The tail just doesn't want to stay in humans, scientists say. A recent study showed that our ancient ancestors lost their tail more than once. The findings, published in Current Biology, will not only help explain why humans don't wag their tails like dogs, but also shed light on the existence of the tailbone, which first becomes a tail and then gradually disappears.
"Fleshy tails are rooted in the earliest vertebrate ancestors and are found in very young embryos, so it would be very difficult to get rid of them completely and not run into problems," says author Lauren Sullan. "As a result, fish and human tails stopped growing, leaving depressed tails like whale legs."
The origin of this mysterious vestigial tail has to do with fish. For the study, Sallan, an assistant professor at the University of Pennsylvania, analyzed fossils of a 350 million-year-old Aetheretmon juvenile fish. This jawed fish is a distant ancestor of modern land animals and possessed a scaly, fleshy tail and flexible tail fin that rested on it.
Sallan found that these structures were completely separated. Comparing juvenile Aetheretmon to juvenile live fish, she found that two "tails" started one on top of the other and then grew by themselves. This discovery overturns at least two hundred years of scientific belief that the caudal fin of modern adult fish was simply added to the end of the ancestral tail, which was also found in land animals.
This upheaval means that the two tails each evolved in their own evolutionary way. The fish have lost their fleshy tail and retained flexibility to improve their swimming process. With only a dorsal fin, they would have developed more "refined movements, and the muscular tail (which is originally present to support the swimming process) collapsed."
The fish, which evolved into a semi-aquatic, and then a land animal, lost its flexible dorsal fin, but retained the fleshy one - and over time it became an appendage familiar to all of us, which we see in dogs, cats, cows and other animals. As dogs show, tails are useful for visual communication, dispersing annoying flying insects, and other functions.
Adult monkeys, including human ancestors, have decided to get rid of their tail altogether, Sallan says, “having lost their remaining bony tail for better vertical movement. Like fish, the remnants of embryonic bony tails are buried in our lower backs - in the tailbone - but they don't receive molecular signals that would lead to limb growth, like arms or legs. Thus, fish and human embryos have similar mechanisms for controlling the shape of the tail."
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ILYA KHEL