Which gas in Earth's atmosphere tells us that there is life on Earth?

Related questions

1. Is the ISS in space?
2. What are three ways that Earth's atmosphere is helpful?
3. How do greenhouse gases cause global warming?
4. Where do CFCs come from?
5. Why is the ozone layer important?
6. How does Earth's atmosphere protect life on Earth?
7. Are gravitons real?
8. Why is atmosphere so important for us?
9. Where did Earth's water originate?
10. Did Mars ever have a magnetic field?

Tags

1. #layer
2. #retained
3. #amounts
4. #trace
5. #surrounding
6. #carbon
7. #vapor
8. #earth
9. #contains
10. #oxygen
11. #water
12. #gases
13. #dioxide
14. #atmosphere
15. #nitrogen
16. #other
17. #roughly
18. #planet
19. #argon
20. #gravity