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 fossil evidence, about how long ago did the first single-celled life form appear on Earth?
A: 130 million years ago
B: 1. 5 billion years ago
C: 2. 5 billion years ago
D: 4. 1 billion years ago
