Chapter 1. Animal Form and Function Module

Introduction

Animals are the focus of several chapters in Biology: How Life Works. Here, we compile all of the text and art on animal form and function from Chapters 5, 10, 28, 35, 37, and 44 into a single module. This module can serve as an introduction to the chapters on animal anatomy and physiology (Chapters 35-43).

Core Concepts:

  • A.1 Animals have a limited number of distinct body plans.
  • A.2 Most animals have four different types of tissue, which combine to form organs.
  • A.3 Homeostasis actively maintains a stable internal environment.
  • A.4 Animals first evolved more than 600 million years ago in the oceans, and by 500 million years ago the major structural and functional body plans were in place.

Animals move, feed, and behave in different ways. Indeed, animals have evolved a remarkable variety of structures to fulfill these and other functions. Consider, for example, the different ways animals move (Chapter 37). Sponges have no muscle cells at all. In jellyfish, muscle fibers contract to squeeze a fluid-filled cavity, powering movement by jet propulsion. Mammals have limbs for locomotion, powered by the coordinated actions of muscles attached to an internal skeleton. Insects do much the same, but their skeleton is external. Other organ systems show comparable variation among animals. The wide range of functions made possible by these anatomical and physiological variations has permitted animals to diversify to a degree unmatched by plants, fungi, algae, or protozoans.

Biologists have described about 1.8 million species of eukaryotic organisms from the world’s forests, deserts, grasslands, and oceans. Of these, about 1.3 million species are animals. There is reason to believe that animal diversification began in the oceans, but today most animal species are found on land, and a majority of all animal species are insects. In this chapter, we look at animals as a group, focusing on their basic form, function, and evolutionary history.

A.1 Animal Body Plans

As discussed in Chapter 23, it is relatively easy to understand how the body plans of humans, chimpanzees, and gorillas show them to be more closely related to one another than any of them is to other animal species. Furthermore, it isn’t hard to see that humans, chimpanzees, and gorillas are more closely related to monkeys than they are to lemurs, and that humans, chimpanzees, gorillas, and monkeys are more closely related to lemurs than to horses. Anatomy and morphology reveal key evolutionary relationships among vertebrate animals, but how do we come to understand the place of vertebrates in a broader tree that includes all animals? More generally, how can we construct an animal phylogeny that includes organisms with body plans as different as those of sponges, sea stars, earthworms, and mussels?

What's an animal?

Let’s begin by asking what an animal is. What features differentiate animals from all other organisms? And, more specifically, what features differentiate animals from their nearest relatives?

As introduced in Chapter 27, the organisms most closely related to animals are choanoflagellates, a type of protist (a single-celled eukaryote). This relationship was first proposed in the nineteenth century on the basis of cell shape and was confirmed in the twenty-first century by molecular sequence comparisons. Choanoflagellates are unicellular, whereas animals are multicellular. However, multicellularity is not a unique feature of animals. Plants and fungi are multicellular as well. Animals are heterotrophs, gaining energy and carbon from preformed organic molecules (Chapter 6). This character differentiates animals from plants, but not from fungi, which are also heterotrophs. Animal cells lack cell walls, allowing them to move during development and in the adult (Chapter 28). This character differentiates animals from both plants and fungi. The ability of animal cells to move allows for a pattern of early embryological development that includes gastrulation (Chapters 20, 28, and 42), a process unique to animals.

Animals can therefore be described as multicellular heterotrophic eukaryotes that form a gastrula during development. Other features, both molecular and biochemical, also distinguish animals from other organisms. For example, unlike other organisms, animals synthesize the structural protein collagen.

Animals can be classified based on type of symmetry.

Early in the history of biology, taxonomists such as Carolus Linnaeus recognized that for all their diversity, animals have a limited number of distinctive body plans. Animals with the same type of body plan can be placed into a group called a phylum. The problem for biologists has long been how to understand the evolutionary relationships among animal phyla, which is still an active field of research.

In Chapter 23, we saw that the fundamental mechanism by which biological diversity increases is speciation, the divergence of two populations from a common ancestor. Played out repeatedly through time, speciation gives rise to a treelike pattern of evolutionary relatedness, with more closely related taxa branching from points close to the tips of the tree and more distantly related taxa diverging from branch points nearer its base. Distinctive features of organisms, called characters, evolve throughout evolutionary history, and the time when they arose can be estimated from their shared presence in the descendants of the population in which the character first evolved. Often the characters analyzed are structural features.

Figure 44.1 shows a simple phylogenetic tree of animals. This tree indicates that animals are closely related to choanoflagellates, but differ from them in the presence (among other features) of persistent multicellularity, the formation of a gastrula during early development, and the synthesis of the structural protein collagen. Sponges (Poriferans) have only a few different cell types, and these cells are not organized into highly organized tissues or organs. Cnidarians (the group that contains jellyfish, corals, and sea anemones) have simple tissues, but do not have complex organs. Bilaterians (most other animals, including humans, insects, and snails) have both well-defined tissues as well as complex organs. These organs are specialized for movement, digestion, and gas exchange, among other functions.

Early biologists could see that animals also differ in the symmetry of their bodies (Fig. 44.2). Sponges are irregular in form, with no plane of symmetry. In contrast, cnidarians (jellyfish, corals, and sea anemones) display radial symmetry, meaning that their bodies have an axis that runs from mouth to base with many planes of symmetry through this axis (Fig. 44.2a). This organization allows jellyfish to move up and down in the water column by flexing muscles around their bell-like bodies, and permits sea anemones and corals to wave their ring of food-gathering tentacles in all directions at once.

Bilaterians, as their name suggests, show bilateral symmetry: Their bodies have a distinct head and tail, front and back, and right and left, with a single plane of symmetry running between right and left at the midline (Fig. 44.2b). Bilateral symmetry enables animals to move in one horizontal direction to capture prey, find shelter, and escape from enemies. It also allows the development of specialized sensory organs at the front end for guidance (Chapter 36), and specialized appendages along both sides for locomotion, grasping, or defense.

Note that the phylogenetic tree shown in Fig. 44.1 does not tell us that sponges are older than more complex animals. Instead, it says that sponges and more complex animals diverged from a common ancestor, but it doesn’t tell us what that common ancestor looked like. Similarly, cnidarians and animals with bilateral symmetry diverged from their last common ancestor at a single point in time and so are equally old, but structurally distinct.

Animals can be classified based on number of germ layers.

New insights about animals became possible with the advent of microscopes that enabled the direct study of early animal embryos. One important observation was that some animals that look very different as adults share patterns of early embryological development. For example, adult sea stars and catfish look very different from each other, but their embryos show a number of key similarities, including the way that the early cell divisions occur and the number of embryonic tissue layers they develop.

In cnidarians (radially symmetrical animals), the embryo has two germ layers, the endoderm and the ectoderm, from which the adult tissues develop. These animals are therefore called diploblastic (Fig. 44.4). In bilaterians (bilaterally symmetrical animals), the embryo has three germ layers, with the mesoderm between the endoderm and ectoderm. These animals are therefore triploblastic (Fig. 44.4).

Comparative embryology also enabled biologists to divide bilaterian animals into the two groups shown in Fig. A.1, which are called protostomes (from the Greek for “first mouth”) and deuterostomes (from the Greek for “second mouth”). In protostomes, the earliest-forming opening to the internal cavity of the developing embryo, called the blastopore, becomes the mouth. Protostomes include such groups as nematodes (roundworms), arthropods (including insects, spiders, and crustaceans), mollusks (gastropods like snails and slugs; bivalves like clams and mussels; and cephalopods like squids and octopus), annelid worms (segmented worms), and flatworms (Fig. A.2).

In deuterostomes, the blastopore becomes the anus, and the mouth forms second. Deuterostomes include chordates (such as vertebrates) and echinoderms (such as sea stars, sea urchins, and sand dollars). However, only in the age of molecular sequence comparisons have the relationships among phyla within each of these two groups become clear.

Molecular sequence comparisons have confirmed some relationships and raised new questions.

Anatomical and embryological observations by early biologists laid the groundwork for understanding animal phylogeny, but, as was true for phylogenetic relationships in Bacteria, Archaea, and Eukarya as a whole, a modern understanding of evolutionary relationships among animals had to wait for the revolution in molecular sequencing. Over the past two decades, comparisons among DNA, RNA, and amino acid sequences have greatly improved our understanding of animal phylogeny. Molecular comparisons support many of the conclusions reached earlier on the basis of comparative anatomy and embryonic development, including the early divergence of sponges and the separation of radially and bilaterally symmetrical animals. Moreover, molecular sequence comparisons confirm the view that choanoflagellates are the closest protistan relatives of animals.

Other hypotheses have been rejected. For example, some biologists pointed to the presence or absence of a cavity, or coelom, surrounding the gut, as a character that could be used to divide bilaterians into three groups: those without a body cavity (acoelomates) and those with a body cavity (coelomates and pseudocoelomates, which differ in the embryonic origin of the cells lining the cavity). These body plans are shown in Fig. 44.3. A body cavity cushions the internal organs against hard blows to the body and enables the body to turn without twisting these organs. However, the traditional phylogenetic division of bilaterians into acoelomate, coelomate, and pseudocoelomate groups gets little support from molecular studies. Similarly, molecular sequence comparisons do not support the once widespread view that segmented bodies indicate a close relationship between earthworms and lobsters.

Many animals have a brain and specialized sensory organs at the front of the body.

A notable feature of many bilaterian animals is that nervous system tissue, including specialized sense organs such as eyes, becomes concentrated at one end of the body. For example, the eyespots of flatworms are located at one end of its body, as are the brain and sense organs of earthworms, squids, and insects (Fig. A.3). The concentration of nervous system components at one end of the body, defined as the “front,” is referred to as cephalization. Cephalization is a key feature of the body plan of most multicellular animals, including vertebrates.

Cephalization evolved independently multiple times in different animal groups and is therefore thought to confer certain advantages. Cephalization is considered an adaptation for forward locomotion because it allows animals to take in sensory information from the environment ahead of them as they move forward. In addition, the nearness of the sensory organs to the brain makes it possible to process this information quickly to enable a suitable behavioral response. As the quality and amount of sensory information taken in increased, brain size and complexity increased. Cephalization is also considered to be an adaptation for predation, allowing animals to better detect and capture prey.

Although cephalization is a feature of many animals, it has been particularly well studied in vertebrates. In vertebrates, the brain, many sense organs, and mouth are all located in the head. Vertebrates also evolved several novel features, including a jaw, teeth, and tongue. These are all thought to be adaptations for predation, or more generally the acquisition and processing of food.

As a result of evolutionary selection for enhanced sensory perception and the ability to respond to important cues in the environment, the brain, sensory organs, and nervous system of many animals are complex in their organization. These are linked to more sophisticated abilities that allow for a broad range of behaviors. These abilities are critical to the success of both predators and prey and underlie the complex interactions that occur among members of a species when they mate, reproduce and disperse, and care for their young.

Some animals also show segmentation.

In addition to cephalization, another broad pattern that is evident when considering the body plans of bilaterian animals is segmentation. Segmentation is the organization of the body into units, or segments, that are repeated from front to back (along the anterior-posterior axis), but modified depending on where they are in the body.

Insects, for example, have a head, thorax, and abdomen, and each of these regions can be further subdivided into segments. The three pairs of legs are a good example of the three segments that make up the thorax. In Chapter 20, we considered some of the genetic mechanisms that generate this segmented pattern in the fruit fly Drosophila. Humans and other vertebrates also develop in a segmented fashion. This is most evident by observing the repeated pattern of vertebrae and nerves that make up the backbone and spinal cord.

Three animal phyla are known for showing a segmented body plan. These are the arthropods (including insects, spiders, and crustaceans), the annelids (ringed or segmented worms), and the chordates (including vertebrates). Some of the genetic and developmental mechanisms underlying segmentation are different in these three groups, suggesting that they might have evolved independently of one another, but this is still an open question and an active area of research. Segmentation may have evolved as an adaptation for specialized forms of movement and development of different body parts.

A.2 Tissues and Organs

As we have seen, animals show a diversity of form. However, most animals organize their cells into just four different types of tissues: epithelial, connective, muscle, and nervous. Tissues are collections of cells that carry out a specific function (Chapter 10). Tissues are combined in various ways to make organs, such as the heart, lung, or kidneys. Most organs are made up of several different tissue types. For example, the skin is made up of two layers: the outer layer is the epidermis, which is epithelial tissue, and the inner layer is the dermis, which is connective tissue (Fig. A.4). In this section, we discuss the four tissue types that combine to make organs in animals.

Most animals have four types of tissue.

With the exception of sponges, animal cells are organized into different kinds of tissue. Epithelial tissue provides a lining for all of the spaces inside and outside of the body. The outer layer of the skin is epithelial tissue, as is the lining of the gut, bladder, and blood vessels. The cells that make up epithelial tissue are closely packed together and connected by cellular junctions (Chapter 10), forming a continuous sheet of cells. This sheet provides a boundary and controls the movement of substances into and out of the body.

Epithelial tissue is classified based on certain characteristics. The first characteristic is layering: A single layer of cells is simple, and more than one layer is stratified. The second characteristic is the shape of the cells: Flat cells are squamous, round or square cells are cuboidal, and tall cells are columnar. For example, a single layer of flat cells is called simple squamous epithelium and this type of epithelial tissue is found lining blood vessels and the interior surface of the lung. Some epithelial tissues have special features, such as a layer of the protein keratin in the case of the skin or cilia in the case of the upper airways.

Epithelial tissue not only forms a boundary, but also may absorb substances from and secrete substances into the space it surrounds. In the gut, for example, the epithelial lining absorbs nutrients. Notably, epithelial tissue has no blood vessels, so it gets its blood supply from the underlying tissue, which is connective tissue.

Connective tissue underlies all epithelial tissues, and is found elsewhere as well. In contrast to epithelial tissue, which is composed of closely packed cells, connective tissue has extensive extracellular matrix and few cells. The extracellular matrix is an insoluble meshwork composed of proteins and polysaccharides (Fig. 10.16). Its components are synthesized, secreted, and modified by the cells that reside within the connective tissue. There are many different forms of extracellular matrix, which differ in the amount, type, and organization of the proteins and polysaccharides that make them up. The extracellular matrix not only contributes structural support, but also provides informational cues that determine the activity of the cells that are in contact with it.

Therefore, connective tissue is characterized by the properties of the extracellular matrix. For example, beneath the epidermis of the skin, which is stratified squamous epithelium, there is a basal lamina (a specialized type of connective tissue) and then the dermis, a type of connective tissue made up of cells called fibroblasts that secrete extracellular matrix (Fig. 10.18). The dermis is strong and flexible because its extracellular matrix is composed of tough protein fibers. The dermis contains blood vessels that nourish both the dermis and the overlying epidermis, which has no blood vessels. The dermis also provides a cushion for the body.

The dermis of the skin is one of several types of connective tissue. The dermis is simply called connective tissue. There is also supporting connective tissue, including bones, cartilage, tendons, and ligaments, which help to support the body and provide a system of levers for movement (Chapter 37). Finally, there is specialized connective tissue, including adipose tissue (fat) and blood.

Muscle tissue is made up of cells (called fibers) that are able to shorten or contract (Chapter 37). These specialized cells contain actin thin filaments and myosin thick filaments. Myosin is a motor protein that uses the energy of ATP to change conformation. This conformational change moves the thin filament relative to the thick filament, resulting in the shortening of individual muscle cells and contraction of the entire muscle tissue (Fig. 37.6).

There are three different types of muscle tissue in vertebrate animals: skeletal muscle attaches to bone and allows for voluntary movements; cardiac muscle is found in the heart and contracts to make the heart beat; and smooth muscle is under involuntary control and can be found in such places as the gut, causing waves of contraction to allow food to move through it, and blood vessels, causing constriction to control blood flow (Fig. 37.2).

Nervous tissue is the fourth type of animal tissue (Chapters 35 and 36). Nervous tissue can be found, for example, in the nerve nets of cnidarians and in the brain, spinal cord, and peripheral nerves of vertebrates. It functions to take in sensory information from the environment, process information, and send signals to target organs to elicit a response. For example, signals sent to muscles allow for movement and signals sent to various organs in the body help to maintain a relatively constant internal state, called homeostasis.

Nervous tissue is made up primarily of neurons (nerve cells) that send electrical impulses from one end of the cell to the other (Fig. 35.4). They communicate with each other at specialized junctions called synapses, where chemical signals are released by one nerve cell and received by another. The ability of nerve cells to communicate quickly and specifically and to form networks allow for rapid decision making and complex behaviors.

One or more tissues combine to form organs.

One or more tissues can combine to make an organ. For example, the intestine includes all four types of tissue – epithelial, connective, muscle, and nervous. In turn, organs may combine to form an organ system. For example, the intestine is one organ of the digestive system, which also includes the stomach, liver, pancreas and other organs (Chapter 40).

Sponges, as we have seen, do not organize their cells into tissues. Jellyfish, corals, and sea anemones (cnidarians) have tissues, but not true organs (Fig. 44.1). They have a set of nerves called a nerve net, but these nerves are not organized into a brain or central nervous system. They exchange gases by diffusion rather than using lungs or gills, and they digest food in a central cavity with a single opening for both eating and excretion.

By contrast, mollusks, arthropods, mammals, and other bilaterians have true organs made up of one or more types of tissue (Fig. 44.1). These include such organs as the heart for pumping blood (Chapter 39), gills or lungs for gas exchange (Chapter 39), stomach and intestine for nutrient absorption as substances pass through a digestive tract with two openings (Chapter 40), and kidneys for water balance and waste excretion (Chapter 41). Many of these organs play important roles in maintaining an internal environment compatible with life, a topic we turn to next.

A.3 Homeostasis

Animals often experience a range of different environments, but all of them are able to maintain many physiological functions such as body temperature and blood pH within a relatively narrow range. The active maintenance of stable conditions is called homeostasis. Homeostasis is a critical feature of cells and of life itself. In this section, we examine how animals are able to maintain homeostasis.

Homeostasis is the active maintenance of stable conditions inside of cells and organisms.

In Chapter 5, we looked at how cells maintain homeostasis. The environment outside of cells may change, but the environment inside of cells is relatively constant. Chemical reactions and protein folding, for example, are carried out efficiently only within a narrow range of conditions, such as pH range or salt concentration.

It is the selectively permeable plasma membrane of cells that actively maintains intracellular conditions compatible with life. For example, the plasma membrane keeps ion concentrations within narrow ranges for normal cell function. The firing of action potentials by neurons is an example of a cell function that requires particular ion concentrations on either side of the membrane. Recent evidence suggests that nerve cell firing rates are maintained at a steady level in the brain of rats, regardless of sensory stimulus or deprivation, or whether the animals are awake or asleep. Temperature and pH are other parameters that do not vary much because enzymes often work effectively only in narrow temperature and pH ranges.

Homeostasis is also maintained for the whole body. Many physiological parameters are maintained in a narrow range of conditions throughout the body, including temperature, heart rate, blood pressure, blood sugar, blood pH, and ion concentrations. Similarly, the water content of the body as a whole is kept stable through the careful regulation of ions and other solutes, as discussed in Chapter 41.

The concept of homeostasis was first described as regulation of the body’s “interior milieu” in the late 1800s by the French physiologist Claude Bernard, who is often credited with bringing the scientific method to the field of medicine. The term “homeostasis” was coined by the American physiologist Walter Cannon, whose book The Wisdom of the Body (first published in 1932) popularized the concept.

Maintaining steady and stable conditions takes work in the face of changing environmental conditions. That is, a cell (or organism) actively maintains homeostasis. For example, long periods of drought challenge an animal’s ability to remain hydrated and maintain a stable water and ion balance. Animals facing drought must respond rapidly by changing the permeability of their skin and respiratory organs so that they can retain as much water as possible.

Homeostasis is often achieved by negative feedback.

How does the body maintain homeostasis? Homeostatic regulation often depends on negative feedback (Fig. 35.18). In negative feedback, a stimulus acts on a sensor that communicates with an effector, which produces a response that opposes the initial stimulus. For example, negative feedback is used to maintain a constant temperature in a house. Cool temperature (the stimulus) is detected by a thermostat (the sensor). The thermostat sends a signal to the heater (the effector), producing heat (the response). The response (heat) opposes the initial stimulus (cool temperature), and no further heat will be produced until the temperature drops below the temperature setting of the thermostat. In this way, a stable temperature is maintained (Fig. 35.18a).

In a similar way, humans and other mammals maintain a steady body temperature even as the temperature outside fluctuates. Nerve cells in the hypothalamus (located in the base of the brain) act as the body’s thermostat (Fig. 35.18b). When a decrease in the temperature in the environment causes a drop in body temperature, the lowered body temperature signals the hypothalamus to activate the nervous system to induce shivering and the production of metabolic heat, as discussed in Chapter 40. At the same time, the hypothalamus activates nerves that cause peripheral blood vessels to constrict. The reduction in blood flow near the body’s surface reduces heat loss to the surrounding air. By contrast, an increase in temperature signals sweat glands to secrete moisture and peripheral blood vessels to dilate to aid heat loss from the skin.

Homeostatic regulation, therefore, relies on negative feedback to maintain a set point, which in this case represents an animal’s preferred body temperature. The ability to maintain a constant body temperature is known as thermoregulation, and it is just one of many physiological set points that the body actively maintains, as we discuss in subsequent chapters.

A.4 The Evolutionary History of Animals

Because many animals form mineralized skeletons, sedimentary rocks deposited throughout their evolutionary history over the past 541 million years contain a rich fossil record of shells and bones. Animals also leave a sedimentary calling card in their tracks, trails, and burrows, again widespread in sedimentary rocks formed after 541 million years ago. Not all animal phyla are well represented in the fossil record—fossil annelids (segmented worms), for example, are rare and fossil flatworms are essentially unknown because they lack mineralized body parts (Chapter 23). In contrast, the fossilized shells and bones of bivalve mollusks (such as clams and mussels), brachiopods (marine shelled animals), echinoderms (such as sea stars and sea urchins), and mammals preserve an excellent record of evolutionary history within these groups.

Phylogenies make predictions about the evolutionary history of animals, and fossils confirm the predictions of comparative biology and record when innovations in morphology and function first appeared. Fossils preserve the remains of now extinct species that often resemble but are distinct from modern groups. They can show intermediate combinations of traits not seen in modern animals. And they underscore the evolutionary importance of extinctions, including a small number of events that removed a majority of existing species, paving the way for renewed diversification among survivors. If we step back and look at the big picture, what do fossils tell us about the evolutionary history of the animal kingdom?

Fossils and phylogeny show that animal forms were initially simple but rapidly evolved complexity.

Phylogeny suggests that animals are relative latecomers in evolutionary history, and the fossil record confirms this hypothesis. Life originated more than 3.5 billion years ago, but microorganisms were the only members of ecosystems for most of our planet’s history. As noted in Chapter 28, macroscopic fossils of organisms thought to be animals first appear in rocks deposited only 579 million years ago. Called Ediacaran fossils after the Ediacara Hills of South Australia where they were discovered, these fossils have simple shapes that are not easily classified among living animal groups (Fig. 44.39).

Phylogeny suggests that we should look for sponges, cnidarians (jellyfish, corals, and sea anemones), and other diploblastic animals among the oldest animal fossils, but sponges, at least, are rare among Ediacaran fossils. This is likely because sponges that do not make mineralized parts have left few fossils. Instead, a majority of Ediacaran fossils show simple, fluid-filled tubes, without identifiable mouths or other organs. Many may have formed colonies, gaining complexity through colonial growth and differentiation, as some living cnidarians do. These early animals had epithelia and are thought to have obtained food by taking in dissolved organic matter or phagocytosing small particles while exchanging gases by diffusion. Most Ediacaran fossils probably form an early branch or branches on the animal tree.

Why did the first animal radiation occur so late in the history of life? Scientists continue to debate this question, but part of the answer appears to lie in Earth’s environmental history. Geochemical data suggest that only during the Ediacaran Period did the atmosphere and oceans come to contain sufficient oxygen to support the metabolism of large, active animals.

The animal body plans we see today emerged during the Cambrian Period.

Ediacaran fossils differ markedly from the shapes of living animals, but in the next interval of geologic history, the Cambrian Period (541–485 million years ago), we begin to see the fossilized remains of animals with familiar body plans (Fig. 44.40). Cambrian fossils commonly include skeletons made of silica, calcium carbonate, and calcium phosphate minerals, and these record the presence of arthropods (insects, spiders, crabs, and their close relatives), echinoderms, mollusks, brachiopods, and other bilaterian animals in the oceans. The rocks also preserve complex tracks and burrows made by organisms with hydrostatic skeletons, muscular feet, and the jointed legs of arthropods. And in a few places, notably at Chengjiang in China and the Burgess Shale in Canada, unusual environmental conditions have preserved a treasure trove of animals that did not form mineralized skeletons (Chapter 23).

These exceptional windows on early animal evolution show that, during the first 40 million years of the Cambrian Period, the body plans characteristic of most bilaterian phyla took shape. This period of rapid diversification in the fossil record is sometimes called the Cambrian explosion. Sponges and cnidarians radiated as well, producing through time the biodiverse habitats of reefs and imparting an ecological structure to life in the sea broadly similar to what we see today.

Scientists sometimes argue about whether the name “Cambrian explosion” is apt. The fossil record makes it clear that bilaterian body plans did not suddenly appear fully formed, so the event was not truly “explosive.” Rather, fossils demonstrate a large accumulation of new characters in a relatively short period of time during which the key attributes of modern animal phyla emerged. For example, living arthropods have segmented bodies with a protective cuticle, jointed legs, other appendages specialized for feeding or sensing the environment, and compound eyes with many lenses. Cambrian fossils include the remains of organisms with some but not all of the major features present today in arthropods. In short, the first 40 million years of Cambrian animal evolution ushered in a world utterly distinct from anything known in the preceding 3 billion years.

Fossils show that animal diversity has been shaped by adaptive radiation and mass extinction over the past 500 million years.

The wealth of paleontological data we now have shows us a dynamic history of animal radiations and extinctions through time (Chapter 23). The graph shown in Fig. 44.41 is a tabulation of fossil occurrences through time, compiled from paleontological literature by American paleontologist Jack Sepkoski since the 1970s. In this figure, we see that despite the burst of body plan evolution recorded by Cambrian fossils, there were still relatively few species at the end of the Cambrian Period.

The following Ordovician Period (485–443 million years ago) was a time of renewed animal diversification, especially the evolution of heavily skeletonized animals in the world’s oceans. The number of genera recorded by fossils increased fivefold, suggesting that species diversity might have increased by an order of magnitude (most genera contain multiple species). The Ordovician radiation established a marine ecosystem that persisted for more than 200 million years.

Interestingly, if you had walked along an Ordovician beach, the shells washing about your feet would have been far different from the ones you see today. The dominant shells were those of brachiopods, not clams. Broken corals in the surf were the skeletons of now-extinct cnidarians only distantly related to modern reef-forming corals. And arthropod shells, molted during growth, were those of now-extinct trilobites, not lobsters or crabs. Why was the Paleozoic world so distinct from our own?

Another look at Fig. 44.41 provides the answer. At the end of the Permian Period, 252 million years ago, environmental catastrophe eliminated most genera in the oceans. Paleozoic coral-like cnidarians become extinct, as did the trilobites. Brachiopods survived as a group, but most species disappeared. As noted in Chapter 23, the likely trigger for this devastation was massive volcanism that unleashed global warming, ocean acidification, and oxygen loss from subsurface oceans.

As ecosystems recovered from this mass extinction, they came to be dominated by new groups descended from survivors of the extinction: Bivalves and gastropods (two groups of mollusks) diversified; new groups of arthropods radiated, including the ancestors of the crabs and shrimps we see today; and surviving cnidarians evolved a new ability to make skeletons of calcium carbonate, resulting in the corals that build modern reefs. In short, mass extinction reset the course of evolution, as it did four other times during the past 500 million years (Fig. 44.41).

Animals began to colonize the land 420 million years ago.

Animals began to colonize land only after plants had established themselves there. Land plants evolved during the Ordovician Period, about 460 million years ago, and arthropods followed onto the land by the time of the Silurian Period, about 420 million years ago, especially chelicerates that included the ancestors of spiders, mites, and scorpions. Insects, descended from crustacean ancestors that independently gained access to the land surface, appeared at about the same time.

The radiation of the major groups of insects, however, began 360 million years ago with a marked diversification of dragonflies and the ancestors of cockroaches and grasshoppers. Some of the dragonfly-like insects that darted among the plants of Carboniferous Period coal swamps had bodies the size of a lobster and 75-cm wing spans! You might well wonder what prevents insects from attaining this size today. These immense insects flew through an atmosphere that contained more oxygen than is present in the atmosphere today, and this higher level of atmospheric oxygen is thought to have been a necessary condition for these gigantic insects. After some time, levels of atmospheric oxygen declined to levels closer to those seen today, and the giant insects disappeared.

Flies, beetles, bees, wasps, butterflies, and moths radiated later, beginning in the early Mesozoic Era. Their rise in diversity parallels that of the flowering plants and reflects coevolution of these pollinators and flowering plants.

Tetrapod fossils first appear in sedimentary rocks deposited near the end of the Devonian Period, about the same time as the early insect radiations 360 million years ago. As discussed in Chapter 23, the fossil record documents in some detail the shifts in skull, trunk, and limb morphology that allowed vertebrate animals to colonize land. These include the evolution of muscled, articulated legs from fins, together with a set of strong, articulated digits where the limbs meet the ground; lungs and a rib cage to support the muscles that control breathing; and an erect, elevated head with eyes oriented for forward vision. The appearance of amphibians and then reptiles follows the predictions of phylogenies based on comparative biology.

Mass extinction has influenced evolution on land as well as in the sea. As new groups of invertebrate animals spread through Triassic oceans, new types of tetrapod diversified on land, including small bipedal reptiles that gave rise to the most remarkable animals ever to walk on land: the dinosaurs. From their beginnings about 210 million years ago, dinosaurs radiated to produce many hundreds of species, dominating terrestrial ecosystems until the end of the Cretaceous Period, 66 million years ago. At that time, another mass extinction, caused by a catastrophic asteroid impact, eliminated nearly all dinosaur species (Chapter 1). We say “nearly all” because the fossil record documents the evolutionary divergence of birds from a specific subgroup of dinosaurs about 150 million years ago.

Mammals have been the dominant vertebrates in most terrestrial ecosystems since the extinction of dinosaurs, but the group originated much earlier, at least 210 million years ago. During the age of dinosaurs, most mammals were small nocturnal or tree-dwelling animals that stayed out of the way of large dinosaurs, although fossils from China show clearly that the largest mammals of this interval ate small dinosaurs and their eggs. Mammals are dominant components of terrestrial ecosystems today in part because they survived the end-Cretaceous mass extinction.

Core Concepts Summary

A.1 Animals have a limited number of distinct body plans.

Early biologists grouped animals on the basis of shared features of adult bodies, such as symmetry.

Sponges have no plane of symmetry; cnidarians (jellyfish, corals, and sea anemones) are radially symmetric; and bilaterians (insects and vertebrates, for example) are bilaterally symmetric.

Cnidarians have two germ layers (ectoderm and endoderm), whereas bilaterians have three germ layers (ectoderm, mesoderm, and endoderm).

Cephalization is the concentration of the nervous system and special sensory organs at the front of the body.

Segmentation is the organization of the body into repeated units along the anterior-posterior (head-tail) axis.

A.2 Most animals have four different types of tissues, which combine to form organs.

Most animals, with the exception of sponges, organize their cells into tissues.

The four tissue types are epithelial, connective, muscle, and nervous.

Epithelial tissue lines spaces, such as cavities or the outside of the body.

Connective tissue is characterized by extensive extracellular matrix.

Muscle tissue is able to contract.

Nervous tissue is specialized for transmitting electrical impulses.

Most animals with tissues, with the exception of cnidarians, have organs, which are collections of tissues that perform a function.

Organs often work with other organs form an organ system.

A.3 Homeostasis actively maintains a stable internal environment.

Many parameters, such as blood pH, ion concentrations, and body temperature, are actively maintained in a narrow range by homeostasis.

Homeostasis is often achieved by negative feedback, in which the response inhibits the stimulus to maintain a set-point.

A.4 Animals first evolved more than 600 million years ago in the oceans, and by 500 million years ago the major structural and functional body plans were in place.

Ediacaran fossils from 579 million years ago provide evidence of early animals.

The Cambrian explosion was an interval of rapid diversification beginning 541 million years ago, during which time most of the animal body plans we see today first evolved.

Chelicerates and insects were the first animals to colonize the land, sometime after 420 million years ago.

Tetrapods first appear in the fossil record about 360 million years ago.

Mammals originated at least 210 million years ago, but became dominant only after the extinction of the nonavian dinosaurs 65 million years ago.

Mass extinctions have repeatedly changed the trajectory of animal evolution during the past 500 million years.

Self-Assessment

1. Draw a simplified animal tree of life, indicating the relationships among sponges, cnidarians, protostomes, and deuterostomes.

2. Name features that distinguish sponges, cnidarians, protostomes, and deuterostomes. Place these features on the phylogeny that you drew above.

3. Explain how cephalization is an adaption for predation.

4. Describe the properties and functions of the four animal tissue types.

5. Define homeostasis and provide one example of a condition that is maintained by homeostasis.

6. Describe the evolutionary significance of the Cambrian explosion.

7. Give two examples of how mass extinctions changed the ecological structure of life on Earth.