What contributes to the variation of protein sequence across different species?
I have found an acceptable answer for my question. Just post here if any interest in it. Pardon my English.

Source:

Nei et. al. 2010. The Neutral Theory of Molecular Evolution in the Genomic Era. Annu. Rev. Genom. Human Genet.

Evolution and point mutation cause the sequence difference of the same functional protein across species. Neutral theory of molecular evolution shows that speciation level of evolution is an accumulation of molecular level of neutral evolution. In neutral evolution, the mutation only occur at a scope of point mutation in sequence in the manner that the function of the protein is retained. Overtime, speciation occurs yet the function carried out by that specific protein is still necessary and is conserved across species. Difference in protein sequence of the same functional protein is the residuals of random point mutation.