How to jump back to the last position of the cursor in emacs?

I believe what you are searching for is pop-global-mark, usually bound to C-x C-@ and C-x C-SPC.

The global mark ring is updated automatically on many occasions such as buffer switching, accidental or otherwise.

You can use the command previous-buffer which in my version (GNU Emacs 23.2.1) is bound to C-x left-arrow by default.

Just type Meta-x describe-key C-x left for further information

There is also winner-mode that binds C-c left arrow to winner-undo which in short allows you to undo any change done to your windows.

In spacemacs C-o is bound to evil-jump-backward.