HUMAN BIOLOGY

444 Chapter 23

are broken, crushed, deformed, or swept away by erosion,

rock slides, and other geologic events.

Fossil-containing layers of sedimentary rock formed

long ago, when silt, volcanic ash, and other materials were

gradually deposited, one above the other (Figure 23.5). This

layering of sediments is called stratification. Although most

sedimentary layers form horizontally, earthquakes or other

geologic disturbances can tilt or break them.

The fossil record is spotty

We currently have fossils of about 250,000 species. How-

ever, judging from present diversity, there must have

been millions of ancient, now-extinct species. For several

reasons we will not be able to recover fossils for most of

them. Most of the important, large-scale movements in the

Earth’s crust have wiped out evidence from crucial periods

in the history of life. In addition, most members of ancient

communities simply have not been preserved. For example,

plenty of hard-shelled mollusks and bony fishes are repre-

sented in the fossil record. Jellyfishes are not, even though

they may have been common. Population density and

body size skew the record more. A population of ancient

plants may have produced millions of pollen grains in each

growing season, while the earliest human ancestors lived

in small groups and produced few young. Therefore, the

chance of finding a fossilized skeleton of an early human

is small compared to the chance of finding spores of plant

species that lived at the same time.

The fossil record is also biased toward certain environ-

ments and locations. Most species for which we have fos-

sils lived on land or in shallow seas that, through geologic

uplifting, became part of continents. We have only a few

fossils from deep ocean sediments. Also, most fossils have

been discovered in the Northern Hemisphere, probably

because most geologists have lived and worked there.

Looking at fossils and Biogeography

Figure 23.5 The Grand Canyon of the American Southwest

reveals sedimentary rock layers that formed over hundreds

of millions of years.

Danny Lehman/Latitude/Corbis

23.4

n The fossil record and biogeography are tools for studying

the evolutionary journey of life on Earth.

A fossil is the remains or traces

(such as tracks) of an organism of

a past geologic age embedded in

the Earth’s crust (Figure  23.4). Sim-

ilarities among fossils and living

organisms—or differences between them—provide strong

evidence of evolution by natural selection as populations

adapted to their surroundings.

Fossils are found in sedimentary rock

When an organism dies, its soft parts usually decompose

first. As a result, the most common fossils are bones, teeth,

shells, seeds, and other hard parts. Fossilization begins

when an organism is buried in sediments or volcanic ash.

With time, water seeps into the organic remains, infus-

ing them with dissolved inorganic compounds. As more

and more sediments accumulate above the burial site, the

remains are subjected to increasing pressure. Over long

spans of time, the chemical changes and growing pressure

transform them to stony hardness.

Organisms are more likely to be preserved when they

are buried quickly in the absence of oxygen. Entombment

by volcanic ash or anaerobic mud (which lacks oxygen) sat-

isfies this condition very well. Preservation also is favored

when a burial site is not disturbed. Usually, though, fossils

Figure 23.4 High-quality fossils such as these are rare.

A Fossilized footprint of one of the flesh-eating dinosaurs

called theropods. B The skeleton of a bat. (A: © Igor Karasi/

Shutterstock.com ; B: Department of Geosciences, Princeton University)

B

A

fossil Remains or traces of
an organism of a past geo-
logic age embedded and
preserved in Earth’s crust.

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