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  • - 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