Why Don’t Humans Have Tails? Exploring the Genetic Mutation That Led to the Disappearance of Our Muscular Appendage

Millions of years ago, the common ancestor of humans and other primates had tails—however, this diverse order of mammals diversified and evolved. As a result, only some primates, like lemurs and monkeys, still have this appendage, while apes and the ancestors of modern humans ditched their tails entirely.

Loss of tail was initially thought to be the result of the evolution of humans to be bipedal. Still, precisely how we lost our seats is a mystery that experts have long sought to answer.

Genetic Clue Behind Taillessness

A team of researchers has discovered the genetic factor that could solve the mystery of a lost human tail. They identified a jumping gene associated with tail growth, which could have leaped into another location in the genome of a primate species millions of years ago. This process created a mutation that took human tails away.

Humans do still have tails, but only when we are embryos. During the early stages of human development, we briefly develop tails and vertebrae, as do all animals with backbones. After about eight weeks, most embryonic human tails disappear entirely through a process known as apoptosis, or programmed cell death.

After that, the only remnant of this lost human appendage is three or four vertebrae, which form the coccyx or tailbone. These protrusions are embryonic leftovers considered pseudo-tails rather than "true tails." Skin-covered pseudo tails contain connective tissue, blood vessels, nerves, and muscles, but they do not have bones and cartilage and are not connected to the spinal cord, as true tails would be.

The earliest known ancestor of humans and apes without tails is a primate genus called Proconsul, which lived in Africa between 23 million and 5.3 million years ago. Proconsul had no sign of caudal vertebrae or the bones found in tails. However, it is believed that tail loss happened even earlier, about 25 million years ago, when the hominoid lineage of humans and apes diverged from Old World monkeys.

In the study "The genetic basis of tail-loss evolution in humans and apes," the researchers compared genetic data from six species of hominoids and nine species of monkeys. They looked for differences linked to the presence or absence of tails. A likely candidate emerged in a short piece of DNA called an Alu element. This type of DNA can jump from one place in the genome to another and get tucked away in the gene TBXT, which regulates tail development. This mutation was discovered in the genomes of apes and humans but not in monkeys.

Biological Significance of Tails

Both apes and humans may have benefitted from tail loss since it helped them transition to two-legged walking. On the other hand, keeping their tails benefited primates in different ways since these appendages perform various functions.

Tails may be extended during leaping and help orient the body through the air as an animal prepares for landing. These muscles aid in maintaining balance while moving, foraging, and even sleeping. They can also brace the body against a surface while the animal hangs from its back legs.

For primates, tails can also serve as a tool. For instance, white-faced capuchins (Cebus capucinus) use these appendages to draw water in a tree hole and drink water from the fur, almost like a sponge. Additionally, primates can snuggle up to their tails as pillows, hide under them for warmth, or even use them in social activities.