Biology Now, 2e

256 ■ CHAPTER 14 The History of Life

BIODIVERSITY

like a bird and the more it looked like a raptor.

Early birds have small, thick skulls and two toes

on each foot; Archaeopteryx, on the other hand,

has a long, almost pointy skull and three toes on

its feet. “Archaeopteryx is just so different from

other early birds,” says Xu.

Xu published his revised evolutionary tree in

2011. It took the scientific world by storm. Some
      researchers embraced the idea: “Perhaps the
      time has come to finally accept that Archaeop-
      teryx was just another small, feathered bird-like
      theropod fluttering around in the Jurassic,” wrote
      one paleontologist in the journal Nature. Others
      disagreed, arguing that Xu’s analysis was not
      convincing. For them, Archaeopteryx remained
      the first bird.

The History of Life on Earth

Xu knew that one new fossil was but a single

shred of evidence and therefore unlikely, by

itself, to convince the scientific community.

“The evidence is not very strong,” he admits,

“but it was a question we wanted to discuss and

add more analyses to, including old and new

fossils.” The identity of the first bird is important

because our understanding of how flight evolved

is based on the classification of Archaeopteryx.

If Archaeopteryx is not the species from which

flight evolved, then our understanding of flight

is wrong.

Xu’s redesign of the early bird evolutionary

tree raised another important question: Did

flight evolve more than once? If Archaeopteryx,

which looked capable of flight, was a bird, then

flight probably evolved just once among dino-

saurs, in the avian lineage. If Archaeopteryx

was not a bird, and was instead simply a raptor

that could fly, then flight evolved at least twice

among dinosaurs, once in raptors and once in

avians—but only avians went on to evolve into

modern birds.

Biological classification helps us answer

important evolutionary questions like these. In

addition to recognizing three broad domains of

life, biologists group the Eukarya into four dis -

tinct kingdoms, the second-highest level in the

hierarchical classification of life (Figure 14.8).

in question was assumed to be Archaeopteryx,

and the shared traits included feathers, clawed

hands, and a long, bony tail.

By comparing Xiaotingia to Archaeopteryx

and other related species, Xu created a new evolu-

tionary tree of early birds (Figure 14.6, bottom).

Suddenly, Archaeopteryx wasn’t in the avian clade.

Instead, Archaeopteryx and Xiaotingia were in a

different clade, grouped with deinonychosaurs—

the small, birdlike, carnivorous dinosaurs

commonly called raptors. (The term “raptor”

refers to carnivorous modern birds like hawks and

owls, but it is also used informally to describe this

group of dinosaurs.) Archaeopteryx, Xu believed,

was not the first bird, but a raptor. It had feathers,

but its descendants did not evolve into birds.

“It was a big change,” says Xu. Yet, he adds, it

wasn’t entirely unexpected. In the last 20 years,

more and more fossils of early avian species have

been discovered—and the more Xu compared

Archaeopteryx, discovered in Europe, to other

early birds discovered in China, the less it looked

Domain

Bacteria

Domain

Archaea

Kingdom

Plantae

Kingdom

Protista

Kingdom

Fungi

Kingdom

Animalia

The history of

life begins with

the common

ancestor of all

living organisms

at the root of

the tree.

Common ancestor

of the Eukarya

Common

ancestral cell

or universal

ancestor

Figure 14.8

The tree of life

The domains Bacteria and Archaea do not have separate kingdoms within

them. The domain Eukarya encompasses four kingdoms: Protista (protists,

an artificial grouping that includes organisms such as amoebas and algae),

Plantae (plants), Fungi (including yeasts and mushroom-producing species),

and Animalia (animals).