From ancient times until today, the art and science of yoga has been handed down from guru (teacher) to disciple (in Sanskrit shishya or chela). Disciples in turn hand down what they have learned to their disciples. Every guru is also simultaneously a disciple not only of their immediate guru, but also of all the preceding gurus in their line of teachers. In this way, the yoga/Hindu Vaishnava culture has been handed down from time immemorial.
Acceptance of a guru or teacher is not undertaken lightly by the serious yoga aspirant. The serious seeker of absolute truth seeks out a guru or yoga teacher and considers what the guru is teaching. They check what the guru is teaching with the Lord in their heart, with yoga scriptures, and with what previous yoga teachers have taught. The bona fide guru will themselves have been a disciple of a guru. Additionally, the serious aspirant looks to see whether the students and disciples of the guru are manifesting the fruits of yoga knowledge. In other words, the yoga aspirant should not blindly accept anyone as guru; rather they must seek confirmation that the guru is in fact bona fide.
In the ancient yoga culture and tradition, the guru-disciple relationship is considered to be natural and necessary; similarly a person without a guru is considered to be like a boat without a rudder.