Breasts are uniquely human — more than 5,000 mammals inhabit this planet, yet Homo sapiens are the only life forms with permanent breasts.

Every other mammal develops temporary breasts during ovulation or nursing in order to produce milk for their young.

Watch the video above to learn why humans evolved to have breasts and what they're actually made of.

Following is a transcript of the video.

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 raises the question: Why are human breasts so big? Were they an evolutionary mistake?

Every other mammal develops temporary breasts during ovulation 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. Why? For example, in 1987 biologist Tim Caro explored seven existing theories on this subject. One was that breasts allowed newborns to nurse from the hip, giving their mothers 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 started walking upright, the sexual organs were no longer as obvious to spot. So males had no obvious 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 they contain 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 No. 1 cause of cancer-related deaths in women worldwide. It affects roughly 1.5 million women each year and 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.

Rena Callahan: Cancer is more common in rapidly dividing tissue. Every time cells are born and die there's an opportunity within the cell cycle to make mistakes in repairing DNA. And essentially a cell with mistakes can become a cancer cell. Breast tissue divides at a rapid pace so there's greater opportunity to make mistakes. That may explain why removing both breasts reduces a woman's risk of breast cancer by at least 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.

EDITOR'S NOTE: This video was originally published on February 15, 2018.