Stealing or not?

Mondays discussion about the Beach Boys and their so called "hits" they wrote really got my attention. It made me wonder how many of the hits out there were actually someone else's songs first. Are most songs written and sung by other artist first, before they are covered by the band that makes them famous. Is it stealing if copy right laws did not exist? I'm sure this whole situation with the Beach Boys has happened with other artists as well I just wonder who.