Wednesday, March 21, 2012

The vestigial human tail

Every once in a while I hear a story of the possibility of a human being born with a tail. I hear a lot of different suggestions as to why this occurs, but my plan for this post is to find the truth behind these reports. If the reports are true and some humans are born with tails, I will them explore why our recent ancestors stopped developing tails, and whether or not the gene still exist in humans.

So when people think of the possibility of a human developing a tail, they usually picture something like this (3).

The reality is that human tails do exist but they hardly look like this!
Some real examples of human tails are as follows (2 & 3).

Normally, a tail is present on the developing human fetus, but usually regresses by the 8th week of development. The true human tail upon birth is caused by a lack of cell destruction of the distal end of the embryonic tail (1). These "true humans tails" are composed of adipose tissue, connective tissue, muscle tissue, various nerves, and blood vessels (like any other true tail) and ranging in size from one to more than 5 inches long. There does exist a spectrum of structure with these "true human tails." While the majority of "true human tails" have neither catilage or developed vertebrae, there have been cases of newborns possessing a "true human tail" with 5 developed vertebrae (1 & 2).

Though what is described above is the true human tail, there is such things as a pseudo tail that accounts for at least one-third of all reports of human tails. These pseudo tails do not develop from the lack of regression of the embryonic tail, but rather arise from complications such as in Spinal Bifida, various lesions, or due to an elongated parasitic fetus (1 & 2).

Now that we know that the "true human tail exists," my nest question is whether or not the "true human tails is coded for genetically or is due to derailed chemical signaling during development. I found the answer in an article of vestigial traits by . Their article on "Evidence for macroevolution" states "true human tails" as an example, and we all now that in order to be an evolving trait, you need to have the present molecular biology, or gene. The article acknowledges a paper by Standfast that accounts three generations of females inheriting a "true human tail." The article also presents 2 papers by Katoh and Roelink, who discovered that the same genes responsible for tails in mice are also present in Humans. These genes are Wnt-3a and Cdx1 (1).

So my next questions while reading were as follows:
1) Do all humans at one point in their life have a "true human tail," and if so how do we lose it?
2) Do we all have human tails, and if so is the organ underdeveloped and unnoticeable?

Thankfully, the article provided the answers to my questions. The answer is no, not all humans have "true human tails." The truth is that all fetuses develop an embryonic tail that is then signaled for cell death, or apoptosis, by the inhibition of the Wnt-3a gene. This means that the cause for the "true human tail" is due to the unsuccessful inhibition of the Wnt-3a gene during the early stages of human development (2 & 3).

So finally I come to my last question about "true human tails." Which one of our recent ancestors was the last to have a tail? The article nicely answers this question too. Go figure! (They got their stuff down.) The article states that currently it is believed among evolutionary biologists that the "true tail" was lost during the evolution of the apes due to due to the lack of Wnt-3a gene expression (1).

But why did the evolution of the apes get rid of the "true tail?" Was the environment that the apes lived in more conducive to not having a tail and actively selecting against apes with more Wnt-3a gene expression, or was this due to a mutation that went to high frequency within populations of apes?

I hope to explore these final questions in my next blog post, but if you would like to read up on possible answers to these questions you can visit:

Thanks for reading, and GOOD LUCK on the next test!

Estevan Delgado



  1. Whoa, this seems like it could have come straight from the Sci-fi channel. I wonder how (or if they could) remove the tumor on the child in the last picture. So sad :( At least this happens pretty rarely.


  2. Other than a cosmetic anomaly, is there any problem with the tail? I guess I'm thinking about my friend who had a 6th finger. He was perfectly fine with it, but he was getting so much grief that he just had the surgery to amputate it. I think it'd be cool to have a tail, but if it detrimental to survival (i.e. more prone to mutate and become cancerous), maybe not.

    1. I think it would be really cool to have a 6th finger! I would definitely try my hand at the piano. That one scene in Gattaca with the piano player using sheet music that could only be played with 6 fingers on each hand always stuck with me.

  3. @Lara- to my knowledge the tail and mass found on the child in the last picture was removed. It is actually pretty simple to remove the tail... I wonder if they then get phantom tail syndrom?

    @Amber Usually there is no problem with the tail. Some people even have the ability- with constraints- to move the tail. I also read that out pelvic bones receives greater support of the the organs by not having the tail because it is able to utilize the muscle around the coccyx better since we are upright beings. I think true tails would not be prone to cancer but psuedo tails seem to me like they could be. I don't have my MD yet though.

  4. In response to Amber's comment, I just wonder if tails get in the way when sitting down or sleeping. It is true that humans have evolved (socially and culturally) to become more and more sedentary.

  5. I think the strongest problem tails (and any abnormal appendage) have is really the need for humans to feel accepted---like Amber's comment hinted at, it's a sad truth that even if an extra appendage is not causing a physical disability, it can disable people from being accepted by others. As social beings, I would argue that this would be just as detrimental to fitness as it would be if it were an actual handicap.

  6. Johanna has a good point. It's just like the point Amber made in her post about how six fingers were viewed as a sign of evil. Even though the finger itself had no direct effect on survival, it indirectly made the person unable to pass on this gene by ostracizing them from society.

    However, I'm not sure if this alone could be enough to remove its prevalence amongst all apes. While ostracism also exists in other species, I feel that there would have to be some kind of catalyst for it. Tails were lost for some reason. Individuals wouldn't be ostracized for having tails when most everyone in the original population had tails. While this explanation describes why tails haven't reemerged, I feel it doesn't explain while they were lost to being with. Can't wait for Estevan to discuss it in his next post :)

  7. Great points and questions! I will work on my follow up post to this same topic this week to hopefully shed some light on the questions you all brought up! :] Time for me to do some more research :]

  8. I was reading an article (found here: that the vestigial tails you mention are in fact NOT true tails. Like my extra digits, a tail is not a true tail unless it is a bony extension of the vertebrae column. The human "tails" as you stated are many of almost every tissue but bone, so therefore can't be real tails. Does that mean these "tails" are not in fact the vestigial organs of evolution, but just errors in development? Hmmm...

    1. Well humans never had tails, so indeed they are errors in development, but we also know that somewhere down evolutionary history one of our recent ancestors had tails, most likely the great apes. The error in development for humans with tails that include bone come from the over expression of a gene that is usually inhibited in humans (why we normally don't have tails with vertebrae). That is why I made a distinction between the psuedo-tails that are mostly tissue and the real tails that include vertebrae. (Did you read that part?)

      But if you are just referring to pseudo-tails, then yes they are just errors in development.

  9. Clarification: most likely the true tails was lost before the emergence of the great apes.

  10. is that first picture real? That is a very well developed tail and would love to learn more about that model and see if there is any story on her and how she dealt with the tail in social life.

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  12. Another quite interesting thought into our "CONTINUING" evolving genome of the human race, is right below us... our very own toes. It does not take a rocket scientist to see the variation of our toes from those of apes, but moreover, there seems to be a large increase of syndactyly (webbed toes) in humans. This is mostly caused by the joining of the second and third toes (counting from the large toe first). Some thing that this is just a strange mutation... whereas others theorize that as more and more of us are being born with this condition, it is just where the human race is heading. Yes, we do NOT need to grasp things with our feet. Yes, our toes are quite shorter then those of the ape. So are we to loose these toes entirely? NO, not quite. Your toes are quite essential for our current bipedal locomotion. But as we loose the ability to individually flex those muscles, they only serve one true purpose... for balance. So what's next in the coming say, 25,000 years or so? What would we see? Most theorize that our 4 smaller toes will be fused together only to serve to give us support for balance, but our nails will also fuse, giving way to... yes, dare I say it... a hoof-like appearance. But even though WE, today, might think it strange... Our future kin will look back at us and think it would be strange to know what it's like to have 5 individual "wiggly" digits on our feet!

  13. i really think a tail will be useful to us i don't know why we lost it but i think we really have to get it back .

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  14. i study biology and i think i have very strong evidence for design in nature

    a) we know that a self replicate robot that made from dna need a designer

    b) from a material prespective the ape is a self replicate robot

    a+b= the ape need a designer

    or even a self replicat watch.the evolution side always say that a watch need a designer because it cant self rplicat. so if we will find a self replicat watch we need to say that is made by itself

    plus: if a self replicate car cant evolve into an airplan, how can a bacteria can evolve into human ?

    the evolution say that small steps for milions years become a big steps. but according to this a lots of small steps in self replicat car (with dna) will evolve into a airplan.

    but there is no step wise from car to airplan

    evolution say that common similarity is evidence for common descent. but according to this 2 similar self replicat car are evolve from each other

    check this site

  15. dcscccc, suppose you are proposing cars which can self replicate, but where there may be tiny errors in the replication which become part of the blueprint for following generations; and suppose these cars exist in a situation where there is evolutionary pressure, such as the need to compete for resources, or the need to flee from dangers. If being more aerodynamic assists with these needs, and a particular car replicates with a slight error which happens to make it slightly more aerodynamic, then this car will outcompete the others and its replications will represent an increasing proportion of the population. Further on down the line, perhaps there will be hundreds of different kinds of tiny errors in replication, but those few which actually make the car more and more aerodynamic will take hold, and after a million generation, some will indeed have evolved into airplanes. But don't take my word on it, all of this can be modeled mathematically.

    As it happens, I do believe in a Creator of our Universe, but mine is not such a weak, pathetic being that it would be incapable of creating evolution by natural selection, or incapable of bringing about all variety of life by setting forth the condition from which a single self-replicating cell would originate.

  16. congratulations guys, quality information you have given!!! evolution definition