1. AP BIOLOGY History of Life on Earth—How Life Probably Emerged

2. Step 1: The earth’s atmosphere formed  Emerging from volcanoes, gases such as NH 3, CH 4, H 2 O vapor, and H2 (but not Oxygen) invaded the atmosphere of the early Earth

3. Step 2: The seas formed  The gases condensed to form the seas as the earth cooled

4. Step 3: Simple organic molecules appeared  Energy (from Ultraviolet light, lightning, heat, and radioactivity) transformed inorganic molecules to organic ones, including amino acids.

5. Step 4: Polymers and self-replicating molecules appeared  These may have formed through dehydration, or the removal of water molecules  Proteinoids (protein-like molecules) can be produced from polypeptides by dehydrating amino acids with heat

6. Step 5: Heterotrophic prokaryotes appeared  Heterotrophs consume organic substances to survive (Example: pathogenic bacteria); since there was a limited amount of organic material on the early Earth, heterotrophs competed and natural selection occurred—these first cells were anaerobic  The buildup of CO 2 from fermentation allowed for plenty of CO 2 to be available for photosynthesis later.

7. Step 6: Autotrophic prokaryotes appeared  A heterotroph mutated and gained the ability to produce its own food using light energy, making it an autotroph (e.g. photosynthetic bacteria); this was a highly successful strategy compared to a heterotroph

8. Step 7: Oxygen and the ozone layer appeared  Photosynthesis produces oxygen, which interacts with UV light to form the ozone layer—this production of oxygen allowed for aerobic respiration  The ozone layer blocks UV light from reaching the earth’s surface

9. Step 8: Eukaryotes appeared  Eukaryotes appeared, particularly those with mitochondria and chloroplasts  Endosymbiotic Theory proposes that groups of prokaryotes associated in symbiotic relationships to form eukaryotes

10. Step 9: Life evolved  Natural selection produced the variety of organisms that have existed throughout the earth’s history