(bright music)
- [Narrator] Breasts come in different shapes and sizes,
but there's one thing they all have in common.
They are unique to humans.
More than 5,000 mammalian species inhabit this planet,
yet homo sapiens are the only life forms
with permanent breasts.
Some may call this human anomaly sexy,
but it also begs the question,
why are human breasts so big,
and were they an evolutionary mistake?
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Every other mammal develops temporary breasts
during ovulation and, or nursing.
Basically their purpose is to produce milk,
so once the milk is gone, the breasts disappear.
But this isn't the case for female humans
whose breasts form during puberty, not pregnancy.
So at some point in our evolution, something changed.
But what?
For example, in 1987, biologist Tim Caro explored
seven existing theories on this subject.
One was that breasts allowed mothers
to nurse newborns from their hip,
giving them more mobility to multitask.
But it doesn't explain why breasts stick around
after the nursing stage is over.
Perhaps the most popular idea was first proposed
by Charles Darwin and later explored by zoologist,
Desmond Morris, in his 1967 book, The Naked Ape.
In it, Morris suggests that breasts evolved
as a sex symbol, to replace the swelling rear end
of other female primates during ovulation.
Once our ancestors starting walking upright,
the sexual organs were no longer as obvious to spot,
so males had no way of knowing
when a female was sexually mature,
and breasts may have formed as a result.
This theory would at least explain
why women's chests swell during puberty,
but it still can't explain why they
stick around after menopause.
Let's take a closer look at the human breast.
The big difference is that it contains more fat
than other female mammals.
The fat fills out the breast tissue,
giving it shape, sort of like milk, but permanent.
Human breasts can become so large
it can cause back and chest pain.
This is why many women get breast reductions.
More than 61,000 received a breast reduction
in 2016, in the US alone.
But breasts aren't only uncomfortable for some,
they can also be deadly.
Breast cancer is the number one cause
of cancer related deaths in women, world wide.
It effects roughly one and a half million women each year
and it killed 570,000 of them in 2015.
Yet as far as scientists can tell,
breast cancer is not common among other primates.
This could be because the risk of cancer increases with age,
and other primates don't live long enough
to develop breast cancer, or perhaps,
it could have something to do
with the permanent breast tissue itself.
- [Dr. Callahan] Cancer is more common
in rapidly dividing tissue.
Every time cells are born and die,
there is an opportunity within the cell cycle
to make mistakes in repairing DNA
and essentially, a cell with mistakes
can become a cancerous cell.
Breast tissue divides at a rapid pace,
so there's a greater opportunity to make mistakes.
- [Narrator] That may explain why removing both breasts
reduces a woman's risk of breast cancer by 95%.
Of course breasts have established their place
in human culture and society.
They can make women feel wanted, liberated, or empowered.
And they've helped build entire empires
based simply on their power to attract.
It looks like, for better or worse,
human breasts are here to stay.
(bright music)