Your Belly Button Is A Scar, Not An Organ

Let’s start with what the navel actually is. The umbilicus, its clinical name, marks the former attachment point of the umbilical cord, the vascular structure that connected you to the placenta during fetal development.

After birth, when the cord is cut and the dried stump detaches, the tissue at the umbilical ring heals over. What remains is a scar, or more specifically, the retracted remnants of four structures:

1. The left umbilical vein (which becomes the round ligament of the liver in adults)
2. The obliterated urachus that runs toward the bladder
3. The two umbilical arteries that become the medial umbilical ligaments

According to a 2019 review in Acta Bio Medica, the navel is, functionally, the most constant scar of the human lifecycle.

The “innie” versus “outie” question, while trivial, is actually also worth dispatching: it has nothing to do with how a midwife or obstetrician cuts the cord. It is determined entirely by how the tissue heals, that is, whether the skin retracts inward or whether a small amount of extra scar tissue, or a minor umbilical hernia, causes the skin to protrude. It is individual biology, not technique.

What Precedes The Belly Button

To understand the navel, you have to understand what preceded it. The umbilical cord is one of fetal biology’s most precisely engineered structures.

It forms around the fifth week of gestation, extending from the fetal abdomen to the center of the placenta. At full term, it averages 50 to 60 centimeters (19 to 23 inches) in length and contains one vein and two arteries, all embedded within Wharton’s jelly — a gelatinous connective tissue — and enclosed in an amniotic sheath.

A common misconception is that the cord connects fetal and maternal bloodstreams directly. In reality, the umbilical vein delivers oxygenated, nutrient-rich blood from the placenta to the fetus; the two arteries carry deoxygenated, waste-bearing blood back. The placenta is the interface that enables exchange between two circulatory systems without ever allowing them to mix.

Together, the cord and placenta constitute one of the most specialized temporary organs in vertebrate biology, functional for precisely the duration required and dismantled at birth. After delivery, the umbilical arteries constrict and degenerate. The stump dries within days, and the scar forms. The navel is what remains.

Why The Belly Button Isn’t Universal

Not all mammals have navels. Egg-laying mammals, or the monotremes, which include the platypus and echidna, don’t. Similarly, marsupials like kangaroos and opossums, don’t develop one in the same sense either: they are viviparous, meaning they give birth to live young, but their gestational period is brief and their placentae are rudimentary. Their offspring are born at an extremely early developmental stage and continue maturing in a pouch. The deep, sustained umbilical connection that leaves a navel simply never forms.

The belly button belongs exclusively to the Eutheria, the placental mammals, and its presence is a direct consequence of their defining reproductive strategy. A 2014 paper in Advances in Biology tracing placental evolution across eutherian clades documents just how radically this group diverged from other mammals.

Eutherians developed increasingly complex hemochorial placentae, in which fetal tissue comes into direct contact with maternal blood, enabling a level of nutrient and oxygen transfer that marsupial placentae cannot match. The result is offspring born at far more advanced developmental stages, with correspondingly higher survival rates.

The tradeoff was immunological. A fetus expresses paternal antigens foreign to the mother’s immune system. Sustaining a pregnancy without triggering rejection required the evolution of elaborate immune tolerance mechanisms, described extensively in the reproductive immunology literature. The fact that evolution solved this problem is, from a biological standpoint, one of the more remarkable achievements in vertebrate history.

The payoff was ecological dominance. Molecular divergence estimates place the origin of Eutheria at around 190 million years ago. After the non-avian dinosaur extinction 66 million years ago, placental mammals diversified explosively.

Today, the group comprises over 4,000 species and occupies nearly every terrestrial and aquatic habitat on Earth. The sustained gestation the placenta enabled supported larger, more metabolically expensive brains. It is not a stretch to say that the navel is, in a quiet way, a prerequisite for human cognition.

The Ecosystem You Carry In Your Belly Button

The story doesn’t end at birth. Because the navel is recessed, sheltered and rarely scrubbed, it turns out to be an unusually stable microhabitat, and what lives there is remarkable.

In 2012, a team led by biologist Jiri Hulcr published findings from the Belly Button Biodiversity Project in PLOS ONE. Using high-throughput 16S rRNA gene sequencing, they sampled the navels of 60 volunteers and identified 2,368 bacterial phylotypes, distinct bacterial lineages, of which an estimated 1,458 were potentially unknown to science. From 60 belly buttons.

Despite this extraordinary diversity, the community was not chaotic. Just eight phylotypes appeared on more than 70% of participants, and those dominant lineages accounted for roughly a third of all bacterial sequences detected. The ecology mirrors what we see in tropical forests: an enormous diversity of rare specialist species, structured by a small oligarchy of well-adapted generalists.

Your navel, in other words, is a field site. A biome. A place where evolution is still doing its work. Of course, the belly button does nothing now. Its vascular connections have long since closed off into ligaments. The cord is gone. The placenta is gone. What remains is scar tissue, shaped by the peculiarities of individual healing.

But that scar carries a very specific meaning. It marks you as a member of Eutheria — the lineage that bet, nearly 200 million years ago, on keeping offspring inside the body longer, nourishing them through a vascular cord, tolerating the immunological complications and producing young capable of surviving and eventually flourishing in a world their ancestors had barely begun to inhabit. That bet paid off on a scale that is difficult to overstate.

Did you know these facts about the belly button already? Take the short and challenging Human Anatomy IQ Test to really put your knowledge about the human body to the test.