Why Don't Humans Have Tails?
Different animals have developed highly specialized tails over time. For instance, horses can use their tails to swat away flies, while the prehensile tails of new world monkeys can help them grip tree branches as they climb. Humans lack tails because they simply don't need them.
from Brain Stuff
Key Facts In This Video
Human's don't need tails since our center of gravity runs down our spine, which helps us balance the weight of our heads. (1:08)
All mammals have a tail at some point in development. (1:53)