"If we follow human evolution far, far back many millions of years, we are confronted by something quite different. The human being is a physical being on the Earth, but there was a time when the being that wandered on the Earth could not yet have taken up a human soul. This soul was on the astral plane. Still further back, we come to a time when it was on the spiritual plane, the devachan plane. In the future it will again climb up to this high level, when it has purified itself on the Earth. From the spirit through the astral to the physical and upward again to the spirit: this is the long evolution of the human being. And yet it seems like a short period when we compare it with the time involved in the evolution of the human being on Saturn and on the other planets. There the human being went through not only a physical transformation but spiritual and astral transformations also.
If one wishes to follow these transformations, one must go up into the spiritual worlds. There one perceives the music of the spheres, tones that flow through space in this spiritual world. When human beings live into this spiritual world again, the harmony of the spheres will sound toward them. In esotericism it is called the trumpet tones of the angels; hence, the trumpets in the third seal. Revelations coming out of the spiritual world are revealed only when the human being progresses ever further. At that point the book with seven seals will be revealed to us. These seals are precisely those we are examining here. They will give up their secrets. For this reason, the book is placed in the middle of the seal, and below it are the four phases of humanity. The four horses are nothing other than the evolutionary stages of humanity through the ages."
Source: Rocicrucianism Renewed, p. 90