When you look in the mirror and wish you were a decade younger, what youre really asking for is reverse development, or reverse aging. Though most animals, including humans, are born, age, and eventually die, some species can break away from this traditional lifecycle: they seemingly defy age and revert to younger versions of themselves. Turritopsis dohrnii, dubbed the immortal jellyfish, is the best-known of such species. A recent study published in Proceedings of the National Academy of Sciences, however, has revealed a new member of this exclusive club with extraordinary abilities: the ctenophore Mnemiopsis leidyi, also called the comb jelly. Now, scientists are wondering how many more time-traveler species there might really be. The work challenges our understanding of early animal development and body plans, opening new avenues for the study of life cycle plasticity and rejuvenation. The fact that we have found a new species that uses this peculiar time-travel machine raises fascinating questions about how spread this capacity is across the animal tree of life, Joan J. Soto-Angel, a postdoctoral fellow at the University of Bergen who co-wrote the study, said in a statement. An unintentional discovery is at the origin of this study. Soto-Angel began investigating the topic after a larval ctenophore suddenly appeared in the place of an adult ctenophore in a tank in his lab. As it turned out, however, it was the same individual. Soto-Angel and his colleagues consequently began trying to reproduce the scenario that might trigger reverse development and discovered that an adult ctenophore can, in fact, revert to a larval stage when experiencing extreme stress.Witnessing how they slowly transition to a typical cydippid larva as if they were going back in time, was simply fascinating, Soto-Angel explained. Over several weeks, they not only reshaped their morphological features, but also had a completely different feeding behavior, typical of a cydippid larva. Comb jellies are ancient animalsin fact, some research suggests that they might be the first animal to have ever existed, emerging roughly 700 million years agoleading the researchers to suggest that reverse development might be a primordial ability.This is a very exciting time for us, Paul Burkhardt, a researcher at the University of Bergen who also co-wrote the study, said in the statement. This fascinating finding will open the door for many important discoveries. It will be interesting to reveal the molecular mechanism driving reverse development, and what happens to the animals nerve net during this process. The two researchers suggest that life cycle plasticityan organisms ability to change aspects of its biology when it experiences specific environmental stimulimight be more present among animals than scientists realized, according to the statement. As for humans, unfortunately, all the creams and products in the world have yet to achieve such enviable anti-aging results. However, future research in developmental biology and aging now has a new example in the animal kingdom to draw inspiration from.