In recent years in my own development, I have been concentrating more on the core teachings of yoga and the history of it's development, based upon available writings and archeological evidence.
From my studies THE foundational text on yoga - probably the most available and oldest surviving writing - is THE YOGA SUTRAS OF PATANJALI. This document seems to be dated to some time between 500 BC to 500 AD; various dates are given by various sources. In this text, it seems (to me) that the core philosophy and the path of yoga, such as the eight limbs (steps or stages) of yoga, are presented in a rather scientific manner by a well-evolved soul.
Next in line, to my knowledge, in terms of core and widely available writings on yoga, were the HATHA YOGA PRADIPIKA, GHERANDA SAMHITA, and THE SHIVA SAMHITA, in that order it seems. These are dated roughly to the 15th to the 18th Centuries AD, at least a thousand years after the YOGA SUTRAS.
Does anyone have any other works to fill in the line of development in yoga from textural evidence?
Particularly, it would be interesting to know of other surviving works or archeological artifacts that might predate the HATHA YOGA PRADIPIKA and YOGA SUTRAS. Samkhya philosophy, the Upanishads, and Adi Shankaracharya's vedantic ideas definitely deserve to be mentioned.