Evolution ❮ 147stasis (no change) (see comparison in Figure 12.3). Because the fossil record is incomplete,
it is very hard to test the two theories—if we find no fossils for a species over a con-
tested period, how can we determine whether change was occurring? The debate there-
fore continues.How Life Probably Emerged
The AP Biology exam often includes questions on how life originated. It is therefore
wise to learn the steps of the heterotroph theory(Figure 12.4), so named because it
posits that the first organisms were heterotrophs,organisms that cannot make their
own food.Volcanoes released NH (^3) UV rays Simple organic molecules
gases to the CH 4 Gases condense (e.g., amino acids, fatty acids,
atmosphere. H
H
2 O to form seas. glycerol, monosaccharides).
2 Lightning
Dehydration
reactions
Cell precursors known as
protobionts formed from
organic molecules.
Autotrophic prokaryotes Heterotrophic prokaryotes
appeared when mutant appeared, with the first
heterotroph gained ability cells existing as anaerobes.
to produce its own food— Buildup of CO 2
from anaerobic resp.
photosynthesis! sets up photosynthesis
O 2 and ozone layer appeared
thanks to photosynthetic output.
Symbiotic relationships Natural selection
of prokaryotes led to the Life continues to
creation of eukaryotes— evolve today.
endosymbiotic theory.
Figure 12.4 Flowchart representation of heterotroph theory.
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