Biology Now, 2e

The First Bird ■ 265

about 65 mya and wiped out three-fourths of

plant and animal species on Earth, including

non-avian dinosaurs. Researchers suspect that

a massive comet or asteroid slammed into the

Gulf of Mexico, choking the skies around the

planet with debris and decreasing the ability of

plants to photosynthesize. As the plants died,

so, too, did animals further up the food chain.

Mass extinctions affect the diversity of life in

two main ways: First, entire groups of organ-

isms perish, changing the history of life forever.

Second, the extinction of one or more dominant

groups of organisms can provide new opportu-

nities for groups that previously were of rela-

tively minor importance, thereby dramatically

altering the course of evolution. When a group

of organisms expands to take on new ecolog-

ical roles and to form new species and higher

taxonomic groups, that group is said to have

undergone an adaptive radiation. Some of the

great adaptive radiations in the history of life

occurred after mass extinctions, such as when

the mammals diversified after the extinction of

the dinosaurs.

To d ay, o n e s p e c i e s o f m a m m a l , Homo sapi-

ens, dominates life on land, and our impact

on biodiversity is unprecedented. Because of

human activities, the world is losing species at

an alarming rate. In July 2016, researchers at

the United Nations Environment Programme

World Conservation Monitoring Centre in the

United Kingdom announced that worldwide

biodiversity had fallen below predetermined

“safe” levels, thresholds below which ecological

function is likely to be negatively affected. An

estimated 58 percent of the world’s land has

lost more than 10 percent of its biodiversity, the

researchers found, with grasslands and biodi-

versity hot spots such as the Amazon rainforest

hit the hardest.

That is alarming news, because nonhuman

organisms provide even the most basic require-

ments for human life. Plants and plantlike

protists produce the oxygen we breathe and

provide us with food. Whole ecosystems provide

so-called “ecosystem services,” environmental

benefits that humans rely on. For example, coast

redwood trees in northern California intercept

fog, mist, and rain, channeling the water onto

and into the ground.

Biologists today assert that we are on our way

toward a new mass extinction, and the cause is

clear: the activities of the ever-increasing number

of humans living on, and exploiting, Earth.

REVIEWING THE SCIENCE

● (^) The Linnaean hierarchy places each species in
successively larger and more inclusive categories.
Closely related species are grouped together into
a genus, related genera are grouped into a family,
related families into an order, related orders into
a class, related classes into a phylum, and finally,
related phyla into a kingdom.
● (^) Organisms are identified by their genus and species
names, together referred to as their scientific name.
● (^) The release of oxygen by photosynthetic bacteria
caused oxygen concentrations in the atmosphere
to increase. Rising oxygen concentrations made
possible the evolution of single-celled eukaryotes
about 2.1 billion years ago. Multicellular eukaryotes
followed about 650 mya.
● (^) Life in the oceans changed dramatically during the
Cambrian explosion, when large predators and well-
defended herbivores suddenly appear in the fossil
record. The Cambrian explosion is an example of
adaptive radiation, in which a group of organisms
● (^) The first single-celled organisms resembled bacteria
and probably evolved about 3.7 billion years ago.
● (^) The most basic and ancient branches of the tree of
life define three domains: Bacteria, Archaea, and
Eukarya. All life-forms fall into one of these three
domains. The Eukarya are further divided into four
kingdoms: Protista, Fungi, Plantae, and Animalia.
The variety of these life-forms, as well as their
interactions with each other and the ecosystems
they inhabit, is biodiversity.
● (^) Scientists use evolutionary trees to model ancestor-
descendant relationships among different organisms.
The tips of branches represent existing groups of
organisms, and each node represents the moment
when an ancestor split into two descendant groups. A
clade is an ancestral species and all its descendants.
● (^) Closely related groups of organisms share distinctive
features that originated in their most recent common
ancestor. These shared derived traits are used to
identify the lineages of a species.