People have probably always eaten wheat, which tastes good and is also a good source of carbohydrates. But for hundreds of thousands of years, people did not grow wheat. They just picked it wild, wherever it happened to grow.
The Greeks ate mainly the Mediterranean triad,wheat (or barley or millet), wine, and olive oil. They also grew vegetables, especially legumes (lentils, beans, peas, chickpeas). Possibly they ate more fish than most other Mediterranean people. Also, because of their feelings about sacrificing meat, they may have eaten meat less than other people did.

Sometime around 10,000 BC, though, the area around Mesopotamia and Egypt became crowded enough that there was no longer enough food to go around just by picking it, and people had to begin growing it on purpose, weeding out all the plants that people couldn't eat like pine trees, and planting the ones that people could eat, like wheat.

The Greeks love to eat. (Who doesn't?) Many Americans have a fear of coming to Greece because they are afraid of the food. There's nothing to be afraid of. It's not like Mexican or food from the far eat. Nothing is so spicy you can't eat it and the seasonings they use are the very same ones you have on the spice shelf in your Kitchen. And unlike the crappy oil that terrorizes the hearts of many Americans, Olive oil is good for you and will keep you alive long after many of your friends have taken the next step in their spiritual evolution.

You will notice that in Greece the waitor will not come up to your table every three minutes to ask how you are doing or if he can get you anything else or fill your glass with every sip you take like they do in restaurants in America. If you want something else it is your job do get his attention and anything goes within reason. And unlike in America where you get your meal and eat it and that's the end, in Greece it is OK to keep ordering. If something delivered to another table looks good, ask what it is and get it. Nobody is going to think you are greedy or a slob. Eating is meant to be enjoyed and restaurant food is cheap (except for some fish). Eating in a Greek restaurant is pretty informal and meant to be relaxing and enjoyable. So relax and enjoy it like the Greeks do.