Believe it or not, some human babies are born with true tails.
It seems to be an atavism. An atavism is a reappearance of an ancestral organ that happens with a rare few organisms in a population. This happens because it took only a small genetic change to disable the development of the organ, but the complete gene line remains, and it takes only a small genetic change to bring it back. For example, there was a dolphin caught and photographed with complete hind limbs (normal dolphins don't have hind limbs), reported on MSNBC.com. And, of course, as a more interesting example, there are babies born with tails.
PubMed has indexed up to 100 reported cases of human tails. The most interesting case is found in the Bergman study (contains photos of infant genitalia). You can see photos of the tail in extended and contracted positions, as though the tail contains relevant muscle.
But the tail was not examined in detail. A more thorough examination is found in a 1980 study by Bar-Maor, for The Journal of Bone and Joint Surgery, which has been made available freely online on PubMed. Click on the "Full Text" button on the right side of the page. The study contains three case reports of human tails, each with bony vertebral segments.
Given that each and every one of us human beings once had a tail, this stuff doesn't have to come as a surprise. Wait, what? Yep, you had a tail, from about 28 to 47 days after conception.
Image source: National Institute of Child Health and Human Development. If you follow that link, each embryo image is a clickable link, and you can view rotated images and animations at many stages in embryo/fetus development. Our tails were removed in later development by the immune system.
Note: these are not about Haeckel's embryo sketches. What you see above is an MRI (magnetic resonance imaging) of an actual human embryo.
EXPLANATIONS FROM CREATIONISTS
So how do creationists explain this stuff? Well, it isn't easy. If I believe that Adam was created on the sixth day and had life breathed into his nostrils by God, who then took a rib out of his abdomen and used it to create Eve, then there really is no way in heck I would expect Adam and Eve to have the complete genes for a tail.
I did a Google search for human tail evolution and, wouldn't you know it, the first link that comes up is a creationist article.
Creationist explanation #1: The "tail" is just a malformation.
This is the most common creationist explanation. The first article in the Google search is a 1982 piece by ICR's Duane Gish.
We would like to emphasize once again the fact that this appendage was not a tail. We have already quoted Ledley's own testimony that the "tail" did not contain even rudimentary vertebral structures. Ledley states in his article that there is no precedent for a vertebral tail without caudal vertebrae. The "tail" was offset from midline with no connection to vertebral structures and contained a soft fibrous fatty core. The resemblance to a tail was highly superficial.
Gish is referring to a study on a human tail by Ledley, which had a caudal appendage WITHOUT vertebral segments. There are "true tails," which contain vertebral segments, muscles, veins--everything we seen in a complete monkey's tail--and then there are "pseudotails," which are nothing more than skin and fat like a tumor. The Lidley tail was a pseudotail. Gish goes on to claim that the appendage is just a malformation. The Bar-Maor study was written two years earlier, in addition to many other cases of true tails.
The largest creationist organization is Answers in Genesis, and they have a 2008 article (Setting the Record Straight on Vestigial Organs) with a little blurb with the same claim, that the "tail" is not really a tail because it is only a fatty tumor. Again, no mention of true human tails, after decades of true human tails being published knowledge.
Creationist explanation #2: The "caudal appendages" are not really appendages.
Creation Ministries International has a 2007 article by Andrew Lamb (Human tails and fairy tales), and this time it includes evidence of knowledge of the Bar-Maor study. How does the author take this?
The x-ray that appears on the TalkOrigins webpage is of Child 3, who had a healthy, well developed coccyx. Being soft tissue, Child 3’s benign caudal appendage does not appear in the x-ray, except perhaps to the trained expert eye. What does appear is the normal healthy coccyx, albeit of only three bonesmost of us have four coccygeal vertebrae; a few percent of people have five and a few percent have three.
He isn't clear, but he notes that the two x-ray images of the tail do not show the appearance of an external appendage, so his writing proceeds as though the vertebral segments are just an internal extension of the coccyx. Well, that would fly, except that the study describes two out of the three tails as a "caudal appendage," meaning an external tail.
Beyond that, the author simply does not have an explanation.
Creationist explanation #3: Eugenie Scott calls it a duplication mistake.
An author on DrDino.com, Kent Hovind's website, rebuts the evolutionary argument about tails by citing the authority of Eugenie Scott, the head of the National Center for Science Education, the pro-evolution thinktank.
Actually, that’s [human "tails"] not an evolutionary issue at all, Brian. It’s um, it’s a matter of developmental biology, it’s a, it’s a matter of what happens when that sperm fertilized that egg, and that egg grew into a baby, and that baby was born. Um, I couldn’t give you the exact precise biochemical explanation but probably at some point where the, um, the genes instructing how many vertebrae to lay down in that vertebral column, um, duplicated itself a couple extra times, by mistake. It was a, uh, faulty transmission of information, so to speak. And this particular individual just added, ended up getting a few extra, extra vertebral, um, segments. Um, and this, this doesn’t happen very frequently, but, you know there are glitches in the genetic material that produce things like this, just as there are glitches in the genetic material that produce people with six fingers. Um, but if somebody was born with six fingers, you don’t think Oh no! That, that takes us all the way back to Acanthostega, with the earliest amphibians some of them had six fingers. It’s not really an evolutionary issue.
So there's the opinion of Dr. Eugenie Scott, who is a well-qualified physical anthropologist, and then there is the evidence. I can't explain why Dr. Scott thinks as she does, but her opinions are not the deciding factor, nor are the opinions of any single expert or textbook. If we must rely on expert authority, then it is best to rely on the consensus of expert authority, which weighs heavily on the side of human "tails" being tails without the quotes.
A "true human tail" is defined by Shifan et al. 2006: "True human tail is a rare event. It is defined as a caudal, vestigial, midline protrusion with skin covering a combination of muscle and adipose tissue." The word, "vestigial," is used, meaning a "true human tail" is defined by the medical establishment as an ancestral leftover. There is disagreement in the medical literature about whether a true tail contains vertebral segments, but there is widespread agreement that it is "vestigial" and it "arises by retention of structures found normally in fetal development." (Pediatric Surgery, Update 8, Vol. 24, No. 01, Jan. 2005). There was at least one dissenting researcher I found who sides with Dr. Scott, saying, "But 'true' tails in humans are not true at all: they do never contain bone, cartilage, notochord or spinal chord, and they are easily removed surgically." (Verhulst, 2004). His disagreement is expressed as claiming that the tails are not really "true," and he is of course mistaken about the lack of bone, etc. I did a search on Google Scholar for "true human tail," and I received 37 results, meaning there have been 37 studies containing that phrase. More of those studies can be found using the phrase, "true tail," and related studies are found using the phrase, "fetal tail."
But of course the most important factor is the evidence. The tails with vertebral segments, skin and organized muscle--that you have seen in the Bergman study, the Bar-Maor study and more--cannot be explained by a mere duplication mistake. After all, they are the same tails that we were conceived with.