Biology (Holt)

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