31.1 What Evidence Indicates the Animals Are Monophyletic?
What traits distinguish the animals from the other groups of organisms?
In contrast to the Bacteria, Archaea, and most microbial eukaryotes, all animals are multicellular.
Animal life cycles feature complex patterns of development from a single-celled zygote into a
In contrast to most plants, all animals are heterotrophs. Animals are able to synthesize very few
organic molecules from inorganic chemicals, so they must take in nutrients from their environment.
The fungi are also heterotrophs. In contrast to the fungi, however, animals use internal processes
to break down materials from their environment into the organic molecules they need most. Most
animals ingest food into an internal gut that is continuous with the outside environment, in which
digestion takes place.
In contrast to plants, most animals can move. Animals must move to find food or bring food to
them. Animals have specialized muscle tissues that allow them to move, and many animal body plans
are specialized for movement.
Animal monophyly is supported by gene sequences and morphology
The most convincing evidence that all the organisms considered to be animals share a common
ancestor comes from their many shared derived molecular and morphological traits.
Many gene sequences, such as the ribosomal RNA genes, support the monophyly of animals.
Animals display similarities in the organization and function of their Hox genes.
Animals have unique types of junctions between their cells (tight junctions, desmosomes, and gap
Animals have a common set of extracellular matrix molecules, including collagen and
Although there are animals in a few clades that lack one or another of these synapomorphies, these
species apparently once possessed the traits and lost them during their later evolution.
The ancestor of the animal clade was probably a colonial flagellated protist similar to existing colonial
choanoflagellates. The most reasonable current scenario postulates a choanoflagellate lineage in
which certain cells within the colony began to be specialized—some for movement, others for
nutrition, others for reproduction, and so on. Once this functional specialization had begun, cells
could have continued to differentiate. Coordination among groups of cells could have improved by
means of specific regulatory molecules that guided the differentiation and migration of cells in
developing embryos. Such coordinated groups of cells eventually evolved into the larger and more
complex organisms that we call animals.
More than a million animal species have been named and described, and there are doubtless millions
of living species that have yet to be named. The synapomorphies that indicate animal monophyly
cannot be used to infer evolutionary relationships among animals, because nearly all animals have
them. It just gives clues if an organism is an animal or not. Clues to the evolutionary relationships
among animal groups thus must be sought in derived traits that are found in some groups but not in
others. Such characteristics can be found in fossils, in patterns of embryonic development, in the
morphology and physiology of living animals, in the structure of animal molecules, and in the
genomes of animals (for example, in mitochondrial and ribosomal RNA genes).
Developmental patterns show evolutionary relationships among animals
Differences in patterns of embryonic development traditionally provided some of the most important
clues to animal phylogeny, although analyses of gene sequences are now showing that some
developmental patterns are more evolutionarily labile than previously thought.
The first few cell divisions of a zygote are known as cleavage. In general, the number of cells in the
embryo doubles with each cleavage. A number of different cleavage patterns exist among animals.
Cleavage patterns are influenced by the configuration of the yolk, the nutritive material that nourishes
the growing embryo. In reptiles, for example, the presence of a large body of a cellular yolk within the
fertilized egg creates an incomplete cleavage pattern in which the dividing cells form an embryo on
top of the yolk mass. In echinoderms such as sea urchins, small yolk particles are evenly distributed
throughout the egg cytoplasm, so cleavage is complete, with the fertilized egg cell dividing in an even
pattern known as radial cleavage. Radial cleavage is the ancestral condition for eumetazoans, so it is
found among many protostomes and diploblastic animals as well as deuterostomes. Spiral cleavage, a
complicated derived permutation of radial cleavage, is found among many lophotrochozoans, such as
earthworms and clams. Lophotrochozoans with spiral cleavage are thus sometimes known as
spiralians. The early branches of the ecdysozoans have radial cleavage, although most ecdysozoans
have an idiosyncratic cleavage pattern that is neither radial nor spiral in organization.
During the early development of most animals, distinct layers of cells form. These cell layers
differentiate into specific organs and organ systems as development continues. The embryos of
diploblastic animals have only two of these cell layers: an outer ectoderm and an inner endoderm.
The embryos of triploblastic animals have, in addition to ectoderm and endoderm, a third distinct cell
layer, the mesoderm, which lies between the ectoderm and the endoderm. The existence of three cell
layers is a synapomorphy of triploblastic animals, whereas the paraphyletic diploblastic animals
(ctenophores and cnidarians) exhibit the ancestral condition.
During early development in many animals, a hollow ball one cell thick indents to form a cup-shaped
structure. This process is known as gastrulation. The opening of the cavity formed by this indentation
is called the blastopore. The pattern of development after formation of the blastopore has been used
to divide the triploblastic animals into two major groups. Among members of the first group, the
protostomes, the mouth arises from the blastopore; the anus forms later. This appears to be the
derived condition. Among the deuterostomes, the blastopore becomes the anus; the mouth forms
later. This is thought to be the ancestral condition. We now know that the developmental patterns of
animals are more varied than suggested by this simple dichotomy, but the protostomes and
deuterostomes are still recognized as distinct animal clades based upon sequence similarities of their
Figure 31.1 A Current Phylogenetic Tree of Animals This phylogenetic tree is used here and in the following two
chapters. It presents a current interpretation based primarily on molecular data, which are particularly useful for identifying
ancient lineage splits. The traits highlighted by red circles will be explained as you read the chapter; you should review this
figure closely after you complete your reading.
The animals are thought to be monophyletic because they share many derived traits, including
multicellularity, mobility, and a heterotrophic lifestyle based on the ingestion of outside
nutrients. Evolutionary relationships among animals are inferred from fossils and from
molecular and developmental traits that are shared by different groups of animals.
31.2 What Are the Features of Animal Body Plans?
The general structure of an animal, the arrangement of its organ systems, and the integrated
functioning of its parts are referred to as its body plan. Although animal body plans are extremely
varied, they can be seen as variations on four key features:
The symmetry of the body
The structure of the body cavity