It has always been believed that the final segment of the human vertebral column, the coccyx, or tailbone, is the remnant of a lost tail. Now, some researchers assert it may be the vestige of not one but two tails, both of which humans and their closest relatives (great apes) managed to shed over millions of years of evolution.
The team, led by Lauren Sallan, an assistant professor of Earth and Environmental Science at the University of Pennsylvania, arrived at this conclusion after analyzing the 350-million year-old fossils of the Aetheretmon. The ancient jawed fish is believed to be a distant common ancestor of modern-day terrestrial animals. When examining the hatchlings of the prehistoric fish, they found two tails, a scaly flexible one, and a fleshy tail fin growing at the same time. This seemingly simple discovery quickly invalidated the centuries-old belief that the modern fish's adult tail fin is an evolutionary add-on to the tail structure shared with ancestral land mammals.
The team, who published their findings in the journal Current Biology in December 2016, have a few theories about the fate of the two tails. They hypothesize that over time, the modern fish shed the fleshy tail fin for the more flexible, scaly one, because it made swimming a lot easier. Land animals, on the other hand, preferred the fleshy one — which evolved into a variety of tails — for a plethora of reasons, from swatting flies, to balance and visual communication.
Apes and humans evolved without either tail as it hindered efficient movement when walking upright. The fossil evidence of the 20 million-year-old ape, Proconsul, shows that it is one of the first primate species to evolve without a tail. The shape of the end of its tailbone is tapered, just as it is in humans, revealing that it had no tail structure.
However, though it has lost much of its ancestral function, the remnants of the embryonic tail is still evident in the tailbone. Its growth is stunted because we no longer have the molecular signals that tell it to grow out like our arms and legs. We wonder how life would have been if we still had at least one of the tails!
Resources: xmescience.com, livescience.com