A time will come in the physical evolution of the earth — it will be after the year 5,700 — when, if he fulfills his rightful evolution, man will no longer tread the earth by incarnating in bodies derived from physical parents. In that epoch, women will be barren; children will no longer be born in the manner of to-day, if evolution on the earth takes its normal course.
There must be no misunderstanding about such a fact as this. Something else, for example, might come about. The Ahrimanic Powers, which under the influence of the impulses working in men to-day are becoming extremely strong, might succeed in preventing earth-evolution in a certain respect. It would then become possible for men — by no means for their good — to be held in the same form of physical life beyond this time in the sixth or seventh millennium. They would become much more like animals, while continuing to be held in the grip of physical incarnation. One of the endeavours of the Ahrimanic Powers is to keep humanity fettered too long to the earth in order to divert it from its normal evolution.
Source: Rudolf Steiner – GA 196 – Some Conditions for Understanding Supersensible Experiences – Dornach, January 18, 1920