"Diagram of the History of the Universe, compressed into a single year. The upper portion of the figure shows the calendar as one row from January to November. Events of special significance have been labeled. Starting at far left under January is labeled ""Big Bang occurs"". Continuing to the right, May is labeled ""Milky Way Galaxy forms"". Under September, ""Our Solar System forms, Life on Earth Begins"". Under October, ""Earth's atmosphere becomes oxygenated"". Finally, under November is ""First complex life forms appear"". The lower portion shows the entire month of December with significant events listed for certain dates. On December 19th ""Vertebrates appear"". Next, ""Land plants appear"" on Dec. 20th. On December 25th ""Dinosaurs appear"". ""Mammals appear"" on the 26th. On the 30th ""Dinosaurs become extinct"", and ""Humans appear"" on December 31st."