We know there are a few species that don't die of old age, like the giant tortoise and naked mole rat. But those species aren't truly immortal — as they still eventually die. These tiny worms might be a different story... one which could have major implications for humans trying to live longer.
Negligible senescence is the term for species where the death rate doesn't increase as individuals get older, and the animals don't experience the lost reproductive capability and diminished functions seen in all other species. That in itself is a plenty impressive biological feat — and figuring out why species like giant tortoise, naked mole rats, lobsters, and rockfish have beaten aging, in a way that all other species haven't, is a major topic of longevity research.
But negligible senescence isn't quite the same thing as true immortality — it's one thing for tortoises to live 250 years, but we wouldn't necessarily expect one to live 2,500 years, even if we (and our descendants) made it our mission in life to protect them from all conceivable harm. Tortoises simply don't get older. For immortality, what we really need is a species that remains young indefinitely.
That may seem like only a minor distinction, so let's look at an example of that second category to better understand what's going on here. Planarians, or flatworms, have basically evolved the Time Lord method of immortality with perpetual regeneration (and no need to bother about any thirteen-body limit). Although their version of regeneration doesn't involve changing their entire bodies all at once, they are able to restore any damaged vital organ, including their brains. That might even scramble their personalities a bit — assuming flatworms had personalities in the first place.
According to new research from the University of Nottingham, these flatworms really can live forever, barring accidents. Biologist Dr. Aziz Aboobaker explains:
"We've been studying two types of planarian worms; those that reproduce sexually, like us, and those that reproduce asexually, simply dividing in two. Both appear to regenerate indefinitely by growing new muscles, skin, guts and even entire brains over and over again. Usually when stem cells divide - to heal wounds, or during reproduction or for growth - they start to show signs of aging. This means that the stem cells are no longer able to divide and so become less able to replace exhausted specialised cells in the tissues of our bodies. Our aging skin is perhaps the most visible example of this effect. Planarian worms and their stem cells are somehow able to avoid the aging process and to keep their cells dividing."
The team's experiments suggest that the enzyme responsible for the flatworms' effective immortality is one known as telomerase. The telomeres are the protective "caps" found on the ends of chromosomes. As cells divide over and over, the telomeres start to fray. And when they get too short, the cells can no longer divide and replicate. This decay process is at the biological root of death of old age.
In many organisms, the enzyme telomerase works to maintain the telomeres during cell division. However, in almost all such species this enzyme is only active during the organism's development. Once it's mature, the enzyme stops functioning, and the telomeres start to winnow away. Aging has officially started.
The researchers were able to locate a possible equivalent of the telomerase enzyme in the asexual flatworms. When they blocked its function, the worms' telomeres shortened during regeneration. When they let the enzyme function normally again, it went into overdrive whenever the worms had to regenerate, meaning all their cells emerged the renewal process with pristine telomeres.
This isn't quite the case with the sexual flatworms, although they too appear able to regenerate indefinitely. There are two possibilities here — either their telomeres will eventually shorten, just very gradually, or they have evolved some other way to maintain their telomeres that doesn't involve telomerase. Whatever the explanation, Dr. Aboobaker says he's optimistic that the flatworm could well reveal the evolution of biological immortality:
"Asexual planarian worms demonstrate the potential to maintain telomere length during regeneration. Our data satisfy one of the predictions about what it would take for an animal to be potentially immortal and that it is possible for this scenario to evolve. The next goals for us are to understand the mechanisms in more detail and to understand more about how you evolve an immortal animal."
Of course, once you know how to evolve an immortal animal, the next trick is figuring out how to adapt that knowledge of regeneration to more complex organisms like... well, pretty much just humans, really. (Maybe dogs too, what with them being our best friends and all, but only if they're good.) So then, here's to regeneration as the key to living forever, particularly because I'm a fan of any path to immortality that might eventually involve turning into Tom Baker.