Earth is very old, and it can be difficult to visualize where key events in the history of life occurred against this backdrop of "deep time." one common way to represent geologic time uses the analogy of a one-hour countdown timer. the timer starts from the top at earth's origin (4.6 billion years ago) and moves clockwise to the present, making one full circle. in this analogy, one hour represents 4.6 billion years, one minute represents roughly 76.67 million years, and one second represents roughly 1.28 million years. (open hint 1 for an explanation of how these calculations are done.)

1.0 billion years old
2.0 billion years old
2.3 billion years old
4.5 billion years old