There is a lot of truth in the simple statement that, if other animals are not similar to us, experimenting on them for human biomedical research is useless and, if they are similar enough to us, then experimenting on them is morally wrong.
This has been shown even more clearly by contemporary evolutionary biology, with its principle of phylogenetic compromise. It means that, when evolution causes a change to occur in a particular structure or process of an organism, other changes will inevitably follow as a compromise to adjust the same organism's other structures or processes that interact with the altered one.
This is a likely origin of biomedically important differences among members of various species. Of course individuals of every species are made from the same biochemical building blocks (DNA, proteins and so on), but members of separate species that have to adapt to different environments will be subject to two basic distinct sources of differentiation: a primary one to adapt to their niche and a secondary one to adapt other parts of their organism to that change. Therefore natural selection has many indirect, as well as direct, consequences for species differentiation, in a ripple effect in evolutionary time.
So, for example, human beings cannot possibly have developed erect standing and larger brains, two of their most distinct characteristics, without having also evolved a series of other changes that would affect other parts of their system.
In this light, the objection often heard from the apologists of animal experimentation that in vitro testing on cell and tissue cultures cannot replace animals because you need a whole organism due to the importance of the correlation among the various parts of the system (the "Intact System Argument") becomes ironically and involuntarily an objection against animal experimentation itself, because in it you have the wrong organism, so the correlation among parts will also be wrong, since the system whose parts are correlated is not the same system we ultimately want to study, which is that of the human species.
The Italian author Dacia Maraini, in her autobiographical book Bagheria (p. 157), recounting how she and other captives were desperate for food while kept in a Japanese concentration camp during the war, writes:
"We even tried to eat some acorns ('If pigs can eat them, why can't we digest them too?'). Instead they turned out to be absolutely inedible."
This reveals the common, widespread assumption about species difference (or rather similarity) in this particular aspect - metabolism - and others. Such an assumption is the core of the idea, which is more implicit than explicitly articulated in a theory, behind animal experimentation. "If pigs can eat them, why can't we digest them too?": it's all in that phrase.
Using animals as guinea pigs may seem intuitive at first.
But science often is not intuitive, it must go well beyond common sense. Think of how intuitive it is to believe that the sun moves and the earth is still. The more we know, the more science progresses, the further away we go from intuitive, commonsensical ideas. For example one of the most advanced sciences we have is modern physics whose quantum mechanics is possibly the most anti-common-sense theory that exists.
In this animal experiments opponents like Ray and Jean Greek have taken the correct approach to the issue.
We can draw an analogy between how our better understanding of the universe and the solar system, caused by the progress in astronomy, has replaced our intuitive notion (the sun moves around the earth) with a counter-intuitive one (the earth orbits around the sun) and how our better understanding of living beings, caused by the progress of various biological disciplines so well described by Ray and Jean Greek's book Sacred Cows and Golden Geese, has made us more aware that our intuitive notion of a fundamental similarity in physical structure (anatomy) and function (physiology) between our species and other animal species equally needs to be replaced by a more counter-intuitive notion of how physically dissimilar all animal species, including ours, are, due to the importance of microscopic and even sometimes non-observable differences among them which were not known before and have been recently and constantly discovered.