The Earth is the planet rich in living beings and these are biologically classified into Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species. Mammal is one of the classes in Kingdom Animalia. Several studies have been conducted to visualize the evolution of mammals from their ancestors. Mammals have several descriptive characteristics which are their evolutionary trend, reproductive behavior, physical appearance, specific blood circulation system and their feeding behavior (refer to Figure 1 in Appendix 1). The first descriptive characteristic of mammals is evolution. “The characteristics of mammals evolved gradually over a period of 200 million years” (Miller & Harley, 2007, p. 359). The beginning of the Tertiary period is defined as the beginning of the “mammalian age” when the extinction of many reptilian ancestors occurred and caused the adaptive radiation of mammals. They evolved during the Triassic period when dinosaurs appeared. These were the first mammals. They were tiny shrew-like mammals with a delicate skeleton. According to Hickman, Robert, Keen, Larson, and Eisenhour (2011), the first placental mammals appeared in the early Cretaceous period and diversified rapidly. Most of the knowledge from early studies on the phylogeny of mammals depends on the analysis of their fossilized teeth and skull fragments. There are several changes in the structure of the sensory organs, bones, teeth and brain to become more adaptive to the dynamic environment. The second characteristic of mammals is reproductive behavior. Most of them have explicit mating seasons, so that birth and young occur at the most favorable time of the year (Villegas-Amtmann, Atkinson, & Costa, 2009). According to Hickman et al. (2011), all male mammals ca...... middle of paper ......l diversity. New York, NY: The McGraw-Hill Companies Inc. Hickman, C. P., Roberts, L. S., Keen, S. L., Eisenhour, D. J., Larson, A., & I' Anson, H. (2011). Integrated principles of zoology. New York, NY: The McGraw-Hill Companies Inc. Hickman, C. P., Roberts, L. S., Keen, S. L., Larson, A., & Eisenhour, D. J. (2011). Animal Diversity (6th ed.). New York, NY: The McGraw-Hill Companies Inc. Martin, R. E., Pine, R. H., & DeBlase, A. F. (2001). A manual of mammalogy: with the keys to the families of the world (3rd ed.). New York, NY: The McGraw-Hill Companies Inc. Miller, S. A., & Harley, J. P. (2007). Zoology (7th ed.). New York, NY: The McGraw-Hill Companies Inc. Villegas-Amtmann, S., Atkinson, S., & Costa, D. P. (2009). Low synchrony in the reproductive cycle of Galapagos sea lions revealed by seasonal progesterone concentration. Journal of mammalogy, 90(5), 1232-1237.