Short answers, pulled from the story.
Thespis became the first protagonist in the year 536 B.C. by stepping out of a chorus of fifty men to speak a single line of dialogue. This event marked the birth of the concept of tragedy and established the protagonist as the central figure whose fate the audience would track.
The protagonist is simply the main character of the narrative whose fate is most closely followed, whereas a hero is a human who has become a semi-divine being with noble qualities. True heroes are distinguished by their strength and courage, while many protagonists are defined by their flaws or lack conventional heroic attributes.
The earliest known example of a protagonist predates the theatrical invention of 536 B.C. and appears in the Epic of Gilgamesh from the third millennium B.C. This ancient text contains a character who functions as a protagonist despite the lack of structured theatrical conventions.
In Euripides play Hippolytus the protagonist role is shared between two characters with Phaedra serving as the protagonist of the first half before her death. Her stepson Hippolytus then assumes the dominant role in the second half of the narrative.
Tybalt serves as the antagonist in Shakespeare play Romeo and Juliet who opposes Romeo and attempts to thwart his relationship with Juliet. This character creates the conflict that drives the narrative forward and provides the necessary friction to test the protagonist.
The antagonist provides the necessary friction that tests the protagonist character revealing their strengths and weaknesses and forcing them to develop as a result. Without the antagonist the protagonist journey would lack tension and the opportunity for growth.