'Avengers' Star Chris Evans -- I Want to be the Next Bear Grylls

"Avengers"-schmavengers ... Chris Evans says his real dream in life is much more primal -- revealing that he secretly longs to eat bark and live in the woods a la "Man vs. Wild."

The "Captain America" Star told Details magazine he dreamed about going all Thoreau back when he was a kid ... saying, "For a long time all I wanted for Christmas were books about outdoor survival. I was convinced that I was going to move to the woods."

But this wasn't just some pipe dream ... the movie star says he was completely committed to the idea and even practiced for the lifestyle -- explaining, "I swear to God, if you saw me when I am by myself in the woods, I'm a lunatic, I do crazy s**t."

"I camped a lot, I took classes" ... Evans said, adding, "At 18, I told myself if I don't live in the woods by the time I'm 25, I have failed."

He's 30 now ... an A-list movie star ... and probably lives in a mansion, or six.