Complex sentence: Atlas Comics grew out of Timely Comics, the company that magazine and paperback-novel publisher Martin Goodman founded in 1939, and which during its wartime peak of popularity published its star characters the Human Torch, the Sub-Mariner, and Captain America.
Split into simpler sentences: Atlas Comics grew out of Timely Comics, the company that magazine and paperback novel publisher Martin Goodman founded in 1939. During its wartime peak of popularity, Timely's most popular characters were the Human Torch, the Sub-Mariner, and Captain America.