Biology Now, 2e

196 ■ CHAPTER 11 Evidence for Evolution

EVOLUTION

which a fossil is found indicates its order in the

fossil record. The ages of fossils correspond to

their order: older fossils are found in deeper,

older rock layers.

The fossil record contains excellent exam-

ples of how major new groups of organisms

arose from previously existing organisms. The

record includes numerous transitional fossils,

evidence of species with some similarities to

the ancestral group (land-living mammals)

and some similarities to the descendant species

(whales). Thewissen spent decades studying

these intermediates—from the first known

whale, the wolflike Pakicetus that waded in

shallow freshwater; to the larger crocodile-like

Ambulocetus that stalked its prey underwater;

to the fully aquatic Dorudon, with its blowhole,

flippers, and tail (Figure 11.6). Yet Thewis-

sen and others had long been searching for the

Indohyus fossils was no easy task. Beginning

in 2003, he made an annual pilgrimage to

Dehradun, India, a city nestled in the foothills

of the Himalayas. There he visited the widow

of A. Ranga Rao, an Indian geologist who had

hoarded piles of fossils excavated from Kash-

mir, a disputed border area between India and

Pakistan. Most early whale fossils have come

from the India-Pakistan region, where whales

first evolved. But because of political tensions,

in recent decades it has been too dangerous

to travel to Kashmir, much less dig for fossils.

Thewissen was frustrated by his inability to

collect fossils in Kashmir.

The fossil record enables biologists to

reconstruct the history of life on Earth, and it

provides some of the strongest evidence that

species have evolved over time. The relative

depth or distance from the surface of Earth at

Figure 11.5

Fossils through the ages

Myriad fossils exist, ranging from imprints of organisms, to preserved organisms, to completely mineralized bone and wood. Each

fossil can be dated, and when the results are compiled, they can tell the life history of Earth.

Soft-bodied animals such as

this one dominated life on Earth

600 million years ago (mya).

A fossil of a trilobite

that lived between

410 and 355 mya.

Fossilized leaf of a

300-million-year-

old seed fern.

This 20-million-year-old

termite is preserved in

amber, the fossilized

resin of a tree.

A fossil of a Velociraptor

entangled with a Protoceratops,

which bit down on the predator’s

claw, locking both in a death grip.

Once solid wood has

fossilized into solid

rock, it is known as

petrified wood.