WEBVTT

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For the microscopic lab worm, C. elegans

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life equates to just 
a few short weeks on Earth.

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Compare that with the tortoise,
which can age to more than 100 years.

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Mice and rats reach the end of their lives
after just four years,

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while for the bowhead whale,
Earth's longest-lived mammal,

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death can come after 200.

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Like most living things,

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the vast majority of animals gradually
degenerate after reaching sexual maturity

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in the process known as aging.

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But what does it really mean to age?

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The drivers behind this process are varied
and complicated,

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but aging is ultimately 
caused by cell death and dysfunction.

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When we're young, 
we constantly regenerate cells

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in order to replace dead and dying ones.

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But as we age, this process slows down.

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In addition, older cells don't perform
their functions as well as young ones.

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That makes our bodies go into a decline,

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which eventually results 
in disease and death.

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But if that's consistently true,

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why the huge variance in aging patterns
and lifespan within the animal kingdom?

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The answer lies in several factors,

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including environment

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and body size.

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These can place powerful evolutionary
pressures on animals to adapt,

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which in turn makes the aging process
different across species.

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Consider the cold depths of the Atlantic
and Arctic Seas,

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where Greenland sharks can live
to over 400 years,

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and the Arctic clam known as the quahog
can live up to 500.

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Perhaps the most impressive of these
ocean-dwelling ancients

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is the Antarctic glass sponge,

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which can survive over 10,000 years
in frigid waters.

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In cold environments like these,
heartbeats and metabolic rates slow down.

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Researchers theorize that this also
causes a slowing of the aging process.

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In this way, the environment 
shapes longevity.

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When it comes to size, 
it's often, but not always,

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the case that larger species have a longer
lifespan than smaller ones.

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For instance, an elephant or whale
will live much longer

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than a mouse, rat, or vole,

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which in turn have years on flies
and worms.

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Some small animals, like worms and flies,

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are also limited by the mechanics 
of their cell division.

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They're mostly made up of cells that can't
divide and be replaced when damaged,

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so their bodies expire more quickly.

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And size is a powerful evolutionary driver
in animals.

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Smaller creatures are more prone
to predators.

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A mouse, for instance, can hardly expect
to survive more than a year in the wild.

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So, it has evolved to grow and reproduce
more rapidly,

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like an evolutionary defense mechanism
against its shorter lifespan.

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Larger animals, by contrast, are better
at fending off predators,

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and so they have the luxury of time
to grow to large sizes

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and reproduce multiple times
during their lives.

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Exceptions to the size rule include bats,
birds, moles, and turtles,

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but in each case, these animals have other
adaptations

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that allow them to escape predators.

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But there are still cases where animals
with similar defining features,

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like size and habitat,

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age at completely different rates.

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In these cases, genetic differences,

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like how each organism's cells
respond to threats,

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often account for the discrepancies
in longevity.

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So it's the combination 
of all these factors

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playing out to differing degrees
in different animals

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that explains the variability we see
in the animal kingdom.

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So what about us?

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Humans currently have 
an average life expectancy of 71 years,

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meaning that we're not even close to being
the longest living inhabitants on Earth.

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But we are very good at increasing
our life expectancy.

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In the early 1900s, humans only lived
an average of 50 years.

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Since then, we've learned to adapt
by managing many of the factors

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that cause deaths,

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like environmental exposure
and nutrition.

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This, and other increases 
in life expectancy

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make us possibly the only species
on Earth

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to take control over our natural fate.