Origins & History of Life

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Across
  1. 2. period in which first birds appear
  2. 4. period in which first flowering plants appear
  3. 8. the number of mass extinctions indicated by the fossil record
  4. 10. the first organisms on the planet; consist of very simple cells with no complex organelles or nucleus
  5. 13. era in which life first formed
  6. 15. period in which dinosaurs arose and the first small mammals appeared
  7. 16. gas that was absent from the atmosphere when Earth formed
  8. 17. the first vertebrates on land
  9. 19. period in which first land animals appeared
Down
  1. 1. organisms that can produce their own food
  2. 3. the first organisms probably belonged to this domain
  3. 5. period in which the petroleum and coal deposits we depend on were produced
  4. 6. primates that walk upright
  5. 7. the current era (age of mammals)
  6. 9. the first organisms (probably), that had to take nutrition from other organic sources
  7. 11. small arboreal mammals with forward-facing eyes that appear in the cenozoic
  8. 12. period in which shelled invertebrates inhabited the ocean
  9. 14. period in which first vertebrates appeared
  10. 18. period dominated by fish