Study the timeline. A timeline. 4.5 billion years ago, Earth is formed. 4.3 billion years ago, Earliest water. 4.1 billion years ago, Single celled life. 3.5 billion years ago, Earliest oxygen and photosynthesis. 2.5 billion years ago, Complex single celled life. 1.5 billion years ago, Multi cellular life. 500 million years ago, Land life. 300 million years ago, Dinosaurs. 200 million years ago, Mammals. 130 million years ago, Flowers. 5 million years ago, Humans. Based on the timeline, about how many years ago could the oldest mammalian organism have lived? 5 million years 200 million years 1.5 billion years 4.5 billion years