258 CHAPTER 12History of Life on Earth
Section 2 The Evolution of
Cellular Life
The Evolution of Prokaryotes
When did the first organisms form? To find out, scientists study the
best evidence of early life that we have, fossils. A is the pre-
served or mineralized remains (bone, tooth, or shell) or imprint of
an organism that lived long ago. The oldest known fossils, which
are microscopic fossils of prokaryotes, come from rock that is 2.5
billion years old.
Recall that prokaryotes are single-celled organisms that lack
internal membrane-bound organelles. Among the first prokaryotes
to appear were marine cyanobacteria. (SIEan oh bak
TIR ee ah) are photosynthetic prokaryotes. Before cyanobacteria
appeared, oxygen gas was scarce on Earth. But as ancient cyanobac-
teria carried out photosynthesis, they released oxygen gas into
Earth’s oceans. After hundreds of millions of years, the oxygen pro-
duced by cyanobacteria began to escape into the air, as shown in
Figure 5. Over time, more oxygen was added to the air. Today oxygen
gas makes up 21 percent of the Earth’s atmosphere.
Two Groups of Prokaryotes
Early in the history of life, two different groups of prokaryotes
evolved—eubacteria (which are commonly called bacteria) and
archaebacteria. Living examples include Escherichia coli,a species of
eubacteria, and Sulfolobus,a group of archaebacteria.
are prokaryotes that contain a chemical called peptidoglycan(PEPtih
doh GLIE kan)in their cell walls. Eubacteria include many bacteria
that cause disease and decay.
are prokaryotes that lack peptidoglycan in their
cell walls and have unique lipids in their cell membranes. Archae-
bacteria are thought to closely resemble the first prokaryotes to have
existed on Earth. Chemical evidence indicates that the first eukary-
otic cells were more likely to have evolved from archaebacteria than
from eubacteria.
Archaebacteria
Eubacteria
Cyanobacteria
fossil
Objectives
Distinguishbetween
the two groups of
prokaryotes.
Describethe evolution of
eukaryotes.
Recognizean evolutionary
advance first seen
in protists.
Summarizehow mass
extinctions have affected the
evolution of life on Earth.
Key Terms
fossil
cyanobacteria
eubacteria
archaebacteria
endosymbiosis
protist
extinction
mass extinction
Figure 5 Evolutionary
timeline.This timeline shows
some of the major events that
occurred during the evolution
of life on Earth.
PRECAMBRIAN ERA
Age(in Millions of Years Ago)
Earliest fossil bacteria Origin of O 2 by photosynthesis
3,500•••••2,500
4A 8B 8C
4A
4A 7B
7B