Info from Marvel.com
First it's Captain America
In World War II patriotic solider Steve Rogers recipient of the "Super Soldier Serum" became the living symbol of freedom, Captain America. Left for dead while frozen in ice, the star-spangled hero with an indestructible shield awoke years later to continue his never-ending battle for liberty.
Natasha Romanoff, a rumored descendant of Russia's royal family. But nothing else is known of her family. During World War II, a Nazi assault upon Stalingrad set the building she was in on fire. A woman, possibly her mother, threw her into the arms of a Russian soldier named Ivan Petrovich before being consumed by the flames.
The Incredible Hulk
Caught in a gamma bomb explosion while trying to save the life of a teenager, Dr. Bruce Banner was transformed into the incredibly powerful creature called the Hulk. Some people say he changed because he tried to redo the Super Soldier Serum. An all too often misunderstood hero, the angrier the Hulk gets, the stronger the Hulk gets
Clint Barton was orphaned at an early age when his parents died in a car accident and was sent to a children's home with his brother Bernard. He soon met the Black Widow, who was working at the time as a costumed villain for her country, and she easily seduced the hot-headed adventurer, making him think it was in his interest to defeat Iron Man. He attempted to do so according to her prompting on several occasions, but soon wised up, regretting his decision. Instead, he hoped to join Iron Man's team of Avengers.
Thor was the god of thunder and of the sky in Norse and early Germanic mythology. Though Odin held a higher rank, Thor seems to have been the best loved and most worshiped of the Norse deities. He belonged to the common people, while Odin appealed to the learned and noble classes. A patron of farmers, Thor was associated with weather and crops.