The First Bird ■ 257fossil record during this time, popularly known
as the Cambrian explosion. The Cambrian
explosion changed the face of life on Earth
from a world of relatively simple, slow-moving,
soft-bodied scavengers and herbivores to a world
dominated by large, fast predators. The presence
of predators sped up the evolution of Cambrian
herbivores, judging by the variety of scales and
shells and other protective body coverings typi-
cal of many Cambrian, but not Precambrian,
fossils (Figure 14.11).
The Cambrian explosion, however, occurred
primarily in the oceans. Because life first evolved
in water, the colonization of land by living organ-
isms posed enormous challenges. Indeed, many
of the functions basic to life, including support,
movement, reproduction, and the regulation of
heat, had to be handled very differently on land
than in water. About 480 mya, near the begin-
ning of the Ordovician period, plants were the
first organisms to meet these challenges. These
early terrestrial colonists were single-celled or
had just a few cells.
Fungi are thought to have made their way
onto land next, according to fossil evidence.
For example, scientists have found fossils of
terrestrial fungi that are 455–460 million years
old. Reconstructing the evolutionary history
of eukaryotes from DNA data, scientists esti-
mate that the common ancestor of fungi and
animals diverged from all other eukaryotes
about 1.5 billion years ago, and fungi diverged
from their closest cousins, the animals, about
10 million years after that.
Next, land plants evolved and diversified
from the original green algae that made it to
land. By 360 mya, at the end of the Devonian
period, Earth was covered with plants. The first
land animals likely emerged about 400 mya.
Many of the early animal colonists on land were
carnivores; others fed on living plants or decay-
ing plant material.
The first vertebrates to colonize land were
amphibians, the earliest fossils of which date
to about 365 mya (see Chapter 17 for more on
vertebrates). Early amphibians descended from
lobe-finned fishes. Amphibians were the most
abundant large organisms on land for about
100 million years. Then, in the late Permian
period, reptiles took over as the most common
vertebrate group. Reptiles were the first groupThese kingdoms are Protista, a diverse group
that includes amoebas and algae; Plantae, which
encompasses all plants; Fungi, which includes
mushrooms, molds, and yeasts; and Animalia,
which encompasses all animals, including dino-
saurs, birds, and humans. The members of each
of these kingdoms share evolutionary innova-
tions that adapted the organisms to their envi-
ronments, enabling them to live and reproduce
successfully (Figure 14.9).
Below the level of kingdom, biological clas-
sification can get even more specific using
the Linnaean hierarchy, a system of biolog-
ical classification devised in the eighteenth
century by a Swedish naturalist named Carolus
Linnaeus. The smallest unit of classification
in the Linnaean hierarchy—the species—
reflects individuals that are the most related
to each other. The most closely related species
are grouped together to form a genus (plural
“genera”). Using these two categories in the hier-
archy, every species is given a unique, two-word
Latin name, called its scientific name. The first
word of the name identifies the genus to which
the organism belongs; the second word defines
the species. Comparing our own species name,
Homo sapiens, to that of the Neanderthals,
Homo neanderthalensis, shows that the two are
classified as separate species but belong to the
same genus, Homo (Figure 14.10).
In the Linnaean hierarchy, each species is
placed in successively larger and more inclu-
sive categories beyond genus. Closely related
genera are grouped into a family. Closely
related families are grouped into an order.
Closely related orders are grouped into a class.
Closely related classes are grouped into a
phylum (plural “phyla”). And, you guessed it,
closely related phyla are grouped together into
a kingdom.
The earliest forms of life evolved in water.
About 650 mya (during the Precambrian period),
the number of organisms appearing in the fossil
record increased. At that time, much of Earth
was covered by shallow seas filled with small,
mostly single-celled organisms floating freely in
the water.
Then, about 540 mya, the world experienced
an astonishing burst of evolutionary activity,
with a dramatic increase in biodiversity. Most of
the major living animal groups first appear in the