Many people and some religions believe that the souls of humans can be reincarnated into animals and then back again. According to The Spirits Book, the answer is a no. But there is a hint of a qualifier.
Here is what The Spirits Book tells us;
613. Can a spirit which has animated a human body be incarnated in an animal?
“No; for such an incarnation would be retrograde step; and a spirit never retrogrades. The river does not flow back to its source.”
Hence, we are told with no uncertainty that a human soul can’t transfer to a lesser form. Now, for the question can an animal, at some point, after a series of reincarnation, begin life in a human body. It is answered;
601. Do animals follow a law of progress like men?
“Yes; and it is for this reason that, in the higher worlds in which men are further advanced, the animals are more advanced also, and possess more developed means of communication. But they are always inferior to man, and subject to him; they are, for him, intelligent servitors.”
Therefore, animals progress via their own path and do not intersect with humans. So it would seem that there is no cross over at all between animals and us. But, the origin of a spirit is unknown, even to the spirits themselves. This hole in the knowledge of where a spirit is first created leaves the door open to some sort of progression from lower life forms.
Again in The Spirits Book, it is written;
“The starting-point of spirit is one of those questions which have reference to the origin of things, and to the secret design of God. It is not given to man to comprehend them completely, and he can only form, in regard to them, suppositions and theoretic systems, more or less probable…
It is thus, for example, that all spirits do not think alike in reference to the relations which exist between man and the animals. According to some, spirit only arrives at the human period after having been elaborated and individualized in the different degrees of the lower beings to creation, According to others, the spirit of man has always belonged to the human race, without passing through the ascensional degrees of the animal world.” [Allan Kardec, The Spirits Book, p. 336]
So we could have started off as an animal and after a long and laborious trial of eons, grow into the ranks of humans, or we could have first just entered the physical body of a primitive human and then started our path. The relationship between our souls and the souls of animals is unclear and we will have to be content with the fact, as Allan Kardec wrote, “as for the mysterious kinship which exist between man and the animals, that we repeat, is God’s secret.” [Allan Kardec, The Spirits Book, p. 337]
But there is a passage in the book, Between Heaven and Earth, by Francisco C. Xavier, inspired by the spirit Andre Luiz, that discusses reincarnation. In it Andre’s team leader, Clarencio, says:
“We tackle multiple experiences and we recapitulate them as many times as needed on our
grand journey to God. Pupae of intelligence in the obscurest sectors of nature evolve toward the realm of fragmentary intelligences inhabited by animals of a higher order, which, in turn, evolve toward the realm of human consciousness, just as human beings, little by little, evolve toward the glorious spheres of the angels.” [Between Heaven and Earth, 2011, p. 186]
One could read the passage above in two ways, one, we start out as a simple speck of rudimentary intelligence and evolve to a human level, by constant reincarnations into the worlds of planets. The other is that animals evolve in a higher state also, but along their own path and do not become a full human type spirit.
To learn more about the entire process of reincarnation and how the spirit world guides us, please read the book, The Case for Reincarnation – Your Path to Perfection.