When did the immortality of the soul start?

Was there ever a period in man's history when his soul was not an immortal entity?

We believe the soul to be co-existent with God, and, therefore, eternal. We believe it has always existed as a distinct entity, and we believe it will always continue to have an existence, but that it will perpetually change its form of manifestations, so that while you recognise it by its external expressions, you will be apt to consider that it has changed states―has lost its priority, but it is not so. It is the same yesterday, today, and forever.

―Hosea Ballou in Spirit