Biology Now, 2e

250 ■ CHAPTER 14 The History of Life

BIODIVERSITY

a hypothesis, but a well-established scientific

theory backed by a mountain of data. And atop

that mountain of data sat Archaeopteryx, the

earliest known bird.

Little did Xu know, as he dug through the dirt

that day, that he would soon be the one to knock

Archaeopteryx from its perch.

Dinosaurs and

Domains

Although the Archaeopteryx fossil is old—150

mil lion years old, in fact—much older fossils

have been found.

Our solar system and Earth formed 4.6 bil -

lion years ago. Some of the oldest known rocks

on Earth are 3.8 billion years old and contain

carbon deposits that hint at life. Cell-like struc-

tures have been found in layered mounds of sedi-

mentary rock called stromatolites that formed

3.7 billion years ago, and projections based on

DNA analysis also support the idea that life

appeared on Earth at that time.

The question of how life arose from nonlife is

one of the greatest riddles in biology, but scien-

tists have little doubt that all life on Earth is

related. As noted in Chapter 1, all living organ-

isms are united by a basic set of characteristics.

I

n 1861, quarry workers in Germany unearthed

the fossil of a crow-sized bird. The 150-million-

year-old preserved skeleton looked bizarre:

it had feathers and clawed hands like a bird, but

teeth and a long, bony tail like a reptile.

After close examination by paleontologists,

the fossil was hailed as a transitional form

between birds and reptiles and labeled the oldest

known bird. Named Archaeopteryx, the feath-

ered dinosaur became famous around the world

as the first solid evidence that birds descended

from dinosaurs (Figure 14.1). Charles Darwin,

who proposed the theory of descent with modifi-

cation, called the discovery “a grand case for me.”

Over 100 years later, a young fossil hunter

named Xu Xing bent over the dirt at a dig site

in Liaoning province in northern China. It was

Xu’s first time in the field since finishing college.

An excellent student, Xu had wanted to study

economics at Peking University in Beijing, but

at the time, students in China did not pick their

majors, and Xu was assigned paleontology.

Luckily, farmers in Liaoning had recently begun

to uncover huge numbers of fossils, and research

was booming.

So, as a grudging paleontologist, Xu began

searching for dinosaurs in Liaoning. He had

a particular interest in feathered dinosaurs.

By the late twentieth century, the idea that

birds descended from dinosaurs was no longer

5 cm

Forelimbs

Hind limbs

Head

Tail

Figure 14.1

The original

Archaeopteryx

fossil

There are 11 complete

Archaeopteryx fossils,

but this one from

Germany was the first

to be found. Note the

faint impressions of

feathers on the wings

and tail.