AP BIOLOGYHistory of Life on Earth—How Life Probably Emerged

Step 1: The earth’s atmosphere formed Emerging from

volcanoes, gases such as NH3, CH4, H2O vapor, and H2 (but not Oxygen) invaded the atmosphere of the early Earth

Step 2: The seas formed

The gases condensed to form the seas as the earth cooled

Step 3: Simple organic molecules appeared Energy (from Ultraviolet light, lightning,

heat, and radioactivity) transformed inorganic molecules to organic ones, including amino acids.

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

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 CO2 from fermentation allowed for plenty of CO2 to be available for photosynthesis later.

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

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

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

Step 9: Life evolved

Natural selection produced the variety of organisms that have existed throughout the earth’s history