Chapter 32 An Introduction to Animal Diversity
Overview: Welcome to Your Kingdom
• Biologists have identified 1.3 million living species of animals.
• Estimates of the total number of animal species run far higher, from
10 to 20 million to as many as 100 to 200 million.
Concept 32.1 Animals are multicellular, heterotrophic eukaryotes with
tissues that develop from embryonic layers
• There are exceptions to nearly every criterion for distinguishing an
animal from other life forms.
• However, five criteria, taken together, comprise a reasonable
1. Animals are multicellular, ingestive heterotrophs.
• Animals take in preformed organic molecules through
ingestion, eating other organisms or organic material that
2. Animal cells lack cell walls that provide structural support for
plants and fungi.
• The multicellular bodies of animals are held together by
extracellular structural proteins, especially collagen.
• Animals have other unique types of intercellular junctions,
including tight junctions, desmosomes, and gap junctions,
which hold tissues together.
• These junctions are also composed of structural proteins.
3. Animals have two unique types of cells: nerve cells for impulse
conduction and muscle cells for movement.
4. Most animals reproduce sexually, with the diploid stage usually
dominating the life cycle.
• In most species, a small flagellated sperm fertilizes a
larger, nonmotile egg. • The zygote undergoes cleavage, a succession of mitotic
cell divisions, leading to the formation of a multicellular,
hollow ball of cells called the blastula.
• During gastrulation, part of the embryo folds inward,
forming layers of embryonic tissues that will develop into
adult body parts.
• The resulting development stage is called a
• Some animals develop directly through transient stages
into adults, but others have a distinct larval stage or
• A larva is a sexually immature stage that is
morphologically distinct from the adult, usually eats
different foods, and may live in a different habitat
from the adult.
• Animal larvae eventually undergo metamorphosis,
transforming the animal into an adult.
• Animals share a unique homeobox-containing family of
genes known as Hox genes.
• All eukaryotes have genes that regulate the expression of other
• Many of these regulatory genes contain common modules
of DNA sequences called homeoboxes.
• All animals share the unique family of Hox genes,
suggesting that this gene family arose in the eukaryotic
lineage that gave rise to animals.
• Hox genes play important roles in the development of animal
embryos, regulating the expression of dozens or hundreds of
• Hox genes control cell division and differentiation,
producing different morphological features of animals. • Hox genes in sponges regulate the formation of channels, the
primary feature of sponge morphology.
• In more complex animals, the Hox gene family underwent
• In bilaterians, Hox genes regulate patterning of the
• The same conserved genetic network governs the
development of a large range of animals.
Concept 32.2 The history of animals may span more than a billion
• Various studies suggest that animals began to diversify more than a
billion years ago.
• Some calculations based on molecular clocks estimate that the
ancestors of animals diverged from the ancestors of fungi as much as
1.5 billion years ago.
• Similar studies suggest that the common ancestor of living animals
lived 1.2 billion to 800 million years ago.
• The common ancestor was probably a colonial flagellated protist and
may have resembled modern choanoflagellates.
Neoproterozoic Era (1 billion–542 million years ago)
• Although molecular data indicates a much earlier origin of animals,
the oldest generally accepted animal fossils are only 575 million
• These fossils are known as the Ediacara fauna, named for the
Ediacara Hills of Australia.
• Ediacara fauna consist primarily of cnidarians, but soft-bodied
mollusks were also present, and numerous fossilized burrows
and tracks indicate the presence of worms.
Paleozoic Era (542–251 million years ago) • Animals underwent considerable diversification between 542–525
million years ago, during the Cambrian period of the Paleozoic Era.
• During this period, known as the Cambrian explosion, about
half of extant animal phyla arose.
• Fossils of Cambrian animals include the first animals with hard,
• There are several hypotheses regarding the cause of the Cambrian
1. The new predator-prey relationships that emerged in the Cambrian
may have generated diversity through natural selection.
• Predators acquired adaptations that helped them catch
• Prey acquired adaptations that helped them resist
2. A rise of atmospheric oxygen preceded the Cambrian explosion.
• More oxygen may have provided opportunities for animals
with higher metabolic rates and larger body sizes.
3. The evolution of the Hox complex provided the developmental
flexibility that resulted in variations in morphology.
• These hypotheses are not mutually exclusive; all may have
played a role.
• In the Silurian and Devonian periods, animal diversity continued to
increase, punctuated by episodes of mass extinction.
• Vertebrates (fishes) became the top predators of marine food
• By 460 million years ago, arthropods began to adapt to terrestrial
• Vertebrates moved to land about 360 million years ago and
diversified into many lineages.
• Two of these survive today: amphibians and amniotes. Mesozoic Era (251–65.5 million years ago)
• Few new animal body plans emerged among animals during the
• Animal phyla began to spread into new ecological niches.
• In the oceans, the first coral reefs formed.
• On land, birds, pterosaurs, dinosaurs, and tiny nocturnal insect-eating
Cenozoic Era (65.5 million years ago to the present)
• Insects and flowering plants both underwent a dramatic diversification
during the Cenozoic era.
• This era began with mass extinctions of terrestrial and marine
• Among the groups of species that disappeared were large, nonflying
dinosaurs and the marine reptiles.
• Large mammalian herbivores and carnivores diversified as mammals
exploited vacated ecological niches.
• Some primate species in Africa adapted to open woodlands and
savannas as global climates cooled.
• Our ancestors were among these grassland apes.
Concept 32.3 Animals can be characterized by “body plans”
• Zoologists may categorize the diversity of animals by general
features of morphology and development.
• A group of animal species that share the same level of organizational
complexity is called a grade.
• Certain body-plan features shared by a group of animals define
2. Animals can be categorized according to the symmetry of their
• Sponges lack symmetry. • Some animals, such as sea anemones, have radial
• Many animals have bilateral symmetry.
• A bilateral animal has a dorsal (top) side and a
ventral (bottom side), a left and right side, and an
anterior (head) end and a posterior (tail) end.
• Linked with bilateral symmetry is cephalization, an
evolutionary trend toward the concentration of sensory
equipment on the anterior end.
• Cephalization also includes the development of a
central nervous system concentrated in the head
and extending toward the tail as a longitudinal nerve
• The symmetry of an animal generally fits its lifestyle.
• Many radial animals are sessile or planktonic and
need to meet the environment equally well from all
• Animals that move actively are generally bilateral.
• Their central nervous system allows them to
coordinate complex movements involved in
crawling, burrowing, flying, and swimming.
3. The animal body plan