Why do humans only have arms and no tail?
Way back in the time, before time itself began, three sorts of creatures began to walk upright, rather than scamper about with four limbs bearing the weight of their body. All three creatures came into being because of the same need, the ability to hunt in tall grassland. Tall prairie grass covered many miles and the grass stood tall, except where grazed short by the wandering herds of bison and antelopes. A hunter need only make one kill a week, from either of these herds, to feed his family, and the rest of his time was his own, which made hunters very keen to stand up and see into the distance. Because the herds could not see over the taller grass, that had not yet been grazed, a humanoid standing on 2 legs could watch the herds from the margins of that which had not yet been grazed, and only had to wait for a beast to lower their head and start to munch away at the taller grasses, then a flint tipped spear brought down the family food. So, standing up on two legs made for an easy life. So, all of the three plains humanoid tribes, started to do it.
One tribe was a humanoid with 2 legs 2 arms and a tail. The tail acted like a balance when the body lent forward to run. This was fine on shorter grass but in the taller stuff the tail got tangled up and became hard to disentangle. The delay meant that this variation was inevitable the last to arrive at a hunting ground and by then the herd had panicked and fled.
Another Tribe was humanoid but with 2 legs 3 arms ( the third one grew from the back reached up over the head and was long enough to be used in conjunction with the left and right arms. ) This was handy since this third arm and hand, could carry a spare spear . This was fine if stalking slowly but at the moment the hunter needed to rush in amongst the herd, select his target and strike with his spear, this third arm unbalanced the hunter; meaning that they missed the target way more often than hit it.
The third tribe were thought to be lesser, by the other two, because they only 2 legs 2 arms and no tail. This variation became the most successful hunters and so their tribe flourished and multiplied, The success of their hunters allowed the tribe to spend time watching how the other animals and birds lived, how to build homes, how to avoid the predators.
The great predators who fed off all three tribes concentrated on the easier two; The predator only had to wait in the long grass for a tailed tribe member to become entangled and they had an easy feast. They soon learnt the if they sent two attackers to approach a three-armed tribal member and if they sent one attacker in front and one behind the three-armed victim would get so unbalanced as they tried to turn round quickly, they fell to the ground and became easy meat, This went on until these two tribes had both been wiped from the face of the Earth. By the time they had done this, and turned to the third tribe for food, this tribe had learnt much about the predators and how to avoid them, even how to lay traps to kill them.