Detecting which user changed content

I currently have a script that emails me every time content on the site changes, with:
$post_id = $post->ID;
$post_type = get_post_type($post_id);
$post_status = $post->post_status

What I’d like to be able to grab is not the post author, but the user that made the change.

I just can’t find how to get that piece of information programmatically.

Thanks!