Difficulty in establishing initial conditions for reseach

I have come across a paper titled: “TheValidity of Animal Experiments in Medical Research” in which the author Gill Langley points out another interesting limitation of animal experiments.  “There are other important discrepancies: the condition in otherwise healthy marmosets starts suddenly and improves variably over time, while in humans the onset of Parkinson’s disease is gradual, of unknown cause and there is no spontaneous recovery. The marmoset model is simplistic compared to the human condition, involving a more limited number and type of brain cells. And marmosets do not develop the pathological hallmark of Parkinson’s disease, the clumps of abnormal protein called Lewy bodies that develop in cells of the brain.”  Langley is citing the fact that not only are animals not suitable models for human medical research on account of their physiological and genetic differences from humans, but also because of the limitation associated with the scientific approach to the scientific method.  That limitation is the fact that when we are researching a human affliction about which we have limited knowledge, such as Parkinson’s disease, it is next to impossible to replicate completely and accurately in animals.  At this point in time, science still does not know what causes Parkinson’s disease, and so all researchers can do is cause an animal to have many of the same symptoms and hope it is the marmoset’s version of Parkinson’s.  At this point, it requires a leap of faith that scientists are actually treating marmoset Parkinson’s disease and not a simple infection, and in the field of science, faith alone is not good enough.