True human tails
Over 100 cases of human tails have been reported in the medical literature. These are not the pseudo-tails that are sometimes reported, consisting of simply various types of caudal lesions. Rather these tails have a complex combination of fat, connective tissue, striated muscle in the core area, blood vessels, nerve fibers, ganglion cells, and pressure sensing nerve organs called Vater-Pacini corpuscles. They are covered by normal skin, with hair follicles, and sweat & sebaceous glands. Although they usually lack bones, several have been found with cartilage and up to five well developed, articulating vertebrae. They have been noted to be anywhere from one to five inches long in the newborn. Newborns with these tails can move them with muscle contractions. During normal human development, apoptosis (cell death at the proper time to form structures) contributes to removing the human embryo tail after it has formed.
The genes for mammalian tail development have been identified, and they have now been identified in us. Obviously they were switched off in our past and now are seen only in rare cases of atavisms.
Importance of atavisms
When observing atavisms it is obvious that these structures are complex and under certain circumstances can be reactivated to form structures that are not normally seen. These individuals thus must possess all the genes to build these remnant organs and all is needed is a mutation or other changes to turn the switch(s) back on that were previously inactivated. Why would a designer put all of the DNA instructions needed to build these complex structures in a species if they were never to be expressed? When atavisms do occur, why do they resemble structures that are either found in fossil ancestors or would be predicted by evolution?
"The occurrence of atavisms (human tails, pelvises and hind leg buds in snake embryos, hind leg buds still seen in whale and dolphin embryos, and the rest) is interesting-but even more so is the way they match up with patterns of genetic data, which in turn corroborate findings of paleontology (the order of appearance of fossils in geologic strata), which also back up patterns of anatomical similarity and vestiges. It is a classic case of many lines of independent evidence coming together, "clicking" like dovetailed joints. Scientists call it consiliance."
~ Kenneth Nahigian, Old Earth Creationist.
Snake grows leg. Atavisms and Phylogeny Link
+29 Evidences for Macroevolution. Part 2: 2. Link
Why Evolution is True. Coyne Link
Evolution: What the Fossils Say and Why It Matters. Prothero Link
Evolutionary Atavisms Link
Atavisms and Fossils (YouTube - DonExodus2) Link