Buddha Nature

I have been thinking of souls today.

At what point in the overall life of a being is the soul merged with the physical being we think of when we talk of a living person?  Is it at the moment of conception?  At the first breath of life? Sometime in between?  When the person is baptized?

Mothers have often told me that they could recognize personalities of their unborn babies by the way they are able to interact with the fetus.  Is a person's personality their soul?  Could it be that the two are separate and distinct?  Is it possible that a fetus is simply a potential life which develops characteristics like arms, legs, and personalities and that their soul is added at the moment God "breaths the breath of life into the being"?  That might be more compatible with the bible's wording, but is seems to slap me in the face of reasonable.

Assuming humans are not special and all living creatures are God's creation, then who is to say a dog does not have a soul?