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.

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~

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.

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.

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

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.

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.

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.

The only thing I can think that would be almost as old is agriculture

Around here it is called world's oldest business.

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.