Humans Have Vestigial Tail Muscles

Every human’s life briefly begins with a tail. During the fifth to eighth week of gestation, all human embryos develop a distinct tail structure, a clear evolutionary echo of our primate (Review) ancestors who used tails for balance and communication. This embryonic tail isn't just a bump; it contains developing vertebrae and nerve cells. Before birth, however, a process of programmed cell death called apoptosis causes the body to reabsorb it, leaving behind only the fused vertebrae of the coccyx, or tailbone. In extremely rare cases, this process is incomplete, and a baby is born with a small, soft tail-like appendage—a striking reminder of our deep evolutionary journey.

While the external tail is gone, its internal machinery was ingeniously repurposed by evolution. The very muscles that our ancestors used to wag and control their tails, such as the coccygeus and parts of the levator ani, remain in our bodies. Instead of controlling an appendage for balance, these muscles now form the pelvic floor. This muscular sling is vital for modern humans, supporting our internal organs from below, assisting in bladder and bowel control, and providing core stability essential for our upright, bipedal posture. What was once used for swinging through trees now provides the foundation (Review) for walking on two feet.