Chronologically, the early Samkhya philosophy arose just before the time of the Buddha, who died in 483 B.C.E. This was the time of pre-Classical Yoga. True Classical Yoga took form over the five hundred years or so after the Buddha. As now, very few people practiced this strict, severe form of yoga. While the ancient sages following this practice believed one could achieve true and final liberation only after leaving the body, there existed a belief within the Upanishads that living liberation was possible. This belief in living liberation  was quite attractive. After all, if there is only the Self, why should we have to get rid of anything else to recognize it?
At this time Buddhism was flourishing, but it was still steeped in the cultural biases of the day. Throughout the world at this time, the prehistoric matriarchal societies had died out. Civilization, technological advancements, and structured political hierarchies were all patriarchal. Women had few or no rights, and very little respect. Even the Buddha refused to allow his stepmother to join his sangha once her husband, his father, died. Ananda (the Buddha’s cousin) pleaded with the Buddha, until the Buddha finally agreed, but only if his stepmother agreed to follow eight precepts that the men didn’t have to follow. Everywhere the feminine was rejected and repressed. 
Tantra arose in reaction to the denial of the body and the denial of the feminine. Why do we have to consider the body or the mind as an enemy? Why do we deny the feminine energies? There must be a way to accommodate all these factors that are so obviously present in life! In Jungian terms, the Indian sages of the early first millennium were the first yogis to recognize and accept the anima: the female part of the soul. It would take European culture a thousand years to come to the same realization. 
Tantra arose out of the seeds of Samkhya, Classical Yoga, and Buddhism but it quickly surpassed all of these philosophies. Where the earlier schools were patriarchal and either dualistic or atheistic, Tantra embraced the feminine, the principle of unity, and offered a way for anyone to practice. Tantra ignored class structure, and as a result had many practitioners from the lower castes. This egalitarianism was unique and energizing.
- — Also known as jivan-mukti.
- — See Karen Armstrong’s book Buddha for a clearly written historical biography of the Buddha. On page 139 she lists the eight conditions the Buddha gave in order for his stepmother, or any female, to join the order: a nun must always stand in the presence of a monk; nuns must always spend retreats with monks (never by themselves); they must receive instructions every two weeks from a monk; nuns must not hold their own ceremonies; nuns must do penance in front of a monk for any offenses; a nun must be ordained by both a nun and a monk; she must never criticize a monk – however, she could be rebuked by a monk; and finally, a nun must not preach to monks. The Buddha only reluctantly allowed his stepmother, and other women, into the sangha, warning that, because he did this, his teachings would now last only five hundred years. Women, he considered, would “fall like mildew on a field of rice,” destroying the order.
- — It wasn’t until the eleventh century that the patriarchal imperative of the Catholic Church began to allow any hint of the feminine. Like the Buddha, the Church never fully embraced women in their hierarchy.