Can you cook or grill chicken totally plain without any marinade or seasoning?

Absolutely. I have been cooking it simply recently, though I prefer to salt it well all over the day before and allow it to rest overnight. Roast at high heat the next day (see today's article on Barbara Kafka, featuring her roast chicken). I start the oven at about 400º, put in the chicken uncovered breast-side down, flip it after about 30 minutes. You can skip this step, and simply put it in breast side up. Salting the day before makes for a deeply flavorful, simply-prepared bird.

Absolutely!

In fact, this is the earliest form of cooking: grilling meat without seasonings.

There's no definitive date in history but ancestors of modern humans (Homo habilus) first used fire 1.5 million years ago. Apart from warmth and providing light in darkness, the usefulness of fire for cooking was probably slowly recognized by these human ancestors.

Of course, back then these hunter-gatherers did not have spice racks, cabinets full of sauces, grocery store aisles of sauces, etc. so they ate things totally plain. Sorry, no ranch dressing for Homo erectus! :-(

So what you are attempting to do was done probably 1 to 1.2 million years ago.

As humans (and their ancestors) created tools, they opened the door to new cooking methods. With a fireproof receptacle, things like poaching and boiling were also new cooking methods. Use of a lid allowed for steaming and the use of flame in an fireproof enclosure resulted in baking/roasting. Once humans were able to render fat from animals, frying became an option.

Have you ever stuck a potato in the oven? Congratulations, you have duplicated something that has been practiced for a hundred thousand years.

Today, the wonders of modern food distribution make it easy for you to simply open a bag of chicken and 1.) dump it on a hot grill, 2.) dunk in boiling water, 3.) drop onto a steamer, or 4.) zap in the microwave.

Thus, you should study the cooking techniques of poaching, boiling, steaming, frying, baking, roasting, and grilling to start.

Cooking is basically the transfer of heat to a food item and fancy sauces and marinades are optional.

Almost any food item you can buy at a store today can be cooked like our human ancestors did a million years ago.

The sky's your limit.

:o)

Best of luck!