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,
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