– Our ancestors had a tail and they lost it twice. What we, modern humans have today is only a “vestigial” tail, an evolutionary leftover. It is visible in embryos, but by the time we are born, we only have a few small bones that cannot be seen from the outside.
The tail bone is vestigial, an organ or trait in humans that have lost their original function through evolution. The tail bone is actually a part of a larger group of features that have gone out of use.
Why we lost our tail has been a scientific mystery, but new research conducted by paleontologists from University of Pennsylvania sheds some light on this evolution enigma.
The origins of this mysterious vestigial tail can be traced back to fish. To find out more about how ancestors lost their tail, researchers studied a 350-million-year-old hatchlings of the fossil fish Aetheretmon that possessed an asymmetrical tail, longer on the top than the bottom, which contains vertebrae.
When Lauren Sallan, an assistant professor in the School of Arts & Science’s Department of Earth and Environmental Science compared the Aetheretmon hatchlings with those of living fish, she discovered something surprising. The two “tails” started out one atop the other and then grew on their own. This discovery is highly significant because for nearly 200 years scientists assumed that the modern adult fish tail fin was simply added to the end of an ancestral tail shared with land animals.
“The tetrapod tail likely started as a limb-like outgrowth in the first vertebrates, while the fish caudal fin started as a co-opted median fin, like the dorsal fin. All vertebrate tail diversity might be explained by the relative growth and loss of these two tails, with the remaining fleshy tail stunted in humans as in fishes,” Sallan, said.
The disconnect means that the two tails went on their own evolutionary paths. Fish lost the fleshy tail and kept the flexible one to improve their swimming. Having just the back fin, she explained, “allows for more refined movements, which a muscular tail (originally present for power swimming) would disrupt.”
Adult apes, including human ancestors, took the tail loss process a step further, Sallan said, “losing the remaining bony tail for better upright movement. Like fish, the remnants of an embryonic bony tail are buried in our lower backs—the coccyx or tailbone—stunted by a loss of molecular signals that would otherwise cause it to grow out like an arm or leg. Thus, humans and fish embryos share mechanisms for controlling tail form.”
Today, most people don’t even know they have tailbones unless they break one. In rare cases, some people are born with a fully functioning tail. This has happened over 20 times in the past 100 years. When it happens the tail is usually removed surgically.