Tailbone Without a Tail

At the base of your spine sits the coccyx, or tailbone. It is the leftover of a real tail your primate ancestors carried before our branch of apes lost it, tens of millions of years ago, as life moved from the trees toward the ground.

The tail never fully vanished from your development. Human embryos grow a small, visible tail in the first weeks of pregnancy. It normally shrinks back as the body forms, leaving just the fused bones we sit on.

The coccyx is not just dead weight. It anchors pelvic floor muscles and helps spread your weight when you sit, so it still pulls its load.

The instructions for building a real tail never left us; they sit dormant in our DNA. Very rarely they switch back on, and a baby is born with a soft, working tail. These rare cases are a striking reminder that the old genetic blueprint is still in there, just usually switched off.