Get top parent category

My category structure is as follows:

- Top Category
---- Sub Category 1
------- Sub Sub Category 1.1
------- Sub Sub Category 1.2
------- Sub Sub Category 1.3
---- Sub Category 2
------- Sub Sub Category 2.1
------- Sub Sub Category 2.2
------- Sub Sub Category 2.3

I’m on a post under 1.2 so it would be:

Top Category -> Sub Category 1 -> Sub Sub Category 1.2 -> Current Post

NB: In the post ONLY “Sub Category 1” and “Sub Sub Category 1.2” are selected as categories (“Top Category” is not checked).

Now, how do I get get the slug of the Top Category (“top-category”), navigating backward?

Thanks!

get_ancestors() returns an array containing the parents of any given object.

This example has two categories. The parent with the id of 447 and the child with a id of 448 and returns the a category hierarchy (with IDs):

get_ancestors( 448, 'category' ); 

returns:

Array
(
    [0] => 447
) 

get_ancestors Codex Page