There are several methods to analyze the evolutionary history of life. For example, by studying fossils we can elucidate the evolutionary history of organisms. Comparative embryology has been providing information on history of life. Genes of extant organisms have also been used to study evolutionary history of organisms. The amino acid sequence of protein is encoded by the corresponding gene with the “alphabet” of DNA. It is possible to construct a phylogenetic tree based on the comparison of gene sequences of different organisms. The phylogenetic tree constructed in this way is called the molecular phylogenetic tree.