Why is prostitution called sometimes world's oldest profession?

This reminds me an experiment made with capuchin monkeys, where the researchers were using small discs as some sort of currency. They could use it to buy stuff like pieces of cucumber (they eat it, but it's meh), jell-o (they like it), grapes (they love it)...
One of the things that they reported is that a female exchanged sex for a disc. Then used said disc to buy a grape.
Conclusion: sex for goods is likely a human behaviour that predates humankind itself.
- Or maybe it's not really unique from other animals, we just have fictional currency and choosy morality. Every animal I can think of has the male doing all the courting work. Male spiders dance, birds sing and survive with brighter colors, bugs may lose their heads, giraffes punch females in the bladder to see if they're even ready for sex, straight female prostitution drastically outweighs straight males, etc. It seems like the one who has to actually develop the offspring has the option to be much choosier. So did the capuchin prostitute herself or is that our morality being imposed on godless creatures who likely see no difference between sex, cleaning, or feeding each other as each is some type of need?
  
  The capuchin engaged in sexual activity for payment, call it what you want but that pretty much the dictionary definition of prostitution. It’s not a moral or god question it’s semantics.
  
  Lmao godless The only difference between you & them is you got a stick up your ass
  
  So many logical fallacies to unpack, and no formatting to assist in legibility? Nah, I'm good.
  
  No wonder I'm still single.

If you take “job” to mean “doing a thing in exchange for another thing”:
Allow me to narrate a brief historical reenactment:
Caveguy:
dragging cavedeer carcus somewhere
Cavegirl:
happens upon the scene
grunting (translation: damn that venison lookin’ tasty af)
Caveguy:
grunting (translation: it’s mine you can’t have it)
Cavegirl:
grunting, but seductively (translation: well, if we smash, can I have some?)
Caveguy:
aroused grunting (translation: hell yeah girl, you can stick around, I’ve been having the worst dry spell)
_fin
- This is cinema - Martin Scorsese
- These Unga bunga bitches always sliding in my DMs trying to get free shit!

It's not just us humans, chimps trade meat for sex: https://www.nationalgeographic.com/animals/article/chimps-behavior-sex-news-animals
Yale taught monkeys about money, and yup, they traded money for sex. From archive of NYT article:
Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.
- ale taught monkeys about money, and yup, they traded money for sex. From archive of NYT article:
  That was a fascinating read! Thanks!
- That would make hunting/gathering the oldest profession, so that you have something you can trade for sex.
  
  It depends on whether you're considering "a profession" to be something that someone does to gain goods/services/"value" from another person, or from nature.

It's a commentary on humanity and society.
There's no records or evidence for the first jobs, so it's mostly humor with a grain of truth. Most likely trading sexual favors for some benefit likely predates most other forms of trade (but it obviously depends on the exact definition of trade, profession, prostitution, etc.)

Because courtship rituals show up before money and across species usually involve the male providing food to the female to demonstrate the males fitness.
Long before language, money, and lots of other shit had been invented, males have traded resources for sex.
In some species the female stays with the young and is unable to get food for herself, she has to rely on her mate providing food or her and the baby die.
- There was an experiment where the researchers introduced "money" to chimps that they could exchange for fruit treats. Some females almost immediately began trading sex for money
  
  Well, yeah.
  Food only lasts so long. Money could be stored and used later.
  So the exchange became a much better deal. In effect the females were always hungry. Because they could exchange money for fresh food later.
  Makes as well could save up money so they didn't have to get food whenever they were horny.
  It really streamlined the process, but long term I'd be interested to see what happened to fitness of the population. Like 4-4 generations down the line, would that population be significantly less fit than a control group?
  Although that would likely have to be done in the wild where threats are and not in a safe enclosure.

More interesting is the origins of that phrase to designate prostitution.
Fortunately, I found an article in worldhistories.net, that shows the first documented time of this phrase. The person who coined the phrase was none other than Ruyard Kipling ("The Jungle Book"):
Lalun is a member of the most ancient profession in the world. Lilith was her very-great-grandmamma, and that was before the days of Eve as every one knows. In the West, people say rude things about Lalun’s profession, and write lectures about it, and distribute the lectures to young persons in order that Morality may be preserved. In the East where the profession is hereditary, descending from mother to daughter, nobody writes lectures or takes any notice, and that is a distinct proof of the inability of the East to manage its own affairs.
On the City Wall, in In Black and White (Allahabad: A. H. Wheeler & Co., 1889), page 78
If you want to know about actual prostitution, we should go far back to ancient Mesopotamian texts.
According to "The Epic of Gilgamesh" (the most ancient epic in the world), the gods created a savage man, Enkidu, who lived in harmony with the animals in the woods. Gilgamesh wants to tame Enkidu, and is told to bring a "harimtu" (a "sacred prostitute") to him.
and he [Enkidu] possessed her ripeness. She was not bashful as she welcomed his ardor. She laid aside her cloth and he rested upon her. She treated him, the savage, to a woman’s task, as his love was drawn unto her.”
Later, as he regrets joining civilization, Enkidu curses the harimtu:
“I will curse you with a great curse… you shall not build a house for your debauch you shall not enter the tavern of girls…. May waste places be your couch, May the shadow of the town-wall be your stand May thorn and bramble skin your feet May drunkard and toper (ed note: someone who drinks alcohol to excess) alike slap your cheek.”
Researcher Gerda Lerner, in her article "The Origin of Prostitution in Ancient Mesoportamia" (Signs, 1986, pp. 245-6), says:
The nature of this curse tells us that the harimtu who mated with Enkidu lived an easier and better life than the harlot who has her stand at the town wall and is abused by her drunken customers.
This would confirm the distinction we made earlier between the women engaged in various forms of sacral sexual service and commercial prostitutes. Such a distinction was more likely to have existed in the earlier period than later.”
So yes, there were prostitutes in ancient Mesopotamia, the cradle of civilization.
EDIT: typo
- And yet recent evidence shows prehistoric women would have been the more likely hunters
  
  Prostitution is not specifically gendered and sex for pleasure's sake is not specific to men. There could be male or female customers seeking male or female prostitutes.
- Cradle*
  
  Thanks!

I liked the analysis in HIMYM. It can't possibly be the world's oldest profession, because in order to exchange something of value for sex, you have to have obtained something of value. Like say this hypothetical first ever prostitute was exchanging blowjobs for berries, then berry gatherer was actually the first profession.
- Unless the offer was a blowjob for him to go get the berries.
  
  Arguably, her giving the blowjob meant she got the berries
- So, sex is as old as mankind, but we didnt invent favors till 2 AD... so, nope... they couldnt pay favors with sex.

It predates human civilization, and likely even predates homo sapiens. Our great chimp ancestors were likely trading spare food for sexual favors, though whether you can consider it the oldest profession depends on your definitions.

It's hard for me to believe that hunter / food seeker isn't the oldest profession... But it's definitely up there.
- Yo if you hunt and gather a little more and share it all you can fuck my cave wife
- What's the last time you met a gatherer doing it as a profession and not a hobby?
  Hunters are a little bit more common, but still, full time food hunters are rare.
  Prostitutes on the other hand.
  So as a simple saying used in a daily life, I can kinda see why it's used :)

I can't answer that question, but I want to know why it's called the oldest profession when sex work is still illegal in most of the world. A profession is usually a legal job. And sex workers should be legal everywhere as a human right.
- Profession doesn't mean legal. It's just as opposed to amateur. ie. You make enough money doing it for it to pay your cost of living.
  But yeah it definitely should be legal, regulated, and unionized
- It has been legal in large parts of the world at some point in history.

Around here it is called world's oldest business.

The only thing I can think that would be almost as old is agriculture
- Nah, prostitution pre-dates agriculture. Hunter-gatherer populations had men trading resources for sex.
  
  That's what agriculture is almost as old as prostitution!

There were probably professions that long predate history, and any of those are a bit hard to prove. There were 'shaman' in pre-history, and good shaman were quite possibly supported by their communities. There may also have been things like dedicated cooks. Trading sex for food however, is clearly hundreds of thousands if not millions of years old, so it's hard to argue that other professions came before it.
- I'm imagining dinosaur hookers, out on the jungle corner in fishnets and halter tops, smoking cigarettes and shaking their tails at passersby.
  Edit: look at that second word before anyone gets snarky about dinos and humans not coexisting. We can have fun that ignores reality, and there's nothing more fun than picking up a t-rex hooker in your Flintstonesmobile, taking her back to your cave and smoking some rock. It's a gneiss day.
  
  ok. I too am now imagining dinosaur hookers, and it's not even the weekend yet. RARHH!
  
  You're focused on the second to first word, I'm focused on that second to last one! Fantastic pun.

Itt: people finally making the argument I've made against the op statement for years.
It's beautiful when society starts to change and it begins to align with your views. 🥲

I couldn't find proofs that prostitution is the oldest

I’ve read an argument (I think I can cite this to David Graeber maybe) that women were one of the original forms of property. Early forms of marriage are essentially master/slave, or a form of prostitution.

The answer is sexism. Before anything else, women were whores.
Not cool actually. It's clearly not true, but that's why it's a saying.