How does the human embryonic tail become the adult coccyx?

During human prenatal development, the embryo exhibits a well-defined tail by the fourth week, which reaches its maximum length at approximately six weeks. While this structure is prominent during early development, it does not persist as a functional tail in humans. Instead, the embryonic tail undergoes a process of shortening and regression, eventually persisting only as a vestigial structure known as the coccyx, or tailbone.1

The adult coccyx is the curved, semiflexible lower end of the vertebral column, composed of three to five small, fused caudal vertebrae. This transformation serves as an example of an evolutionary vestige, reflecting the shared ancestry between humans and other vertebrates that possess functional tails. In early adulthood, the individual coccygeal vertebrae typically fuse together, and in later life, the coccyx may fuse with the sacrum. The coccyx also serves as an attachment point for muscles, such as the coccygeus muscle, which helps support the pelvic viscera.213