The umbilicus, also known as the belly button or navel, is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All placental mammals have a navel. It is fairly conspicuous in humans.
In humans, the scar can appear as a depression (often referred to as an innie) or as a protrusion (outie). As navels are scars, and not in any way defined by genetics, they can serve as a way of distinguishing between identical twins in the absence of other identifiable marks.
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