GREP Find/Change - how to change mixed uppercase to lowercase?

I'm doing some final proofreading and correcting some text in a book I've helped design.

I've had some success using GREP Find/Change.  But I can't get it to work in the following case.

What I want to do is find all instances of 'Nature' and change them to 'nature'. (Simple uppercase to lowercase substitution.)

BUT I want to exclude any instances of 'Mother Nature'. (Keep uppercase 'N' in Nature.)

According to Adobe help, I ought to be able to use a Negative Lookbehind expression.  I've tried typing in the Find box... Nature(?<!Mother )

But it finds everything... 'nature', 'Nature', 'Mother Nature', which isn't much help.

Can somebody please tell me where my mistake is and how to correct it?