The cliche supposes that prostitution was around before ancient and prehistoric people could figure out any other possible endeavor they wanted to do for a living. I have no doubt at all that back in some cave a million or so years ago, some guy [it's always the guys] wanted to have sex and someone else [a layyyyydeyyyy?] didn't, or before she agreed to [if she wasn't just clubbed over the head by her suitor] thought for a second and said, "what's in it for me"? This also supposes that there was money to be exchanged, and while humans today are different in many ways from humans millions of years ago, I have no doubts that given the option, a bunch of prehistoric sorts sitting around the cave saying, "OK, guys, we need to invent something of value we can give to one another in exchange for goods or services -- we can make it out of stone or metal and put our faces on it and some birds or other animals and if one of us wants that leg of buffalo the other guy has, I can give him a handful of this ... what should we call it ... 'money' for his trouble." At which point of of the other guys in the cave most likely said, "Or we can have more sex." I'm thinking the cavemen went for the sex before the money. So, without money, there's no prostitution, right?
If you're a biblical sort, and think the world began with two humans, a man and a woman, who had dominion over everything, obviously the first profession, was farming, even if all they did was pick fruit off of trees. According to the bible, though, since Adam and Eve were in the Garden of Eden, they didn't have to work, but as soon as they took that damned apple, they were cast out and instantly realized they were naked and they were shameful, not hungry, so they reached for something to cover themselves up with. So, biblically, it would seem that the tailors and clothiers were the world's first profession, likely soon followed up by farming.
People way back when, bible folk or not, needed to eat, clothe themselves and have shelter. I mean, who's thinking of sex if they are starving, cold or homeless? (OK, I know. Men.) So in terms of things you need to do, it seems sex, and sex for money, had to be at least fourth or fifth on the list of jobs. [Let's see now, World's Oldest Profession -- tailor; then farmer, then hunter, then contractor, then carpenter, then thief, because you know there had to be some thievery going on.]
If you think of what prehistoric people etched on the walls of their caves, you had hunters, farmers, and the animals they hunted. No sex or prostitutes, (until those naughty Pompeiians!)
As far as doing things that weren't absolutely necessary in order to live, unlike hunting and gathering, you could say the first activity humans engaged in was that cave etching/drawing. So, as far as chosen professions go, it would seem that journalism is the world's oldest profession. [Insert punchline here].