The POSIX (Single UNIX Specification 4) standard is not helpful:
If the first line of a file of shell commands starts with the characters "#!" , the results are unspecified.
So, the standard implies that if you don't have #! then it should run a POSIX shell. But modern shells are not POSIX compliant. The old Korn Shell 88 (ksh88) ran the Bourne shell (close to a POSIX shell) with no #! line, but ksh93 breaks that, and so does Bash. With both ksh93 and Bash, they run their own shell if no #! line is present.
Despite popular opinion, Bash and Korn shells differ. When you write a shell script you can never be sure what shell you will be run from, or even if it will be run from another shell at all (most programming languages can run other programs). The minute you use something outside of Bourne/POSIX syntax you will be scuppered.
Always use a #! line, don't leave it to chance.
