Does all Great Apes have tails? If not why?

Question

Do Gorilla have tail?

Answer 1

The answer is no, and simple equations can summarize it:

All Great Apes (Gorilla) = no tail
Monkey = tail

Since Great Apes share a common ancestor with humans, they do not have tails. Monkey is the common name for those lesser primates (on the evolutionary scale) that are related to Great Apes that do have tails.

Comments

So is that the whole reason behind the Tail :D