1. The first cells probably arose when ______________________ formed around aggregates of organic compounds.

2. _______________ are modern bacteria and ancestors of mitochondria & chloroplasts.

3. In the _____________________ period (600 to 500 million years ago) most major groups of animals evolved.

4. ______________ are ancestors of eukaryotes.

5. Scientists think that life began on earth about _____________________ years ago.

6. _____________________ showed that organic compounds can be formed from inorganic compounds.

7. About 3 billion years ago, cyanobacteria evolved and released _____________________ into the oceans and air.

8. About _____________________ years ago, the first eukaryotes (protists) evolved from bacteria.  

9. _____________________ developed into more complex, multicellular life forms, such as plants, animals, and fungi.

10. About 400 million years ago the _____________________ developed;  this allowed organisms to live on land.

11. _____________________ (especially insects) were the first animals to thrive on land.

12. _____________________ are organisms that have a vertebral column or backbone.

13. The first vertebrates were _____________________.

14. In the history of the earth, there have been _____________________ mass extinctions.

15. Simpler vertebrates like fish, amphibians, and reptiles are _____________________ (cold-blooded).

16. More complex vertebrates like birds and mammals are _____________________ (warm-blooded).
 

List 3 of the 7 groups of vertebrates.

   1. _________________________ 2. _________________________ 3. _________________________

ESSAY: Describe 3 ways that life may have begun on earth.