Why Don't Big Animals Get More Cancer?
- Animals come in all shapes and sizes,
but every complex multicellular creature
has one thing in common,
it started life as one cell.
That cell and all of the cells that it spawns,
copy themselves, divide and transform,
again, and again, and again,
to build specialized complex bodies,
containing anywhere from millions of cells
to more than a thousand trillion individual living units.
A body like yours contains around 37 trillion cells.
To build a human, factoring in the huge number of cells
that die and are lost along the way,
scientists estimate it takes 10 to the 16th cell visions.
During your lifetime, that's 10 million billion times,
that the machinery of the cell and its DNA instructions
will be copied.
Which is a lot of chances to make mistakes.
Every cell division carries a risk
of creating errors and mutations.
Most of these are harmless.
But a few rare mutations can destroy
the genetic programs or break down the machinery
that control a cell's life and death,
giving it the power to divide uncontrollably.
We call this cancer.
So logically speaking, the larger an animal is
and the longer that it lives,
it should be more likely to get cancer.
In fact, because humans are living longer and longer,
the odds of developing cancer at some point in your lifetime
are about 20%.
And it's been found
that for every 10 centimeters taller a person is,
their risk of developing cancer goes up by 10%.
(somber music)
When you consider all of this the obvious conclusion
is biologically and mathematically speaking,
giant long-lived animals like elephants
should have all the cancer. (chuckles)
So should blue whales.
And hippos.
And giraffes.
And rhinoceroses.
And megalodon.
(gentle music)
Hey smart people, Joe here.
Let's run some quick numbers.
The average human weighs about 70 kilograms
and has approximately 37 trillion cells in their body.
An elephant that weighs 5,000 kilograms.
Well that's 70 times more cells than a human has.
A blue whale, the largest thing to live on earth ever,
weighs about 150,000 kilograms.
Which means it has over 2000 times as many cells as a human.
About 79 million billion cells, which is nuts.
A giant animal means more cells.
More cells equals more cell divisions.
And more cell divisions means more chances of mutations
that could let a cell become cancerous.
So an elephant should have like,
70 times higher odds of cancer than you or me.
And for a blue whale then,
it should be like 2000 times higher.
I think that math works out.
Well, logic says that giant animals
should get tons of cancer.
But the weird thing is they just don't.
This puzzle is known as Peto's paradox,
because scientists love alliteration.
And because it's named after a scientist
named Richard Peto.
Peto noticed that even though mice
have a thousand times fewer cells than human,
and have lifespans 30 times shorter,
both species get cancer at about the same rate.
And as scientists have looked across the animal kingdom,
body size, lifespan, and cancer rates
just don't seem to be associated
like you'd predict from numbers alone.
So what is protecting
the animal kingdom's utmost examples of enormity and old age
from an unfortunate end, thanks to oncological illness?
Was that too much?
Why big thing not get cancer?
It's possible
that large animals have evolved better mechanisms
for catching and correcting mutations
before they get too dangerous.
Now for a cell to go down the pathway to cancer,
it usually requires more than one mutation
in more than one type of cancer causing gene.
There are so called oncogenic or tumor producing genes.
Now, when these are mutated and broken,
they're basically always on and they're telling the cell
to keep dividing uncontrollably.
But they are all also genes
whose job it is to keep cells from doing that.
The so-called tumor suppressor genes.
Tumor suppressor genes work
by acting as a kind of roadblock,
keeping cell division from moving forward.
Or as signals to self destruct the cell, if things go wrong.
Now, interestingly,
when scientists looked at the genome of elephants,
for one particular tumor suppressor gene,
they had 20 copies of it, and we have just one copy of it.
This means that an elephant's
tumor preventing security system
has like 20 layer redundancy.
Larger animals also have slower metabolic rates
than smaller animals.
So smaller animals create more DNA damaging byproducts
which means larger animals may get fewer mutations per cell.
Now, of course,
no one's out there putting blue whales in CAT scans.
So it could be that some large animals
actually do get cancer,
but it just doesn't kill them, so we don't notice it.
And this is where giant size could be a lifesaver.
You can imagine that in an animal to size of a bus,
a tumor needs to be pretty gigantic
to actually impact the animal's health.
But hungry cancer cells
also compete viciously with each other for resources.
So the larger that a tumor gets,
smaller hyper tumors may act as kind of parasites
on the original tumor and starve it
so that it can't grow large enough to be deadly.
Cancer is probably as ancient as multicellular life.
I mean, as soon as an organism evolves
to have multiple specialized cells working together
for the good of the whole,
there's serious evolutionary pressure
to make sure that one cell doesn't mutate
and try to out-compete all the rest.
Especially at the expense of killing the whole organism.
So, any animal that evolved to be giant,
also had to evolve stronger defenses against cancer
or else it probably didn't survive.
Basically, if your species gets big and lives a long time,
that means you figured out a way not to be a walking tumor.
The paradox actually makes a lot of sense
when you put it that way.
But what scientists hope
is that by studying how these giant animals
don't die of cancer, maybe we can learn some new tricks
for fighting cancer in our own species.
Because part of the paradox
may just be that cancer rates in humans are shockingly high.
And this may have something to do
with our modern diet and lifestyle.
The scientists have found across the animal kingdom,
cancer mortality is tightly linked to what an animal eats.
With mammals that eat other mammals
facing the highest rates of cancer death.
Now cancer isn't new in humans.
A handful of ancient mummies
have been diagnosed with tumors.
But cancer rate in people are higher today than ever before.
In modern industrialized nations,
thanks to a combination of less physical activity
and diets that are loaded
with calories and sugar and salt and fat,
increased weight alone puts humans at risk
of at least 13 types of cancer.
And this lifestyle has spread around the world
with pretty sad results.
Within a couple of decades
of switching to a Western-type diet,
people living on Pacific and Indian ocean islands,
experienced a massive surge
in so-called diseases of civilization
like diabetes and cancer.
Another major factor is how much pollution
and cancer causing chemicals
we've put into the environment around us.
These environmental carcinogens,
they play a role in up to 75% of human cancers.
Even whales are not immune from this one.
The one place in the world
where whales get as much cancer as humans,
is Canada's Saint Lawrence river estuary.
Where agricultural and industrial pollutants
have accumulated for decades.
And of course, a few lifestyle habits
that can lead to cancer are uniquely human.
I mean, whales don't use tobacco products or drink alcohol.
That we know of.
It's pretty clear there's not one single answer
to Peto's paradox.
As usual, evolution has come up with many answers
to the cancer problem in large and long living animals.
Life as they say, finds a way.
Humans, well, we can't accelerate our own evolution
to become naturally cancer-free.
But we do have one unique advantage.
We can study evolution's answers
and figure out new ways to treat the cancers that we do get.
It wouldn't hurt to eat a little better too, I guess.
Maybe krill's the next cancer fighting super food.
Stay curious.
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Hmm?
Very large bison.
And polar bears.
Hmm? Okay?
What? Yeah. Mm-hmm.
Huh? You done yet.
Okay.
That's my, cries and tall person, face.
Okay. One take (indistinct)
