It apparently goes back further than mankind itself. The New York Times reports the first observed case of monkey prostitution:

Something else happened during that chaotic scene, something that convinced Chen of the monkeys' true grasp of money. Perhaps the most distinguishing characteristic of money, after all, is its fungibility, the fact that it can be used to buy not just food but anything. During the chaos in the monkey cage, Chen saw something out of the corner of his eye that he would later try to play down but in his heart of hearts he knew to be true. What he witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.)

Immediately thereafter, several of the other monkeys banded together and demanded the prostitute monkey hand nover 20% of her grape to be redistributed to the rest of the troop -- while a different segment banded together and demanded that monkeys not be allowed to trade sex for money.

She should've gone the route this decadent, immoral woman went...

http://www.theonion.com/news/index.php?issue=4127&n=2&id=4251

"while a different segment banded together and demanded that monkeys not be allowed to trade sex for money."

That was the _humans_ behavior ;)

Maybe Tim Burton could incorporate these findings into another Planet of the Apes remake.

Al, check this out.

The thing that puzzled me is why they shied away from exploring the prostitution angle. Frankly, I never understood why prostitution is considered immoral, but I accept that a portion of humanity has these more or less immutable beliefs. However, hindering the study of one of the closest species to humans because of someone's beliefs about prostitution? Especially when said prostitution is a key feature of human society? It's absurd.