This begs the question, what does the phrase "their present time period mean?" If a soul is not incarnated and can visit any time period in soul form, it seems to me that means a soul can also incarnate in any time period yet only in one time period/physical life at a time. I long ago decided my understanding has to be valid because I know this is my 17,043rd human incarnation and unless science is way way way off on the time that homo sapiens sapiens have walked the earth, my explanation seems very close to correct. Even if my guides meant to include Neandethals in the definition of human, 17,043 lives and times periods still don't make sense together.