The attitude of some of our earliest ancestors (6)
I believe the answer is:
stance
'the attitude' is the definition.
(stance is a kind of attitude)
'some of our earliest ancestors' is the wordplay.
'some of' indicates a hidden word.
'STANCE' is hidden in the letters of 'our earliest ancestors'.
'of' acts as a link.
(Other definitions for stance that I've seen before include "position when batting" , "driver preparing to strike" , "Position assumed by upright person" , "Position or attitude taken" , "Posture; point of view" .)