GCG and type 2 diabetes mellitus: A focused review of the literature selecting information from these threefields of knowledge in databases: Cochrane Library, Medline and SciELO,articles and book chapters in English and Portuguese, between 1955 and 2019,using the headings “GIP, GLP-1, PYY, type 2 diabetes, vertebrates digestivesystem, hominid evolution, obesity, bariatric surgery “.