Explanation
Each number in this mnemonic means "5 × 10 to that power, in years ago," and every step is 10 times closer to today than the last. So 5x10^9 means 5 followed by 9 zeros: 5,000,000,000 (5 billion) years ago. As the little number on top (the exponent) counts down from 9 to 0, you move from the birth of the Earth all the way to today:
- 5x10^9 = 5,000,000,000 (5 billion) years ago: the birth of the Earth
- 5x10^8 = 500,000,000 (500 million) years ago: the first fish
- 5x10^7 = 50,000,000 (50 million) years ago: the first monkeys
- 5x10^6 = 5,000,000 (5 million) years ago: the first humans
- 5x10^5 = 500,000 years ago: the first babies with brains, getting much bigger after birth
- 5x10^4 = 50,000 years ago: the first language
- 5x10^3 = 5,000 years ago: the first writing, the first cities, the first armies
- 5x10^2 = 500 years ago: the first printed books, the hemispheres merge
- 5x10^1 = 50 years ago: the first commercial computers, everything escalates
- 5x10^0 = 5 years ago: the last 5 years, global warming & warring