Here is a parsimonious scientifically justifiable model.
The Universe has developed from a state of singularity to one of great complexity as matter time and space were created at the big bang.
Strings, quarks, bosons, atoms, molecules, complex molecules, organic molecules have developed over billions of years.
Some of these complex molecules became self replicators, producing offspring that could develop new behaviors over many generations, and slime became sub cellular particles like prions and DNA, creating viruses and single celled organisms.
Single celled organisms started cooperating in massive arrays and became lichens and fungi and plants, first ferns and then sexual reproducers.
Plants became active and became lower animals, lower animals became higher lower animals, invertebrates and vertebrates, lizards and birds, then mammals.
Mammals became apes and other higher mammals.
Let us stop there- after 14 billion years there has been created a varied flora and fauna with nary a sign of free will- all behavior is determined.
The higher apes become chimpanzees, missing links, neanderthals and Homo sapiens. Even here there is an argument that Free Will is still obviously missing.
Homo Sapiens eventually becomes one of the only entities that is fully self conscious- not only a perceiver of the world but a perceiver of itself in the world.
Ah, it says, these internal states that I perceive are what cause my behavior, the amateur philosopher/scientist decides. I shall call that Free Will.
Despite that, nature continues its deterministic course with early (and later) mankind deluded about the nature of their nature and the real scientific explanation of the world.
Evolution wrought a complex decision maker, conscious and self conscious, capable of making behaviors that are not easily predictable, but determined all the same.
It is only when advanced Mankind with the tools of modern science and modern philosophy considers the nature of Free Will, that it becomes obvious that Free Will is a myth and that all behavior is determined despite what our perceptions and cognitions insist to be the case