The problem of free will reads:
How to explain the subjective experience of free will (first person’s stance) by a scientific theory, dealing with objective concepts (third person’s stance).
Due to our subjective experience we do not need further arguments “for humans having free will” (first person's stance). What we need, is a scientific explanation for the capability of free will (third person's stance).
Presently, all scientific theories dealing with objects on the scale of neural assemblies deal with deterministic heuristics. The approaches by Eccles, Penrose and his collaborators, employing certain mechanisms on the quantum level, do not convince the maiority of neuroscientists.
As you write, we do not ascribe free will to very basic creatures. On the other end, in accordance with the general relationship of species the most simple hypothesis ascribes free will to animals from species that are very similar to the human species.
The capability of free will requires certain organs like a nerve system with a brain. The brain must be able to create an internal model of the environment of the animal. It must facilitate to simulate different behaviours as reaction to a given situation. In addition, the brain must have the capability to evaluate different alternatives and to decide for one of them.
In the end of your blog you pose three questions explicitly:
Ad 1: Every explanation of mental capabilites should not restrict itself to the question “What is?” Always it should also consider the question “How did the capability evolve?” That has been done for the explanation of mental capabilites in general.
But I do not know whether an evolutionary consideration of free will has already created any important result. For more details one can search e.g. for the keyword “Evolutionary Psychology” and consider Walter, Henrik: Neurophilosophy of Free Will: From Libertarian Illusions to a Concept of Natural Autonomy (2009)
Ad 2 and 3): Libertarians hold three views: a) We make free decisions, we could have decided also differently. b) We can give reasons, why we act as we do. c) We ourselves are the originator of our actions. A physicalist searches for an explanation within a scientific theory.
A compatibilist holds that free will is compatible with determinism.
I do not see any reason why the theory of evolution - a scientific theory based on determinism in general and on deterministic chaos in particular - does sharpen the problem of free will, as stated above.
The capabilities concerning affect, behaviour and cognition have developed during the evolution of animals. The development of these capabilites is never a binary, no-yes process in the sense: This species lacks the capability, but the next evolving species has it. Evolution is a continuous process which develops capabilities from small beginnings.
All this holds also for the capability of free will. Hence I do not consider evolution an objection against the capability of free will, as you write “Proponents of freewill would need to provide an explanation of why some classes of living creatures have it and others don't.“
Added: I learned that Goschke has published several papers on a neurobiological model of volition. For him the viewpoint of evolution is a natural presupposition, see