Did the presentation of bhakti evolve or even its definition evolve over time?
Is this the way of presentation of Bhakti that evolved? Or did the definitions of Bhakti themselves evolve? Actually the principle of Bhakti of the heart's attraction towards Krishna is an eternal fact. But how it is revealed by the Lord through his various representatives that way.
Just like Krishna is eternally involved. Now Ramacharya, they may talk about Vishnu and Chaitanya Mahaprabhu are in Krishna's glory. This does not mean that Krishna was not supreme earlier.
Krishna was not supreme at the time of Ramacharya. Krishna became supreme at the time of Chaitanya Mahaprabhu. Krishna is the Krishna to Bhagwan of all things.
But that particular truth revealed later. So it is the revelation of the nature of Bhakti that develops. Bhakti itself remains eternally the same.
But how the Bhakti is understood and explained, that has evolved. Chaitanya Mahaprabhu has given us the culmination.