Ancient Ancestors to Humans Had Not One, But Two Tails

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Artist’s rendition of the ancient Aetheretmon in the foreground (Photo Credit: John Megahan)

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.

Fossils of the Aetheretmon show evidence of two growths which evolved into tails in the mature animals (pictured is a fossil from a juvenile fish) Photo Credit: Lauren Sallan

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.

As the animals matured the growths formed the asymmetrical tail in the adult fish, with a stiff top part and a shorter, more flexible bottom (pictured is a fossil of a mature fish) Photo Credit: Lauren Sallan)

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

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259 Comments
  • bsc
    bsc9 months
    Wow that is so fascinating!
    • noiwantthecandy
      noiwantthecandyabout 1 year
      we don't have tails. if our ancestors had 1 or 2 tails and lost it, the tails would still be in their DNA and their child would have that tail, and so on. plus if we are fish we wouldn't be mammals. so it's science vs. science
      • Roseyabout 2 years
        this is not true
        • Bonnieabout 2 years
          seriously?! we were NOT fish! by the way, if we were decended from fish, WHY COULDNT WE BRRATH UNDER WATER! *breath+
          • cfj coolabout 2 years
            cool but not realy are ansesters
            • Cool m8!over 2 years
              very good study
              • Blackopps 4 fanalmost 3 years
                This was a good text
                • yummyyumabout 3 years
                  better than fortnite.
                  • yummyyumabout 3 years
                    That is so cool!
                    • ayylienabout 3 years
                      very cool