Hi,
Good question. This is one I answered before: “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.

Why did the ancestor of the apes and people lose their tails? No one really knows. Things don't disappear just because they are not being used. One gene can cause many different things. It is possible that a gene that was helpful to this species had a few effects, including the loss of the tail. It is also possible that having a tail came with a price. Individuals that had small or no tails would have been able to leave more offspring with the no-tail gene. Can you think of any ways that a tail might be a problem?

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

I would also add that monkeys in the Americas have prehensile tails, but the monkeys in Africa don’t. Some can’t really grab with them at all. Others can sort of hold onto things, but can’t really grip them and can’t hang from them.

If you’re interested in questions like this, there are all sorts of scientists that study this kind of question. Developmental biologists study how things form before they are born (or hatched, or whatever). Evolutionary biologists try to figure out how species ended up the way they did. Anthropologists study people.

That's a good question and I doubt anybody knows the answer. We're apes, and no apes have tails, but our more distant relatives among the primates, the monkeys, lemurs, tarsiers, and so on all do. Why we lost it is an interesting evolutionary question.

Excellent question, and challenging to answer. Tails serve a variety of purposes in animals. Some, like peacocks, use tails to to attract mates. Other animals use tails to swim, like fishes. Some of our close relatives use their tails for climbing, manipulation, and balance- like monkeys. Most people think that humans lost their tails when our ancestors transitioned from living in trees to living on the ground, where a tail was no longer useful- in fact, a tail cost energy to grow! Because ontogeny recapitulates phylogeny- a fancy way of saying development mirrors evolutionary history- humans do still have tails during development! However, the tails are gone by the time we are born.