Essentials of Ecology

CONCEPTS 4-2A AND 4-2B 81

The Fossil Record Tells Much

of the Story of Evolution

Most of what we know of the earth’s life history comes
from fossils: mineralized or petrified replicas of skel-

etons, bones, teeth, shells, leaves, and seeds, or impres-
sions of such items found in rocks. Also, scientists drill

cores from glacial ice at the earth’s poles and on moun-

taintops and examine the kinds of life found at differ-
ent layers. Fossils provide physical evidence of ancient

organisms and reveal what their internal structures
looked like (Figure 4-4, p. 82).

The world’s cumulative body of fossils found is

called the fossil record. This record is uneven and incom-
plete. Some forms of life left no fossils, and some fos-

sils have decomposed. The fossils found so far probably

represent only 1% of all species that have ever lived.

Trying to reconstruct the development of life with

so little evidence—a challenging scientific detective

game—is the work of paleontologists. GREEN CAREER:

Paleontologist

The Genetic Makeup

of a Population Can Change

The process of biological evolution by natural selec-

tion involves changes in a population’s genetic makeup

through successive generations. Note that populations—

not individuals—evolve by becoming genetically different.

Cenozoic

Mesozoic

Paleozoic

Millions of years ago

Precambrian

Origin of Earth

Earth cool enough

for crust to solidify

Oldest prokaryotic

fossils

Accumulation of

O 2 in atmosphere

from photosynthetic

cyanobacterium

Oldest

eukaryotic fossils

Origin of

multicellular

organisms

Plants

colonize land

Extinction of dinosaurs

First humans

Eubacteria Archaebacteria Protists Plants Fungi Animals

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

Prokaryotes Eukaryotes

Figure 4-3 Overview of the evolution of life on the earth into six major kingdoms of species as a result of natural selection.
For more details, see p. S46 in Supplement 7.