The First Bird ■ 251could contain nearly a trillion species. So far,
about 1.5 million species have been collected,
named, and placed into an evolutionary tree,
meaning it is possible that only one ten-thou-
sandth of 1 percent of species are known to us.
In other words, 99.9999 percent of species are
yet undiscovered.
Life throughout history and today is so diverse
that biologists created a classification system
to organize it into categories. The domains
form the highest, most inclusive hierarchical
level in the organization of life, describing the
most basic and ancient divisions among living
organisms. There are three domains of life
(Figure 14.2):● (^) Bacteria, which includes familiar disease-
causing bacteria such as E. coli
Life shares this set of common properties because
all living organisms descended from a common
ancestor, known as the universal ancestor. This
hypothetical ancestral cell is placed at the base
of the tree of life. From that cell, all life emerged.
Biological diversity, or biodiversity, embraces
the variety of all the world’s living things, as
well as their interactions with each other and
the ecosystems they inhabit. Biodiversity can
be described at the level of genes, or species, or
entire ecosystems.
In spite of intense worldwide interest, scien-
tists do not know the exact number of species
alive today. Most traditional estimates have
fallen in the range of 3–30 million species,
but in 2016, scientists combined microbial,
plant, and animal data sets from around the
world, scaled up, and estimated that Earth
Bacteria
Archaea
Eukarya
Common
ancestral cell
Domain Bacteria’s key evolutionary
innovations include diverse lifestyles
and communication and coordination
within populations. Bacteria are
microscopic and single-celled, and
they reproduce asexually by splitting
in two.
Domain Archaea’s key evolutionary
innovations include adaptation to
extreme environments, which has
allowed them to become widespread,
diverse, and abundant. Like the
bacteria, archaeans are microscopic
and single-celled, and they reproduce
asexually by splitting in two.
Domain Eukarya’s key evolutionary
innovations include a larger cell size,
the presence of a nucleus and other
membrane-enclosed internal
compartments, and multicellularity in
some groups. Most eukaryotes have
the ability to reproduce sexually,
through meiosis and the combination
of gametes into a zygote.
Time
Figure 14.2
Three domains of life
This “tree of life” shows the relationships of the three domains of life.
Q1: Why is there a shared line from the universal ancestor for Archaea and Eukarya?
Q2: Where would birds be found within this figure? What about humans?
Q3: To which domain would you expect a disease-causing organism to belong? What if the
organism was multicellular?