From How Humans Lost Their Tails:
Even if geneticists are beginning to explain how our tail disappeared, the question of why still baffles scientists.
The first apes were bigger than monkeys, and their increased size would have made it easier for them to fall off branches, and more likely for those falls to be fatal. It’s hard to explain why apes without tails to help them balance wouldn’t have suffered a significant evolutionary disadvantage.
And losing a tail could have brought other dangers, too. Mr. Xia and his colleagues found that the TBXT mutation doesn’t just shorten tails but also sometimes causes spinal cord defects. And yet, somehow, losing a tail proved a major evolutionary advantage.
“It’s very confusing why they lost their tail,” said Gabrielle Russo, an evolutionary morphologist at Stony Brook University in New York who was not involved in the study. “That’s the next outstanding question: What on earth would the advantage be?”
