Most animals, mammals included, sport tails. As well they should. Tails are truly excellent appendages -- essential equipment for survival in many cases. Most primates have tails, but alas, the apes, our most immediate cousins, lack these useful "limbs." Some random mutation, way back when, and zap, no more tails. As a result, we humans live in a tragically diminished universe. And deep down, we feel the loss -- it's a fact that humans have been desperately trying to grow tails for countless millennia. Embryo after embryo puts his or her whole heart into the effort. Tails are there, prominently, between day 31 to 37 of the gestational process, but then they disappear -- leaving the poor embryo dispirited and regretful. In mourning! How much happier a species we would be if we could only wag.
Tails have many uses. For locomotion, obviously. We'd be much better tree and monkey-bar climbers if we had good prehensile tails. We wouldn't be nearly so earthbound. There would be fewer dangerous falls if we had tails -- they're essential for balance. Just watch a cat or a squirrel. Plus a human with a tail could grab on to something to keep himself upright. A tail can also be used for defense. Grab the bad guy's arm with a good strong grasping tail. Think about how much more interesting the martial arts or wrestling would be if were were betailed. Imagine how useful a prehensile tail would be at a cocktail party; both your hands are occupied spreading the green stuff on the cracker, so you hold your wine glass with your tail.
Tails are also good communicators; they're expressive. Observe canis domesticus: submission, aggression, defiance, excitement, puzzlement. Tails would be important for parent child interactions. "Don''t you dare swish your tail at me, young lady." If dogs can do it, why couldn't we do it better?
And also sexual activity. Imagine the possibilities.
Style: "how are you wearing your tail this year?" Billion dollar industries revolve around haircare. Tailcare offers even greater possibilities. I'm sure that the expense of tail grooming, tail coloring, tail enhancing, and tail bejeweling could lift us out of most recessions. Tailcare would be one of the major drivers of the economy.
Human life would have been infinitely richer.