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 talkorgins.org . 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!