As a sommelier I naturally think grapes fermented before grains. What fermented first often strikes arguments amongst beverage geeks.
It is similar to the chicken and egg argument—I think the chicken came first—single celled organisms had to split and split enough times to become chickens—then they could lay eggs. The origins of human fermentation may be traced to the Neolithic period in Asia Minor, Mesopotamia, and Egypt.
Grains are more complex than fruits. Grains have complex carbohydrates. Fruits have simple sugars. So, deductively speaking, grains are older and more evolved, and theoretically fermented before fruits—in the state of nature.
The most recent DNA evidence shows grape origins to be in the Stans of the former Soviet Republic, most specifically Kazakhstan. Man most likely fermented grapes before grains because grapes (fruits) are better looking!
Grains have to be soaked in water and processed to ferment. Before man figured out how to make bread, they procured grains, added water they heated over a fire, and a porridge like substance resulted. They let the porridge sit out in the sun to ferment with water and a few weeks later—beer.
Today’s beer has a few more steps involved, but you can get a relative idea. Before brewing became prominent along the Nile River Basin and the Fertile Crescent of the Tigris and Euphrates, man had to figure out how to grow grains. This was a lot of work. Agrarian Societies evolved and people lived in the same place year after year. Fermented beverages became a good form of currency and useful for bartering.
I think of primitive man with club in hand, spending half the day making fire, living in a cave-like dwelling. They gather the good-looking grapes (or any colorful fruit) from vines into a carved out bowl made of wood or stone. They forget about it, leaving it in the sun. Several weeks later they find it and it is now a delicious liquid. They naturally want to drink more and more.
Animals are naturally attracted to sweet colorful fruits—not grains—as honey bees are attracted to beautiful flowers. And, a sommelier will always argue man fermented wine first.