In classic thinking there is the body, soul and spirit. The soul, by itself is classically considered inanimate. The soul is animated by the spirit.
Our static soul, which makes us unique, appears to be connected to our memory. It is our unique memories that would allow us to know we are who we are, and what brought us to this point in time. The soul or memory begins when brain waves appear. The soul begin by making memories of feedback from the internal control systems of the growing body.
Our DNA is also a type of soul; animal soul. Our DNA has all the memories needed to sculpture an entire physical and material you. However, by itself the DNA will not do anything. It is a static soul.
The DNA needs to be acted upon and animated by interaction with the external environment and the internal protein grid environment. When two daughter cells separate, both contain the same DNA, plus very similar protein grids, that are carried forward to help animate the soul of the DNA. The DNA is like a hard drive that needs the CPU to make it useful. The protein grid is like the CPU.
In the case of female gamete cells, preparing for fertilization, when 3/4 of the doubled DNA is extruded from that cell, this DNA is forever static memory, or soul, since it lacks the needed protein grid to animate it. It is like a backup copy of the DNA soul.
The elaborate protein grid within the ovum proper is the spirit of the CPU. When a sperm enters, and this adds the other half of the DNA; fully functional human animal soul, the combined DNA soul or memory becomes animated; genes are shuffled to reflect the grid.
Humans have two centers of consciousness; inner self and the ego. The inner self which is common to all animals is connected to our DNA. The inner self is the center consciousness connected to our species as a human; human nature. The inner self is like the spirit that animates neural memory soul, which can then be used to animate sub-memory within organs; spirit, soul and body.
The second center of consciousness is the ego, which is much newer in the time scale of evolution. It appears to have formed about the time of the rise of civilization. The ego is who we consciously know as ourself. The ego is also spirit; CPU, in the sense that through will and choice, it directs us and our sensory organs into life to interact and perceive, which impacts our stored memories; our soul.
The eternal soul is connected to our DNA, in the sense that it has memories from the most remote past. This soul has a connection to the inner self, since the inner self can animate the divine soul; primal urges.
In terms of when the soul enters the body, the answer depends on which soul; animal or human, species dependent or uniquely you. There are many lawyers of soul and spirit in the body.