1. densely populated.
2. ran out.
3. dates back.
4. impact.
5. prehistoric...tool.
6. climate change...collapse.
7. civilisation.
8. hand in.
9. do without.
10. lessened.
11. challenges.
12. maintains.
13. rely on.
14. average.
15. separate.
16. miniature.
17. invented.
18. archeology.