Humans today lack an external tail, a common feature in many animals. A tail is a flexible, posterior appendage extending from the backbone, typically containing vertebrae but no internal organs. The absence of a visible tail in humans prompts curiosity about our evolutionary past and how we diverged from other tailed creatures.
Our Tail-Bearing Ancestors
Our lineage shares a common ancestry with primates that possessed tails, which served various functions in their arboreal (tree-dwelling) environments. Many primates use their tails for balance when moving through trees or leaping between branches. Some, like spider monkeys, have prehensile tails capable of grasping and holding onto branches, acting as a fifth limb.
Beyond locomotion and stability, tails in primates and other animals also play roles in communication, defense, and temperature regulation. The presence of tails in our distant ancestors points to a time when these functions were beneficial for their way of life.
The Evolutionary Loss of the Tail
The evolutionary timeline indicates that our hominoid ancestors, diverging from Old World monkeys, began losing their tails approximately 25 million years ago. This anatomical shift occurred over a period of nearly 20 million years, between the Oligocene and Miocene epochs. The transition from tailed to tailless primates is evident in species like Proconsul heseloni (around 18 million years ago) and Nacholapithecus kerioi (around 15.5 million years ago), which had already lost their tails.
Recent scientific research has shed light on the genetic changes underlying this loss. An alteration in the TBXT gene, which plays a role in embryonic development and tail formation, is a significant finding. A short, “jumping” gene fragment known as an Alu sequence inserted itself into a non-coding region of the TBXT gene. This genetic insertion altered the gene’s function, leading to the suppression of tail growth during embryonic development. The loss of the tail may have been favored by selective pressures related to adaptations for bipedalism, as a tail might hinder upright walking.
The Coccyx: A Vestige of Our Past
The coccyx, commonly known as the tailbone, serves as anatomical evidence of our tailed ancestry. Located at the bottom of the vertebral column, the coccyx is typically composed of three to five fused vertebrae. It represents a greatly reduced remnant of the tail present in our evolutionary predecessors.
Despite its small size, the coccyx still has minor functions. It provides an attachment point for several muscles and ligaments, including those of the pelvic floor. It also offers some support when a person is sitting. Its primary significance lies in its status as a vestigial organ, a structure that has lost most or all of its original function through evolution.
Rare Cases: Human “Tails” Today
While humans are generally born without external tails, rare instances occur where infants present with tail-like appendages. These occurrences are broadly categorized into two types: true tails and pseudotails. True tails are considered a form of atavism, meaning they represent the reappearance of an ancestral trait. These benign growths contain caudal vertebrae, similar to the skeletal elements found in the tails of other vertebrates.
Pseudotails do not contain bone and are typically fatty tumors, lipomas, or other spinal anomalies. True tails arise from a failure of the embryonic tail to fully regress during development. During early embryonic stages, humans develop a temporary tail-like structure that normally reabsorbs before birth. When this regression process is incomplete, a true tail may remain. These appendages are generally surgically removed shortly after birth.