Good news for that monster you dated in undergrad who refused to go down on you: he'll likely be edited out of the gene pool soon. In an in-depth piece about the evolutionary history of oral sex, Medical Daily suggested that women evolved to be with partners who performed oral sex on them. There's also some boring stuff about how performing fellatio may have evolved because ingesting semen has been shown to reduce the risk of preeclampsia, but that's kind of giving our ancestors' deductive reasoning skills a lot of credit.
The study Medical Daily cited was a 2013 survey of 243 heterosexual (presumably cisgender) men in long-term relationships, which found that the more likely they were to believe their female partner was highly sought after by other dudes trying to impregnate her, the more likely to perform oral sex on her they were:
The research showed that men who were most likely to report getting their partner to orgasm during oral sex were more likely to think their woman was sought after by other men. Therefore, the more eager men are to please partners believed to have better options the more likely they may be to perform cunnilingus to keep their mates from cheating.
(I like to think that Bernie would be shocked and disappointed by this toxic masculine behavior.) So really, this is less about women sneakily evolving to be with partners who are better at giving oral sex, and more about men being so obsessed with their fears of being cuckholded (the terminology used by the study) that they will go to great, tongue-dextrous lengths to avoid being cheated on. That doesn't really have a ton to do with women at all, and it seems sort of ridiculous to determine whether or not women have evolved to score more oral sex by speaking exclusively with 243 men.
The good news is, it's 2016 and we are free from the biological threat of failing to propagate the species, so we can stop making fear-based decisions about sex!
Bottom line: oral sex is not a bank you make deposits in and withdrawals from to buy your partner's fidelity. It's a thing you do because they like it and want it and because you are invested in their pleasure and sexual agency.
Images: Andrew Zaeh for Bustle; Giphy