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Hi, this is Emily from MinuteEarth.
Not to sound super morbid, but with each passing second of life, the cells in our bodies accrue
a little more damage, which is at least partly why, as people get older, their odds of dying
increase: today, a 40 year-old has a .3% chance of dying in the next year, while a 60-year
old has a 1% chance of dying in the next year, an 80 year old has a 5 percent chance, and
a 100-year-old has a 50% chance.
But around that age, there's some evidence that the odds of dying level off.
So, what would make the mortality curve flatten out like that?
Well, ne theory is that it has to do with natural selection and bad jeans – the other
kind.
Take a hypothetical gene mutation that proves fatal during childhood.
Because it kills its host before they get old enough to reproduce, it never gets passed
on – natural selection weeds it out.
On the other hand, a nasty mutation that tends to kill people after they grow up and reproduce
can get passed on, but here's where things get complicated, because human kids depend
on their parents for survival.
So a bad gene that tends to kill relatively young adults is also likely to indirectly
kill their young kids, and thus be weeded from the gene pool, too.
But a bad gene that kills slightly older adults has a lower chance of indirectly killing their
slightly older, more self-sufficient kids, and thus has a lower chance of getting weeded
out of the gene pool, and a gene that kills even older adults has an even lower chance
of also killing their kids.
In short, natural selection's ability to eliminate harmful genes gets lower and lower
as the age at which those genes strike gets higher and higher.
And this could explain the weird death rate curve.
Like, think of natural selection as a magical force field that protects people from the
grim reaper, but it gets weaker as people get older, so their odds of dying go up.
Eventually, the force field wears off entirely, so while people's chances of dying at that
age are really high, they don't go up any further because there's no protection left
to lose.
And if the death rate really does level out - say, somewhere around 50 percent once we
hit 100 years of age - how long humans can ultimately live is just a numbers game: right
now, there are about half a million hundred-year-olds on Earth.
If half of them die each year, only about 15 thousand will make it to 105, only about
500 will make it to 110,and odds are none of the current centenarians will beat the
old-age record of 122 . But if we count all 7.7 billion people living on the planet today,
odds are that 15 people will beat that record – and one lucky human will live to the ripe
old age of 127.
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