Human embryos briefly grow a tail that is normally absorbed before birth

Every human embryo starts life with a tail. Around the fourth week of development, a tail bud forms at the posterior end of the embryo, extending beyond the point where the legs will later appear. This transient structure is a normal part of our shared vertebrate blueprint, not a sign of anything going wrong. ^[1][2]

A tail that disappears

By the eighth week, the human tail is at its longest, but it never develops into a functional appendage. Instead, it stops growing and begins to regress. The tail's vertebrae shrink and fuse into a small, triangular bone called the coccyx, or tailbone, which remains as a vestigial remnant in adults. ^[2][3]

1. Week 4

   Tail bud appears at the embryo's posterior end.

2. Weeks 5–6

   Tail reaches its maximum length, with up to 10–12 somites forming.

3. Weeks 7–8

   Tail begins to regress as programmed cell death removes cells.

4. Birth

   Remaining vertebrae have fused into the coccyx; no external tail is present.

When a tail persists

In very rare cases, a baby is born with a soft, fleshy tail. This is not a true vertebrate tail: it contains no vertebrae, only skin, fat, and sometimes muscle. Such appendages are usually harmless and can be surgically removed. They are distinct from the coccyx, which is a normal part of the skeleton. ^[3][4]

• The coccyx is the fused remnant of the embryonic tail and serves as an attachment point for pelvic muscles. ^[3]
• Human tails at birth are extremely rare and typically lack bony elements, unlike the tails of other primates. ^[4]
• The tail's regression is driven by apoptosis, or programmed cell death, a normal developmental process. ^[2]

The brief existence of a human tail is a striking reminder of our evolutionary history. It shows how development can produce structures that are later dismantled, leaving behind only a small, functional remnant. ^[1][3]