The Avengers #91

In "Take One Giant Step—Backward!", the Avengers face their most bizarre threat yet as Ronan the Accuser unleashes a plan to de-evolve humanity into simian forms, turning Earth into a Kree-Skrull battleground. With Mar-Vell captured and the odds stacked, the team must rely on their wits—and the mysterious Sentry #459—to stop the countdown to primal chaos. Written by Roy Thomas and brought to life with dynamic art by Sal Buscema, this 1971 classic features a cover by Buscema that captures the escalating tension perfectly.