To understand the atman or the soul of a human one needs to consider consciousness. In doing so we ask is not the atom the smallest functional unit of matter and has electrons that move according to scientific theory that forms the basis of chemical and biological reactions? Further do not these electrons reside in different orbitals around the nucleus of protons, only two electrons (maximum) in the first orbital, and eight electrons (maximum) in the next orbitals? That is scientific theory which makes atoms conscious, so all matter including rocks are conscious. We then ask what is consciousness composed of? It is a kind of energy, something that gives it movement (life force) and something that gives it direction (soul force).
Evolution of matter into organisms, such as protozoa, plants, lower animals and higher animals like us is therefore nothing more than the evolution of the consciousness through the direction of the soul force of atoms. It is this soul energy component that develop body parts into nerve tissue in the brains of higher animals and eventually gives us humans thoughts. The soul force of consciousness energy is what is differentiated into the three gunas, of sattvic, rajasic and tamasic nature, so that we humans acquire these attributes. Once we have individually acquired these attributes we have a character, and we call that our soul, namely the heart of our individual selves. It is our atma.
Every individual therefore has a different soul, depending on the guna consciousness energy that he or she is accessing. And this soul dies by dissolving into its constituent atoms when we stop living and are buried or cremated. The soul is not something that goes into the atmosphere and then waits for another human being or an animal into which it deserves to go for its next birth. That would be impossible for even God to implement, and a ridiculously complicated way to run Nature. God is not like that. He just advices people from time to time that He is in control of humanity if things do not go well on this planet.