The belly button is the remnant of the place where the umbilical cord connects a baby to the placenta. The word “umbilical” springs from the Latin word meaning middle or center, originally referring to a scroll’s ornamental end.

Every mammal with a placenta also has a belly button: elephants, dolphins, bats. Some mammals may have a very faint belly button, including the platypus, which lays an egg, and marsupials, where the cord falls off while the baby is still in the pouch.

Birds and reptiles, while not being mammals, also have a faint belly button that marks the spot where they were once attached to an egg yolk. Even dinosaurs had belly buttons.

The human umbilical cord forms in the 2nd month of gestation. Four things are running through it: a vein that brings oxygen and nutrients to the baby; an artery taking waste away from the baby; a duct that turns into the intestinal tract; and a tube that develops into the bladder.

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Blood moves through the umbilical cord at around 4 mph (6.5 km/h). At full term, about a quart (1 l.) of blood flows through the umbilical cord every minute.

After birth, the umbilical cord is clamped before being cut. The belly button forms when the stump of the cord dries up and falls off. It’s a baby’s first scar. The difference between the innie and the outie is not due to the doctor tying the umbilical cord wrong, or cutting it differently. It’s simply a matter of how the skin grows around the cord. If there is less skin, it folds inward making an innie; if there is more skin, it folds outward to make an outie. Often, a baby has an outie at first that turns into an innie later on. Plastic surgery can improve the appearance of a belly button after reaching adulthood.

Just like fingerprints, no two belly buttons are alike.

Around 90% of humans have “innie” belly buttons and 10% have “outies.”

In a survey, oval belly buttons were voted most attractive. The outie was least attractive.

A pregnant woman’s stomach protrudes so much that an innie often temporarily turns to an outie, reverting after the baby is born.

In 2011, researchers studied the bacteria that live in the human navel. They asked 60 volunteers to swab their belly buttons. The bacteria was cultured and the DNA was analyzed. Called the Belly Button Biodiversity project, researchers found about    2,300 different strains of bacteria in the navels of the 60 participants.

The average person has about 67 types of bacteria in the navel. Some of the strains were completely unknown to science. Just eight of those species were frequent and abundant across the individuals sampled (they were present in more than 70% of people). The rest of the species were rare, often appearing in just a single navel. Furthermore, not a single type of bacteria was found in all 60 people. Each person had their own unique “fingerprint” of belly button flora. In November 2012, the team published their findings in a paper called “A Jungle in There: Bacteria in Belly Buttons are Highly Diverse, But Predictable.

The town of Shibukawa in Japan is referred to as the belly button of the country because of its location in the center of the country. Townfolk capitalize on this by throwing a Bellybutton Festival every year in late July. Participants paint faces on their torsos, with the belly button as the mouth. A kimono and a large hat hide the rest of the body. There are contests, parades, food, souvenirs, and festivities.