Transcript AP Biology

AP BIOLOGY
History 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