The reason that the brains of so many animals look so similar is not so much because of the similarities of their genetics, but the result of the way nerve cells follow a particular language of logic that grows these brains in similar ways using that logic to make sense of the different information presented by different types of bodies. He managed to trace the signaling pathways used by the chemical. The chemical does not use pathways. The chemical is one of the markers in the brain. The amount of that chemical in a synapse has an effect on the firings of the cell receiving this chemical.
Different cells use different amounts of all chemicals.
Different cells make different decisions. Such a statement is a leap to conclusion. Yes, drugs do influence the way the brain works, but the brain doesn't necessarily use drugs to think. The drugs in the brain are the medium of the process that is involved, but the drugs are not that process. His studies in the mid-1950s at the University of Lund laid the groundwork for the discovery of dopamine depletion in Parkinson's and subsequent treatment with levadopa, which is still the principal drug used by doctors to delay the disease's devastating advance.