the FOXP2 protein, which has been shown to be involved in language, has only 2 out of about 700 amino acids which are different between chimpanzees and humans.
15 This means they are 99.7 percent identical. While this might seem like a trivial difference, consider exactly what those differences are. In the FOXP2 protein, humans have the amino acid asparagine instead of threonine at position 303 and then a serine that is in place of an asparagine at 325. Although apparently a minor alteration, the second change can make a significant difference in the way the protein functions and is regulated.
16 Thus, a very high degree of sequence similarity can be irrelevant if the amino acid that is different plays a crucial role. Indeed, many genetic defects are the result of a single change in an amino acid.
For example, sickle cell anemia results from a valine replacing glutamic acid in the hemoglobin protein. It does not matter that every other amino acid is exactly the same.