Monotremes are a unique group of mammals that lay eggs rather than giving birth to live young. Synonyms for the word monotremes include egg-laying mammals, prototherians, or monotremata. The term prototheria comes from the ancient Greek word for "first beast", as monotremes are considered to be the most primitive mammalian group, dating back to the dinosaur age. Other synonyms for monotremes include platypus and echidnas, which are the only two living species of this group. Although monotremes are not well-known, they are important for understanding the evolution of mammals and the diversity of life on Earth.