I has been shown that individual primates often will trade sexual favors for food or even grooming. I find it unlikely that we didn’t follow the same path on our way toward houses and refrigerators. This may be loosely described and hunter/gatherer activity but not a profession since it is on an individual level. 
Science: bringing to us all the creation of the monkey prostitute.
Is there nothing that science cannot achieve? 
… which IS just a form of prostitution, after all, isn’t it?
It would probably not do to advance that theory while actually on a date. 
“So, you have agreed to meet me for the price of a cup of coffee and a donut. Would you agree to a blowjob for $200?”
“What kind of woman do you think I am?!”
“We’ve established that, now we’re just haggling over the price.”
I view a “profession” in this context as a specialized occupation that allows the professional to support himself without participating in survival activities like hunting, gathering, or farming.
I would guess that shaman/holy man/priest is the oldest profession.