Biology Ch 32

Vertebrates colonized land around

365 million years ago and diversified into numerous terrestrial groups. Two of these survive today: the amphibians (such as frogs and salamanders) and the amniotes (reptiles, including birds, and mammals).

Bilateral symmetry

A bilateral animal has two axes of orientation: front to back and top to bottom. Such animals have a dorsal (top) side and a ventral (bottom) side, a left side and a right side, and an anterior (front) end and a posterior (back) end. Nearly all animals with a bilaterally symmetrical body plan (such as arthropods and mammals) have sensory equipment concentrated at their anterior end, including a central nervous system ("brain") in the head.

Although some animals, including humans, develop directly into adults, the life cycles of most animals include at least one ____ stage

A larva is a sexually immature form of an animal that is morphologically distinct from the adult, usually eats different food, and may even have a different habitat than the adult, as in the case of the aquatic larva of a mosquito or dragonfly

Are acoelomate flatworms basal bilaterians?

A series of recent molecular papers have indicated that acoelomate flatworms (phylum Acoela) are basal bilaterians, as shown in Figure 32.11. A different conclusion was supported by a 2011 analysis, which placed acoelomates within Deuterostomia. Researchers are currently sequencing the genomes of several acoelomates and species from closely related groups to provide a more definitive test of the hypothesis that acoelomate flatworms are basal bilaterians. If further evidence supports this hypothesis, this would suggest that the bilaterians may have descended from a common ancestor that resembled living acoelomate flatworms—that is, from an ancestor that had a simple nervous system, a saclike gut with a single opening (the "mouth"), and no excretory system.

Eumetazoa is a clade of animals with tissues

All animals except for sponges and a few others belong to a clade of eumetazoans ("true animals"). Members of this group have tissues, such as muscle tissue and nervous tissue. Basal eumetazoans, which include the phyla Ctenophora (comb jellies) and Cnidaria, are diploblastic and generally have radial symmetry.

Mesoderm

All bilaterally symmetrical animals have a third germ layer, called the mesoderm, which fills much of the space between the ectoderm and endoderm.

Cell Structure and Specialization

Animals are multicellular. Animals lack the structural support of cell walls. Instead, proteins external to the cell membrane provide structural support to animal cells and connect them to one another. The most abundant of these proteins is collagen which isn't found in plants or fungi.

A basic feature of animal bodies is their type of symmetry—or absence of symmetry.

Animals have radial symmetry. Sea anemones, for example, have a top side (where the mouth is located) and a bottom side. But they have no front and back ends and no left and right sides.

Most animal phyla belong to the clade Bilateria

Bilateral symmetry and the presence of three prominent germ layers are shared derived characters that help define the clade Bilateria. This clade contains the majority of animal phyla, and its members are known as bilaterians. The Cambrian explosion was primarily a rapid diversification of bilaterians.

There are three major clades of bilaterian animals

Bilaterians have diversified into three main lineages, Deuterostomia, Lophotrochozoa, and Ecdysozoa. With one exception, the phyla in these clades consist entirely of invertebrates, animals that lack a backbone; Chordata is the only phylum that includes vertebrates, animals with a backbone.

Fate of the Blastopore

Blastopore is the indentation that during gastrulation leads to the formation of the archenteron. After the archenteron develops, in most animals a second opening forms at the opposite end of the gastrula. In many species, the blastopore and this second opening become the two openings of the digestive tube: the mouth and the anus. In protostome development, the mouth generally develops from the first opening, the blastopore, and it is for this characteristic that the term protostome derives. In deuterostome development the mouth is derived from the secondary opening, and the blastopore usually forms the anus.

During the development of most animals, cleavage leads to the formation of a multicellular embryonic stage called a

Blastula, which in many animals takes the form of a hollow ball.

Another wave of animal diversification occurred 535-525 million years ago, during the

Cambrian period of the Paleozoic era—a phenomenon referred to as the Cambrian explosion. In strata formed before the Cambrian explosion, only a few animal phyla have been observed. But in strata that are 535-525 million years old, paleontologists have found the oldest fossils of about half of all extant animal phyla, including the first arthropods, chordates, and echinoderms. Many of these fossils, which include the first large animals with hard, mineralized skeletons, look very different from most living animals. Even so, paleontologists have established that these Cambrian fossils are members of extant animal phyla, or at least are close relatives. In particular, most of the fossils from the Cambrian explosion are of bilaterians, an enormous clade whose members (unlike sponges and cnidarians) typically have a two-sided or bilaterally symmetric form and a complete digestive tract, an efficient digestive system that has a mouth at one end and an anus at the other. As we'll discuss later in the chapter, bilaterians include molluscs, arthropods, chordates, and most other living animal phyla.

The protist ________ is the closest living relative of animals

Choanoflagellate. Based on such evidence, researchers have hypothesized that the common ancestor of choanoflagellates and living animals may have been a suspension feeder similar to present-day choanoflagellates. Morphologically, choanoflagellate cells and the collar cells (or choanocytes) of sponges are almost indistinguishable. Scientists exploring how animals may have arisen from their single-celled ancestors have noted that the origin of multicellularity requires the evolution of new ways for cells to adhere and signal (communicate) to each other. To learn more about such mechanisms, researchers compared the genome of the unicellular choanoflagellate Monosiga brevicollis with those of representative animals. This analysis uncovered 78 protein domains in M. brevicollis that were otherwise only known to occur in animals. (A domain is a key structural or functional region of a protein.) For example, M. brevicollis has genes that encode domains of certain proteins (known as cadherins) that play key roles in how animal cells attach to one another, as well as genes that encode protein domains that animals (and only animals) use in cell-signaling pathways.

Diploblastic

Cnidarians and a few other animal groups that have only these two germ layers are said to be diploblastic

All animals share a common ancestor

Current evidence indicates that animals are monophyletic, forming a clade called Metazoa. All extant and extinct animal lineages have descended from a common ancestor.

hemichordates (acorn worms), echinoderms (sea stars and relatives), and chordates are members of the bilaterian clade

Deuterostomia; thus, the term deuterostome refers not only to a mode of animal development, but also to the members of this clade. (The dual meaning of this term can be confusing since some organisms with a deuterostome developmental pattern are not members of clade Deuterostomia.) Hemichordates share some characteristics with chordates, such as gill slits and a dorsal nerve cord; echinoderms lack these characteristics. These shared traits may have been present in the common ancestor of the deuterostome clade (and lost in the echinoderm lineage). As mentioned above, phylum Chordata, the only phylum with vertebrate members, also includes invertebrates.

Coelom Formation

During gastrulation, an embryo's developing digestive tube initially forms as a blind pouch, the archenteron, which becomes the gut. As the archenteron forms in protostome development, initially solid masses of mesoderm split and form the coelom. In contrast, in deuterostome development, the mesoderm buds from the wall of the archenteron, and its cavity becomes the coelom.

As the diversity of animal phyla increased during the Cambrian, the diversity of

Ediacaran life-forms declined. Fossil evidence suggests that during the Cambrian period, predators acquired novel adaptations, such as forms of locomotion that helped them catch prey, while prey species acquired new defenses, such as protective shells. As new predator-prey relationships emerged, natural selection may have led to the decline of the softbodied Ediacaran species and the rise of various bilaterian phyla. Another hypothesis focuses on an increase in atmospheric oxygen that preceded the Cambrian explosion. More plentiful oxygen would have enabled animals with higher metabolic rates and larger body sizes to thrive, while potentially harming other species. A third hypothesis proposes that genetic changes affecting development, such as the origin of Hox genes and the addition of new microRNAs (small RNAs involved in gene regulation), facilitated the evolution of new body forms.

Acoelomates

Finally, some triploblastic animals lack a body cavity altogether. They are known collectively as acoelomates A body cavity has many functions. Its fluid cushions the suspended organs, helping to prevent internal injury. In softbodied coelomates, such as earthworms, the coelom contains noncompressible fluid that acts like a skeleton against which muscles can work. The cavity also enables the internal organs to grow and move independently of the outer body wall. If it were not for your coelom, for example, every beat of your heart or ripple of your intestine would warp your body's surface.

All animals have developmental genes that regulate the expression of other genes, and many of these regulatory genes contain sets of DNA sequences called

Homeoboxes

Hox Genes

Hox genes play important roles in the development of animal embryos, controlling the expression of many other genes that influence morphology

Sponges, which are among the simplest extant (living) animals, lack

Hox genes. However, they have other homeobox genes that influence their shape, such as those that regulate the formation of water channels in the body wall, a key feature of sponge morphology

Like all features of organisms, animal body plans have evolved over time.

In some cases, including key stages in gastrulation, novel body plans emerged early in the history of animal life and have not changed since. As we'll discuss, however, other aspects of animal body plans have changed multiple times over the course of evolution. As we explore the major features of animal body plans, bear in mind that similar body forms may have evolved independently in different lineages. In addition, body features can be lost over the course of evolution, causing some closely related species to look very different from one another.

The symmetry of an animal generally fits its lifestyle.

Many radial animals are sessile (living attached to a substrate) or planktonic (drifting or weakly swimming, such as jellies, commonly called jellyfishes). Their symmetry equips them to meet the environment equally well from all sides. In contrast, bilateral animals typically move actively from place to place. Most bilateral animals have a central nervous system that enables them to coordinate the complex movements involved in crawling, burrowing, flying, or swimming. Fossil evidence indicates that these two fundamentally different kinds of symmetry have existed for at least 550 million years.

Are ctenophores basal metazoans?

Many researchers have concluded that sponges are basal metazoans (see Figure 32.11). This conclusion was supported in a 2016 phylogenomic analysis, but several other recent studies have placed the comb jellies (phylum Ctenophora) at the base of the animal tree. In addition to the most recent phylogenomic results, data consistent with placing sponges at the base of the animal tree include fossil steroid evidence, molecular clock analyses, the morphological similarity of sponge collar cells to the cells of choanoflagellates (see Figure 32.3), and the fact that sponges are one of the few animal groups that lack tissues (as might be expected for basal animals). Ctenophores, on the other hand, have tissues, and their cells do not resemble the cells of choanoflagellates. At present, the idea that ctenophores are basal metazoans remains an intriguing but controversial hypothesis.

Cenozoic Era (66 Million Years Ago to Present)

Mass extinctions of both terrestrial and marine animals ushered in a new era, the Cenozoic. Among the groups of species that disappeared were the large, nonflying dinosaurs and the marine reptiles. The fossil record of the early Cenozoic documents the rise of large mammalian herbivores and predators as mammals began to exploit the vacated ecological niches. The global climate gradually cooled throughout the Cenozoic, triggering significant shifts in many animal lineages. Among primates, for example, some species in Africa adapted to the open woodlands and savannas that replaced many of the former dense forests. The ancestors of our own species were among those grassland apes.

Body Cavity

Most triploblastic animals have a body cavity, a fluid- or airfilled space located between the digestive tract and the outer body wall. This body cavity is also called a coelom (from the Greek koilos, hollow). A so-called "true" coelom forms from tissue derived from mesoderm. The inner and outer layers of tissue that surround the cavity connect and form structures that suspend the internal organs. Animals with a true coelom are known as coelomates

Main characteristics of animals

Multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers

The Cambrian period was followed by the

Ordovician, Silurian, and Devonian periods, when animal diversity continued to increase, although punctuated by episodes of mass extinction (see Figure 25.17). Vertebrates (fishes) emerged as the top predators of the marine food web.

The fossil record from the Ediacaran period (635-541 million years ago) also provides early evidence of

Predation

Most animals reproduce _____

Sexually and the diploid stage usually dominates the life cycle. In the haploid stage, sperm and egg cells are produced directly by meiotic division, unlike what occurs in plants and fungi (see Figure 13.6). In most animal species, a small, flagellated sperm fertilizes a larger, nonmotile egg, forming a diploid zygote.

Consider Cloudina, a small animal whose body was protected by a shell resembling a series of nested cones

Some Cloudina fossils show signs of attack: round "bore holes" that resemble those formed today by predators that drill through the shells of their prey to gain access to the soft-bodied organisms lying within.

Pseudocoelom

Some triploblastic animals have a body cavity that is formed from mesoderm and endoderm (Figure 32.9b). Such a cavity is called a "pseudocoelom" and the animals that have one are called pseudocoelomates. Despite its name, however, a pseudocoelom is not false; it is a fully functional body cavity.

Sponges are the sister group to all other animals

Sponges (phylum Porifera) are basal animals, having diverged from all other animals early in the history of the group. Recent morphological and molecular analyses indicate that sponges are monophyletic, as shown here.

Animal body plans also vary with regard to tissue organization.

Sponges and a few other groups lack tissues. In all other animals, the embryo becomes layered during gastrulation (see Figure 47.8, "Visualizing Gastrulation," which will help you understand this three-dimensional folding process). As development progresses, these layers, called germ layers, form the various tissues and organs of the body.

Mesozoic Era (252-66 Million Years Ago)

The animal phyla that had evolved during the Paleozoic now began to spread into new habitats. In the oceans, the first coral reefs formed, providing other marine animals with new places to live. Some reptiles returned to the water, leaving plesiosaurs (see Figure 25.5) and other large aquatic predators as their descendants. On land, descent with modification in some tetrapods led to the origin of wings and other flight equipment in pterosaurs and birds. Large and small dinosaurs emerged, both as predators and herbivores. At the same time, the first mammals—tiny nocturnal insect-eaters—appeared on the scene. Angiosperms and insects both underwent dramatic diversifications during the late Mesozoic.

Ediacaran biota

The name comes from the Ediacara Hills of Australia, where fossils of these organisms were first discovered (Figure 32.5). Similar fossils have since been found on other continents. Among the oldest Ediacaran fossils that resemble animals, some have been classified as molluscs (snails and their relatives) or close relatives of the molluscs, while others are thought to be sponges or cnidarians (sea anemones and their relatives). Still others have proved difficult to classify, as they do not seem to be closely related to any living animal or algal groups. In addition to these macroscopic fossils, Neoproterozoic rocks have also yielded what may be microscopic fossils of early animal embryos. Although these microfossils appear to exhibit the basic structural organization of present-day animal embryos, debate continues about whether these fossils are indeed of animals.

Triploblastic

Thus, animals with bilateral symmetry are also said to be triploblastic (having three germ layers). In triploblasts, the mesoderm forms the muscles and most other organs between the digestive tract and the outer covering of the animal. Triploblasts include a broad range of animals, from flatworms to arthropods to vertebrates. (Although some diploblasts actually do have a third germ layer, it is not nearly as well developed as the mesoderm of animals considered to be triploblastic.)

The cells of most animals are organized into

Tissues, groups of similar cells that act as a functional unit. Ex- muscle tissue and nervous tissue are responsible for moving the body and conducting nerve impulses, respectively. The ability to move and conduct nerve impulses underlies many of the adaptations that differentiate animals from plants and fungi (which lack muscle and nerve cells). For this reason, muscle and nerve cells are central to the animal lifestyle.

Are sponges monophyletic?

Traditionally, sponges were placed in a single phylum, Porifera. This view was challenged in the 1990s, when molecular studies indicated that sponges were paraphyletic; as a result, sponges were placed into several different phyla that branched near the base of the animal tree. Since 2009, however, several morphological and molecular studies have concluded that sponges are monophyletic after all, as traditionally thought and as shown in Figure 32.11. Researchers are currently sequencing the entire genomes of various sponges to investigate whether sponges are indeed monophyletic.

Protostome Development (Cleavage)

Undergoes spiral cleavage in which the planes of cell division are diagonal to the vertical axis of the embryo; as seen in the eight-cell stage of the embryo, smaller cells are centered over the grooves between larger, underlying cells (Figure 32.10a, left). Furthermore, the so-called determinate cleavage of some animals with protostome development rigidly casts ("determines") the developmental fate of each embryonic cell very early. A cell isolated from a snail at the four-cell stage, for example, cannot develop into a whole animal. Instead, after repeated divisions, such a cell will form an inviable embryo that lacks many parts. Deuterostome Development (Cleavage) Radial cleavage. The cleavage planes are either parallel or perpendicular to the vertical axis of the embryo; as seen at the eight-cell stage, the tiers of cells are aligned, one directly above the other. Most animals with deuterostome development also have indeterminate cleavage, meaning that each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo. For example, if the cells of a sea urchin embryo are separated at the four-cell stage, each can form a complete larva. Similarly, it is the indeterminate cleavage of the human zygote that makes identical twins possible.

Nutritional mode of animals

Unlike plants, animals cannot construct all of their own organic molecules, and so, in most cases, they ingest them—either by eating other living organisms or by eating nonliving organic material. But unlike fungi, most animals feed by ingesting their food and then using enzymes to digest it within their bodies.

Animal larvae eventually undergo metamorphosis

a developmental transformation that turns the animal into a juvenile that resembles an adult but is not yet sexually mature. Though adult animals vary widely in morphology, the genes that control animal development are similar across a broad range of taxa.

The zygote then undergoes cleavage

a succession of mitotic cell divisions without cell growth between the divisions.

In the ancestors of more complex animals, the Hox gene family arose via

a the duplication of earlier homeobox genes. Over time, the Hox gene family underwent a series of duplications, yielding a versatile "toolkit" for regulating development.

Although data from fossil steroids and molecular clocks indicate an earlier origin, the first generally accepted macroscopic fossils of animals date from

about 560 million years ago. These fossils are members of an early group of softbodied multicellular eukaryotes, known collectively as the Ediacaran biota

Zoologists currently recognize about three dozen phyla of extant animals. Researchers infer evolutionary relationships among these phyla by

analyzing whole genomes, as well as morphological traits, ribosomal RNA (rRNA) genes, Hox genes, proteincoding nuclear genes, and mitochondrial genes.

Fossils indicate that fern galls

date back at least 302 million years, suggesting that insects and plants were influencing each other's evolution by that time.

DNA sequence analyses show that animal cadherin proteins are composed primarily of

domains that are also found in a cadherin-like protein of choanoflagellates (Figure 32.4). However, animal cadherin proteins also contain a highly conserved region not found in the choanoflagellate protein (the "CCD" domain labeled in Figure 32.4). These data suggest that the cadherin attachment protein originated by the rearrangement of protein domains found in choanoflagellates plus the incorporation of a novel domain, the conserved CCD region. Overall, comparisons of choanoflagellate and animal genomes suggest that key steps in the transition to multicellularity in animals involved new ways of using proteins or parts of proteins that were encoded by genes found in choanoflagellates.

Bilaterians also diversified in two major clades that are composed entirely of invertebrates: the

ecdysozoans and the lophotrochozoans

Animal species vary tremendously in morphology, but their great diversity in form can be described by a relatively small number of

major "body plans." A body plan is a particular set of morphological and developmental traits, integrated into a functional whole—the living animal. The term plan here does not imply that animal forms are the result of conscious planning or invention. But body plans do provide a succinct way to compare and contrast key animal features. They also are of interest in the study of evo-devo, the interface between evolution and development.

By 450 million years ago, groups that diversified during the Cambrian period began to

make an impact on land. Arthropods were the first animals to adapt to terrestrial habitats, as indicated by fragments of arthropod remains and by well-preserved fossils from several continents of millipedes, centipedes, and spiders. Another clue is seen in fossilized fern galls—enlarged cavities that fern plants form in response to stimulation by resident insects, which then use the galls for protection.

To date, biologists have identified 1.3 million extant species of animals, and estimates of the actual number run far higher. This vast diversity encompasses a spectacular range of

morphological variation, from corals to cockroaches to crocodiles. Various studies suggest that this great diversity originated during the last billion years. For example, researchers have unearthed 710-million-year-old sediments containing chemical evidence of steroids that today are primarily produced by a particular group of sponges. Since sponges are animals, these "fossil steroids" suggest that animals had arisen by 710 million years ago.

DNA analyses generally agree with this fossil biochemical evidence; for example,

one recent molecular clock study estimated that sponges originated about 700 million years ago. These findings are also consistent with molecular analyses suggesting that the common ancestor of all extant animal species lived about 770 million years ago.

Ecdysozoa

refers to a characteristic shared by nematodes, arthropods, and some of the other ecdysozoan phyla that are not included in our survey. These animals secrete external skeletons (exoskeletons); the stiff covering of a cricket and the flexible cuticle of a nematode are examples. As the animal grows, it molts, squirming out of its old exoskeleton and secreting a larger one. The process of shedding the old exoskeleton is called ecdysis. Though named for this characteristic, the clade was proposed mainly on the basis of molecular data that support the common ancestry of its members. Furthermore, some taxa excluded from this clade by their molecular data, such as certain species of leeches, do in fact molt.

Lophotrochozoa

refers to two different features observed in some animals belonging to this clade. Some lophotrochozoans, such as ectoprocts, develop a unique structure called a lophophore (from the Greek lophos, crest, and pherein, to carry), a crown of ciliated tentacles that function in feeding (Figure 32.12a). Individuals in other phyla, including molluscs and annelids, go through a distinctive developmental stage called the trochophore larva (Figure 32.12b)—hence the name lophotrochozoan.

Like Cloudina, some other small Ediacaran animals had

shells or other defensive structures that may have been selected for by predators. Overall, the fossil evidence indicates that the Ediacaran was a time of increasing animal diversity—a trend that continued in the Paleozoic.

In most animals, hox genes regulate

the formation of the anterior-posterior (front-to-back) axis, as well as other aspects of development. Similar sets of conserved genes govern the development of both flies and humans, despite their obvious differences and hundreds of millions of years of divergent evolution.

Ectoderm

the germ layer covering the surface of the embryo, gives rise to the outer covering of the animal and, in some phyla, to the central nervous system.

Endoderm

the innermost germ layer, lines the pouch that forms during gastrulation (the archenteron) and gives rise to the lining of the digestive tract (or cavity) and to the lining of organs such as the liver and lungs of vertebrates.

Gastrulation

the layers of embryonic tissues that will develop into adult body parts are produced. The resulting developmental stage is called a gastrula.

Terms such as coelomates and pseudocoelomates refer to organisms that have a similar body plan and hence belong to the same grade. However, phylogenetic studies show that

true coeloms and pseudocoeloms have been independently gained or lost multiple times in the course of animal evolution. As shown by this example, a grade is not necessarily equivalent to a clade (a group that includes an ancestral species and all of its descendants). Thus, while terms such as coelomate or pseudocoelomate can be helpful in describing an organism's features, these terms must be interpreted with caution when seeking to understand evolutionary history.