If Earth's entire 4.6-billion-year history were compressed into a single 24-hour day, when would modern humans first appear?
AAround noon — halfway through Earth's history
BAround 6:00 PM — in the last quarter of the day
CAround 11:00 PM — in the last hour
DLess than one second before midnight
Modern humans have existed for roughly 300,000 years. On a 24-hour scale representing 4.6 billion years, that is about 0.006% of the total — less than one second before midnight. This dramatically illustrates how recent humans are in Earth's story.
Question 2 True / False
In undisturbed sedimentary rock layers, the oldest layers are at the bottom and the youngest are at the top.
TTrue
FFalse
Answer: True
This is the law of superposition — one of the fundamental principles of geology. Each new layer of sediment is deposited on top of older layers, so in undisturbed sequences, deeper means older.
Question 3 Short Answer
Why do geologists divide Earth's history into different eras and periods instead of just using years?
Think about your answer, then reveal below.
Model answer: The time spans are so enormous that raw numbers of years are hard to grasp and work with. Dividing history into named eras and periods (like the Mesozoic Era or the Jurassic Period) based on major events like mass extinctions and the appearance of new life forms makes it easier to discuss and organize Earth's history.
Named time divisions also reflect real changes in life and geology. The boundary between the Mesozoic and Cenozoic, for example, marks the mass extinction that killed the dinosaurs — a genuine turning point in Earth's history, not an arbitrary line.