I think it would be really cool to have a tail that could grab onto things (a prehensile tail). Unfortunately, humans and our closest relatives (the apes) don't. We do have what's called a "vestigial" tail, meaning that it's a sort of evolutionary leftover. It's visible in embryos, but by the time we're born, we just have a few small bones that can't be seen from the outside. Most people don't even know they have tailbones unless they break one. Occasionally, a baby will be born with a tail, but it is usually removed surgically.

People often confuse apes (chimps, gorillas, orangutans, gibbons, etc.) with monkeys. They're not. Apes don't have tails.

A very interesting question with a subtle answer. If you look at the end of the spine of a human, you'll find a peculiar feature....

There is something that looks strikingly similar to a tail, which is appropriately named the tail bone (aka coccyx); in rare cases, some people are born with something tail-like, which is considered a congenital defect. So, what is this tail bone exactly?

It is accepted that the tail bone is vestigial, an organ or trait in humans that have lost their original function through evolution. The tail bone is actually a part of a larger group of features that have gone out of use. You can read more about it vestigiality. You may even be familiar with other vestigial parts of the human body, such as the appendix or wisdom teeth, which people often have removed for health or medical reasons.

Why did the tail bone become a vestigial feature?
There are several theories as to why this happened. If you think about a few animals with tails, you might notice that they tend to move on four legs (e.g. cat) or use the tail itself for locomotion (e.g. fish). Tails in animals are observed to have function, such as balance, brush away insects, or grabbing things. A possible explanation is that humans became bipedal (i.e. walk on two legs) and so the need for a tail faded out.

Why did humans become bipedal?
There are also a lot of theories for this as well. You can read a more in-depth article click here to read article.

Theories range from being able to carry objects (the wonders of opposable thumbs! There's even a whole Wikipedia article about the thumb) to exposing less surface area to the sun where there's a lot of sun.

Here's an excerpt from the hyperlinked article:
Twentieth-century theories proposed a wide array of other factors that might have driven the evolution of hominin bipedalism: carrying objects, wading to forage aquatic foods and to avoid shoreline predators, vigilantly standing in tall grass, presenting phallic or other sexual display, following migrant herds on the savanna, and conserving energy (bipedalism expends less energy than quadrupedism). Furthermore, if the early bipeds were regularly exposed to direct midday tropical sunlight, they would benefit from standing upright in two ways: less body surface would be exposed to damaging solar rays, and they would find relief in the cooler air above the ground.

Who knows, with a few more years of evolution, the tailbone might disappear altogether! What do you think could end up being like a vestigial trait with our ever changing lifestyle and usage of technology?

Hope this helps!
Best,

Good question.
Humans are apes, and apes for reasons of evolution that are not currently known have lost their tails, unlike their closest relatives, the monkeys, who still retain their tails. Sometimes humans are born with very short tails, but they are cut off by the doctors when it happens.

The easiest answer to this is that there was a genetic pressure to no longer have a tail. That means, at some point in evolution, it became more favorable for survival to have no tail. Thus, monkeys without tails were more successful at reproducing, creating more monkeys without tails, and leading them to take over. What was this pressure? We don't know for certain, but with our ancestors being able to swing from trees with hands, tails became redundant and a waste of energy. When walking, we also did not need tails. So, it became more common to not have tails over millions of years.

What is REALLY crazy is that the genes that make tails are turned off in humans, but random mutations in a baby's development can turn them back on. This is probably how we lost tails in the first place (random mutations turning off tail development). But even today, very very rarely a human will be born with a fully functioning tail. This points to our evolutionary past, but can be pretty scary for parents. This has happened over 20 times in the past 100 years.

Source: click here to read more

This is an interesting question! Actually, a tail is present for a few weeks during human embryonic development, and is most visible around the time the embryo about 31-35 days old! Eventually, however, the tail disappears and all that is left is the coccyx, which is a sort of "vestigial tail" if you will. The coccyx itself is a point of attachment for several soft tissues, such as muscles, ligaments, and tendons.

While the answer to your question is not known exactly, it may be useful to think about what other animals use tails for. Depending on the animal, tails can be used for balance, as something to rest on, for swatting insects away, keeping cool, or as an extra appendage that can grab/hold things. Because of our bone structure, we are able to balance without a tail. We also don't need a tail to swat insects away or grab things, because we have hands (with opposable thumbs). So, having an extra body part that that does things other body parts can do just as well would be costing the body extra energy to sustain that part without it actually doing anything uniquely useful.