The term "soul" as used in the OP assumes the concept of duality. I.e. a 1 to 1 relationship between a physical body and something imaterial that it depends upon somehow, e.g. a symbiotic relationship, or perhaps a dependent parasite.
a body only requires a soul if there is an expectation for it to have consciousness.
a soul only requires a (material) body if they have a desire that requires expression in the conditioned world (for instance, to be a gross materialist, there is a very real need for a soul to have a material body)
The idea is that this soul is the entity that represents the essense of "I" and is considered immortal. The question then becomes when do these two entities come together to make the whole.
at conception of course
The Christians believe it occurs at conception and the Muslims believe it is some weeks later. Curiously this is why Bush tried to outlaw stemcell research and limit science while the perceived backwardness of Islam embraced such research.
Suggesting that its only humans who have souls poses problems (which find their way into issues of application and so on)
But before we can truly address the OP we need to understand what this "soul" thing is and how it relates to the physical. For example LG maintains that every living cell requires a "soul" to keep it alive. In which case a "soul" never enters a person but is an integral part of life, e.g. something alive cannot occur without this essential component.
Its not my idea.
Its an idea prominent throughout the Vedas
In Leo's case, soul is more like "The Force" in the Star Wars movies, and somehow our physical body's have some type of networking connection with this universal entity. The concept of a soul entering the body again makes no sense in this case.
This is also an idea present in the vedas. Sometimes it is referred to as "brahman". Usually such advocates argue that the self has no fundamental substance and that it is all an illusion which finds its perfection when individuality ceases to exist. Basically the idea was re-introduced in post- buddhist india (by Sankaracharya - more commonly known as Mayavadi philosophy) so it shares a few similarities.
In short, it requires non-existence (or "abhava") to come to the table in order to be taken seriously.
Needless to say, there are numerous criticisms of such a take.
So once we have established the ideas the next step should be justifications and evidence, which is somewhat where we are in the topic.
which means you have to address the normative boundaries of the question - IOW "how one has to
be" in order to know it.
For instance if one advocates that the standard tools of empiricism are sufficient (ie the senses), it begs the question how the senses could ever hope to see what they are seeing with.