What’s the purpose of tails in modern mammals? When did the tail originate?
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You could argue that the tail is one of the first kinds of body types ever evolved. The first kinds of animals were basically worms and sponges; little more that tubes.
As fish and other aquatic life evolved, that end part of the body would become a more functional tail, providing locomotion.
When animals moved to land, a tail was no longer needed for locomotion but evolution doesn’t get rid of something just because it’s no longer needed. The tail was basically repurposed. It can be used as a weapon, an aid to balance and stability, contribute to heat regulation, a disposable escape mechanism, another sensory tool. Some species evolved in a way that enhanced the tail, making it an appendage with which they can manipulate their environment while with others it gradually evolved away until it was no more than a few bones hidden inside the body.
Different animals = vastly different purposes.
Tail is an extremely specialised organ because it evolved extremely early in vertebrate evolutionary history, and as such, it had the time to specialize over different species – some of the first Chordata already had some resemblance of tail specialisation even before we evolved a proper spine.
Tails originate before the evolution of mammals. They are ancestral to all vertebrates, which is why lizards, amphibians, and fish have them too.
They have several purposes in mammals, including balance, signaling, swatting insects, underwater locomotion, and grasping in the case of animals with prehensile tails. Of course in some lineages, namely the apes, the tail is reduced to the point of being lost.
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