The digestive system is extremely complex. For it to evolve in a mere 2.5 million years is simply not possible. The above is an unsupported rationalization. Consider that the human brain is far more complex than the digestive system, and that the physiology of the entire body is regulated via the autonomic nervous system, which is controlled by the brain. Thus, the bizarre claim that the physiology of the human digestive system could not have evolved during the period is easily seen as the ludicrous fantasy that it is.
Expert opinion is that physiological evolution is highly unlikely.
Any mutation that changes the shape of such molecules will almost always be at a disadvantage as it reduces their ability to interact with each other or with their substrate. Stabilising selection of this kind can be a very conservative force. Stabilising selection is a powerful agent that leads to genetic homogeneity. To explain how genetic variation is maintained in the face of such widespread censorship by natural selection is one of the main problems of population genetics. The last sentence in the expert opinion quoted indicates, of course, that physiological evolution happens anyway.