Most of us know what a tail is until we try to define it. From a biologist’s and anatomist’s point of view, a tail is a bony extension from the sacrum and coccyx at posterior end of an organism’s body. The sacrum and coccyx, however, are a part of the vertebrae, so what about invertebrates? Do they have tails? Well, no, not really. They have tail-like appendages because there is no bone in them (no matter how hard you think a scorpion’s “tail” is).
So, what prompted the evolution of the tail? How come mainly vertebrates have tails? Was it acquired and then died out?
Tails are first and foremost functional. For kangaroos, it’s used in maintaining balance and steering. Birds’ tails are also used for steering, mostly. Some birds (i.e. widowbirds and peacocks use their tails in mating displays). In boa constrictors, the tail (which is most of its body) is used to strangle other animals. In most marine life (and snakes), tails are used for locomotion. In many monkeys, tails are used as another appendage, using it to grab objects out of reach. In livestock, other than shooing insects, there is debate about the tail function, mainly because horses can have their tails shorn to no adverse effects on their existence. Then again, the horses with shorn tails are in captivity instead of in the wild. There is thus strong argument that tails evolved due to do environmental demands, and the similarities on the different continents is due to similar environmental demands. Additionally, tails are thought to be leftover from the reptilian ancestors of mammals and continued their function.
One type of tail is the prehensile tail, or one that can grasp or hold objects, found in mainly New World monkeys and all snakes (a prehensile body). A tail is partly prehensile if cannot be used to pick up objects, but rather anchor an animal usually in a tree or in climbing, seen in the prehensile-tailed porcupine (New World animal) and opossums. Why is it only found in the New World? Some scientists argue that the forest is more dense in the New World, and so ability to maneuver quickly through this dense foliage was granted to those who had an extra, functional appendage. Additionally the absence of gliding vertebrates in the New World forests, seen in the less dense forests elsewhere promote this dense-forest explanation. Additionally, there was isolation. When Pangaea split, Australia wasn’t the only place isolated. The Americas were essentially a large island, and even with the Bering Straits being iced over, the prehensile tails of primates in South America were still able to evolve without too much outside interference. Tails evolved differently to suit environmental needs, but all were the same in that they were a bony outgrowth of the spinal column, making it necessary to have vertebrates to have a true tail (why invertebrates do not have a true tail).
The coccyx in humans is thought to be the remnants of a tail we once had, due to its location and fusion, yet the “tails” that some are born with do not contain bone, so are not true tails. The coccyx in other vertebrates is the beginning of tails, where the coccyx is not fused, unlike the wax it is fused in us. The coccyx could’ve evolved for other reasons, and is not be useless as many vestigial traits are deemed. The coccyx serves as an anchor for ligaments, and muscles like gluteus maximus and levator ani muscles, which converge from the ring-like arrangement of the pelvic (hip) bones. This forms a bowl-shaped muscular floor of the pelvis called the pelvic diaphragm or pelvic floor. This structure is necessary for keeping lower abdominal organs in our abdomen as well as activities such as defecation, urination, sitting down, and ambulation (walking). There is debate about our tailbone being the remnants of a tail, but, in the meantime, we can look at it as a necessary structure for bipedalism, because it anchors the muscles needed for us walk upright. Therefore, it most likely contributed more to our bipedalism than anything else. The question is, if the coccyx is indeed the remnants of a tail, did we give up a tail in order to walk upright? The last bipedal animals with tails were most likely the dinosaurs and it is believed our mammalian ancestor walked an all fours.
Lemelin, P. Comparative and Functional Myology of the Prehensile Tail in New World Monkeys. http://www.ncbi.nlm.nih.gov/pubmed/7595958
German, Rebecca. The Functional Morphology of Caudal Vertebrae in New World Monkeys. http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330580414/pdf