Did you know that human embryos early in development have tails that later fail to grow for a lack of signaling from the genes? We end up with the coccyx at the end of our spines that protrudes a bit from our rear ends. The tailbone is barely noticeable, but it serves as a reminder of our ancient past.
Biologists believe we and our relatives on the evolutionary tree, the great apes, lost our tails for ease of walking and other upright movements. We might add ease of sitting, too, though most species and breeds of monkeys, cats and dogs sit despite having tails. That said, it would be difficult to sit in a chair or wear concealing clothing, recent additions to the Homo species’ props, if we did have tails.
A human embryo showing a tail in the 5 th week of pregnancy ( Wikimedia Commons /Dr. Ed Uthman)
Seeker.com reports on a new article in the journal Current Biology that says human embryos’ tails go back to our evolutionary ancestors—fish.
Lauren Sallan is the author of the study. She is an assistant professor in the Earth and Environmental Science Department at the University of Pennsylvania. She examined fossils of Aetheretmon fish hatchlings, which are distant relatives of today’s land animals. These hatchlings had a flexible tailfin below and a scaly fleshy tail above.
She concluded the two types of tails on Aetheretmon were separate. She compared Aetheretmon hatchlings to modern fish and found that the tailfin and scaly tail above eventually grew separately thought they had started out one on top of the other.
“This discovery overturns at least two centuries of scientific belief that the modern adult fish tailfin was simply added to the end of an ancestral tail shared with land animals,” Seeker.com says.
The two tails then went on to develop differently in different animals. Fish kept the flexible tail that helps them swim better and allows for more precise and refined movements. The other tail, a more muscular appendage, allowed for power swimming but disrupted refined movements, Seeker.com says.
This graphic giving the evolution of tetrapods (four-legged land or hybrid land-sea animals) shows some transitional fossils. It shows Eusthenopteron at the bottom, indisputably still a fish, through several transitional animals to Pederpes at the top, indisputably a tetrapod. (Wikimedia Commons /Maija Karala)
Some fish emerged from the water, first to be semi-aquatic and then to evolve into land animals. These animals lost the swimming fin but kept the fleshier one that cows, monkeys, felines, canines and other animals have today. Tails can be used to communicate, shoo insects and other purposes, Seeker.com says.
Many tails on monkeys that move around upright are small, which lends credence to the theory that we lost our tails to facilitate upright movement.
Most apes and humans and their ancestors lost even a vestige of a visual tail. We have the remnants of a bony tail that develops early in our embryonic stage, Sallan says. But the genes that control tail growth have stopped signaling them to grow in most people, unlike legs and arms, for example, which still receive the signals to grow.
Rani’s Southamerican Monkey. Mewar, ca. 1700, anonymous ( Wikimedia Commons )
We say “most humans” because some people are born with vestigial tails.
A 1984 article in the journal Human Pathology states that a true or persistent tail in humans comes from the remnant of the embryonic tail and contains fat, connective tissue, muscles, blood vessels, nerves and is covered in skin. But bones, cartilage and spinal cord are lacking.
It may be as long as 13 cm (5.1 inches), can move and contract, and occurs twice as often in males as in females. A true tail is easily removed surgically, without residual effects. It is rarely familial. Pseudotails are varied lesions having in common a lumbosacral protrusion and a superficial resemblance to persistent vestigial tails.
An example of a vestigial tail
Featured image: A graphic showing what a human would look like with a tail ( The Daily Omnivore )