The Midrash Rabbah (Genesis 23, 9) says: “Four things changed in the times of Enosh: one, the mountains became rocky, two, the deceased began to rot, three, people’s faces became similar to those of apes, and four, people became susceptible to demons.”
In Sanhedrin (109a) it says about the generation of the “Tower of Babel” one third of the people wanted to wage war against G-d and He turned them into monkeys, …
The Malechet Shlomo asks: “Why are these animals (monkeys and elephants) singled out for a special blessing? He answers, look in the Meiri who says they are similar to man in certain aspects.
The Shevet Mussar (chapter47) says we should look at the monkeys and it should arouse us to repent. Why? Because monkeys originally were men and for this reason when they see people they copy them (“monkey see, monkey do”). They want to go back to their original state, to being human again but they can’t so they copy our actions and gestures.”
“So why should a person sin and cause himself irreversible damage … and he will be embarrassed to be seen by people? He will desire to undo the damage but he won’t be able to… today is the time to do them (the mitzvoth) and tomorrow is not the time.. so do them now and don’t wait. Don’t be like the monkey that wants go back to his original state but can’t.”