License of modifications

[Alexander Toresson <alexander.toresson@gmail.com>]

Hello!

One file of a software (software A) I'm working on packaging was
originally taken from another project (software B), and had a public
domain license.
Then a contributor of software A modified it. The main license of
software A is GPL-2+. There's no copyright or license statement in the
file itself, but I've had to hunt it down from the readme of software
B.
Do I assume that the changes of the contributor are GPL-2+, or public
domain? Can different parts of a source file have different licenses?

Regards, Alexander Toresson