits and can reproduce in its larval state, in which it has gills. Some fish can grow back their fins and spiders can regenerate legs. Some starfish can regrow an entire central disk from just one arm and a fragment of the central disk. Aquatic flatworms called planarians have an unlimited capacity for regeneration thanks to their stem cells.
What do all these creatures have in common? They are fish, amphibians or invertebrates—primitive animals, on the whole, that belong to the anamniote group, animals that don’t have an amniotic sac in the embryonic stage.
These species have a simpler immune system than mammals, one that doesn’t trigger the complex inflammatory responses that limit scar tissue formation in amniotes. “It’s easier for them to regenerate a limb because, when their cells divide to reform a body part, there isn’t such an intense immune response to block it,” Vergneau-Grosset explained.
So these creatures have special abilities not because they are more evolved than mammals but because their immune systems are more permissive. For example, spiders can grow back their legs due to the simplicity of their anatomy: a chitin shell filled with fluid (hemolymph). During molting, the chitin forms a new leg, and the hemolymph simply fills the new space.
Impressive, but...
Vergneau-Grosset pointed out that when animals grow back body parts, the new limb is rarely perfect. In the axolotl, some toes may be missing on a regenerated leg, and a fish’s new fin often has deformed bony rays. “They’re little monsters,” she joked.
In geckos, which can voluntarily shed their tails to escape predators, the new tail is thicker than the original. Geckos can do this, Vergneau-Grosset explained, because they have precise break points in their vertebrae and satellite stem cells in their muscles. The new tail can’t grow back again if it breaks, but it does retain its sensitivity and mobility; the nerve regeneration that this implies is particularly intriguing for research.
What about humans?
Could humans benefit from—or at least take inspiration from— these fascinating animal traits? Are there genetic mechanisms here that could be useful for human medicine? Could we too conquer time?
“It’s very difficult to apply these processes to humans because mammals are very complex organisms,” Vergneau-Grosset said. “To regenerate limbs, we would need to inhibit the immune system, which isn’t always desirable for the body as a whole. Transplant rejection, in which the new tissue is attacked by the immune system, is a good example of this mechanism.”
Moreover, some of the animals mentioned have tissues composed largely of water and hyaluronic acid, a molecule that promotes water retention and tissue elasticity. The cosmetics industry makes extensive use of its hydrating capacity, especially in “anti-aging” creams.
“But for now, that’s where it ends,” said Vergneau-Grosset. “Stem cells are a burgeoning field of research, but we’re not yet regenerating limbs or implanting them, and we’re venturing into territory fraught with ethical questions.”
For now, we are left with October 31 to don our spookiest costumes and pretend we can defy the sands of time.
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