In Neil Gaiman’s run of The Sandman, the character  Death famously says, “When the first living thing

existed, I was there waiting.

When the last  living thing dies, my job will be finished. I’ll put the chairs on the tables, turn off  the lights, and lock the universe behind me.”  And when that happens, the last creature  .

Death will see through the door will  likely be—as the first one was—a microbe. Their large numbers certainly stack the  odds in their favor. But it doesn’t hurt

that many microbes have their own ways of  evading what we have deemed the inevitable.

Defying death is not as clear-cut as it  might initially seem. What we define as

immortality depends a bit on what you think it  means to die—whether it’s the loss of your own

individual self, or the loss of  something that has come to exceed you. It also depends on what you think it means  to survive.

Immortality is not a guarantee

of the microcosmos. There are predators to  contend with, and many simple forces of nature. The tardigrade is famously resilient to these  forces, able to withstand extremes of temperature,

radiation, and pressure.

They can survive these  conditions by entering cryptobiosis, desiccating

their bodies and slowing down their metabolic  rate as they become a dried out ball called a tun. The tun state protects the tardigrade and lets  them wait out the worst for long periods of time. And that’s part of the legend of the invincible  tardigrade, driving our speculation that they will

outlive us through any apocalypse.

Researchers  have even predicted that tardigrades could survive

astronomical catastrophes like an  asteroid crashing into our planet. But invincibility is not the same as immortality. Tuns have been revived after 30 years of

cryptobiosis.

But if we exclude the time spent  in cryptobiosis, the lifespan of most tardigrade

species ranges from a few months to a few years. When these tardigrades are in their tun,

they’re not just protected from the  elements. They’re also protected

from senescence, the thing that we call aging.

Senescence is a combination of many deteriorations

both within cells and across them. Cells acquire  damage, and with time, that damage accumulates

and accumulates. Our bodies experience that in the  decline of different physiological functions, and

it connects to an increased mortality as we age.

When not in a tun, even the mighty tardigrade

experiences senescence. But we know of  at least one organism that does not. Scientists studying hydra over several  years found that the mortality rates

over those years were quite low.

But  even more striking, the hydra showed

no evidence of aging. You might even say  that the hydra remained forever young. They do this by replacing their cells every 20  days, relying on three core populations of stem

cells that are themselves continuously and  indefinitely self-renewing.

These stem cells

allow the hydra to reproduce asexually by  budding off, and they’re also behind the

hydra’s legendary powers of regeneration. And these stem cells make the hydra

biologically immortal. Because its cells are  always being replaced, the hydra is able to

avoid the accumulation of damage that  other multicellular organisms experience.

The hydra doesn’t age. And so until another  force or predator intervenes, it doesn’t die. So we have one definition of immortality that  depends on aging in multicellular organisms, but

how does that apply to single-celled creatures  that reproduce asexually?

What does death mean

to a single cell that divides and divides and  divides, creating a legacy that is recursively

itself. Is it enough to just be one of thousands  (or tens of thousands) of clones to ensure that

even if you die, you still, in essence, live? In the case of the paramecium, no.

As each paramecium divides, it ages. And it will  continue to do so with each division. With damage

accumulating in its DNA, the paramecium will  start to divide more slowly, and eventually

the clonal daughter cells will die off.

However, they can avoid this fate

through conjugation, the protist equivalent of  sex. This act is its own sort of rejuvenation,

rewinding the clock for the paramecium. Scientists have argued that senescence

originated with protists like the paramecium.

But there are signs that some bacteria experience

aging too. They just forestall the effects of an  aging population by dividing asymmetrically. With

each split, one daughter cell will acquire more  damage than the other.

And so with each division,

one cell will rejuvenate while the other  will age. The longevity of the group

is thus built on creating differences  between the seemingly identical individuals. How does this fit in with our vision of  immortality?

It’s hard to know. At this point,

the question of whether we should consider  a flask of bacteria immortal starts to feel

like a matter of semantics. It’s informed as much  by personal and philosophical ideas as it is by

biological ones.

But of course, some of the most  interesting questions of science and philosophy

boil down to issues of semantics. These aren’t  just questions of how the world exists, but also

of how we see the world, how we describe  it to each other, how we imagine it. For millennia, we’ve been constructing  immortals as part of our religions

and myths to understand the world around  us.

In the stories we tell, these immortals

dictate our lives in invisible ways, and  they defy the thing we do not want to face. But microbes don’t have the same grandiosity as  the mythical figures we create. They don’t need

to.

The hydra may be biologically immortal,  but at the end of the day, it’s a tube with

arms. And a lack of senescence aside, if a  predator is able to make the hydra its prey,

the hydra is still quite capable of dying. Thank you for coming on this journey with us

as we explore the unseen world that surrounds  us.

And thank you especially to all these

people. Our patrons on Patreon who make  it possible for us to make this show. If you want to see more from our  master of microscopes James Weiss,

you can check out Jam & Germs on Instagram.

And if you want to see more from us,

there’s always a subscribe button somewhere nearby.