That was exactly my point. The molecule is not novel. It indeed existed and we had the opportunity to evolve with its ambient presence. If the laundry list of advantages were that important, it would have created selective pressure on those who didn’t consume it. But it didn’t prove itself as indispensable while other nutrients did; that’s why they are nutrients and caffeine isn’t.
>It indeed existed and we had the opportunity to evolve with its ambient presence.
It may exist in a number of plants all over the world but that doesn't mean that people were harvesting and using it outside of certain isolated cultures until fairly recently, compared to evolutionary timescales.
If anything I'd expect significant genetic variation representing adaptations only present in some groups.
Carbs are not essential indeed. But consuming carbs when available gave us net selective advantage, which is why it's been the biggest part of our post-agricultural diet. And those carbs didn't exist in isolation but in the context of other macro and micronutrients including vitamins.
Can't say the same about caffeine sources, they weren't consumed for their nutritional content, and the supposed selective advantage of their consumption apparently didn't matter as much as carbs'. We didn't witness a supremacy of caffeine using populations, but we witnessed ones that could utilize carbs.
Maybe we didn't have a gap for millions of years. The African kola nut contains caffeine and is supposedly a popular stimulant.