148 ❯ STEP 4. Review the Knowledge You Need to Score High
STEP DESCRIPTION
The earth’s atmosphere formed. Emerging from volcanoes, gases such as NH 3 , CH 4 ,
H 2 O(g), and H 2 (but not oxygen) invaded the
atmosphere.
The seas formed. The gases condensed to form the seas as the earth
cooled.
Simple organic molecules appeared. Energy (from UV light, lightning, heat, radioactivity)
transformed inorganic molecules to organic ones,
including amino acids.
Polymers and self-replicating These may have been formed through dehydration,
molecules appeared. or the removal of water molecules; e.g.,
proteinoids can be produced from polypeptides
by dehydrating amino acids with heat.
Protobionts appeared. These are cell precursors formed from organic
molecules; they are unable to reproduce, but can
carry out chemical reactions and have permeable
membranes.
Heterotrophic prokaryotes appeared. Heterotrophs consume organic substances to survive
(an example is pathogenic bacteria); since there
was a limited amount of organic material,
heterotrophs competed and natural selection
occurred—these first cells were anaerobic; thus,
the buildup of CO 2 from fermentation allowed
for plenty of CO 2 to be available for
photosynthesis.
Autotrophic prokaryotes appeared. A heterotroph mutated and gained the ability to
produce its own food using light energy, making
it a photo autotroph (e.g., photosynthetic
bacteria); this was a highly successful strategy
compared to the heterotroph’s.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.
Eukaryotes appeared (specifically Endosymbiotic theoryproposes that groups of
mitochondria types and prokaryotes associated in symbiotic relationships
chloroplasttypes). to form eukaryotes (the various organelles in cells
today invaded a cell and eventually became one
organism).
Life evolved. Natural selection produced the variety of organisms
that have existed throughout the earth’s history.