Comparative Anatomy

I

Introduction

Comparative Anatomy, scientific study of the similarities and differences in the structure of living things. Comparative anatomy helps to show how organisms function, how they develop, and how they are linked by evolution, the process by which organisms change over many generations. The theory of evolution, one of the fundamental tenets of modern biology, states that new types of organisms develop from common ancestral types over long periods. Studying the body structures of various organisms often helps scientists determine how different species, or distinct kinds of organisms, are related to each other, as well as how and when they diverged from a common ancestor (see Species and Speciation).

Comparative anatomy can be used to investigate plants and simple microorganisms, but its most important role is in the study of animals. In animals, comparative anatomy usually focuses on living species, but scientists also investigate extinct species by examining fossils, body remains trapped in sediment or amber. With extinct animals, anatomists rarely have a chance to study soft body parts because these parts normally decay before they have a chance to fossilize. With living species, the entire body can be examined, giving a much fuller picture of how it functions. Anatomists also compare existing species with fossils to trace the path that evolution has followed and to gather information that is used in animal classification. Anatomical studies usually involve adult animals, but anatomists also investigate the way animal bodies reach their adult shape in a field of study called developmental anatomy.

Many important physical features can be seen on the outside of animal bodies, but often the most revealing ones are hidden inside. These hidden features provide valuable clues about an animal's distant ancestors. For example, an endangered reptile from New Zealand called the tuatara looks much like a lizard, and it was originally classified as a lizard back in the early 19th century. But in 1867 anatomist Albert Gunther, working at the British Museum in London, noticed that tuataras have some unusual features. Among these features are teeth that are permanently fused to their jaws rather than separate from the jaw like the teeth of lizards. From this evidence and other anatomical observations he concluded that tuataras are not lizards at all, but sphenodonts—the only surviving members of an ancient group of reptiles that flourished alongside the dinosaurs.

Most comparative anatomy studies involve gross anatomy, which deals with structures that are big enough to be seen with the naked eye. Smaller structures, such as individual cells, may also be investigated using the magnifying power of various types of microscopes. This field of study is called microscopic anatomy. In recent years, progress in molecular biology has enabled scientists to investigate still smaller structures, particularly deoxyribonucleic acid (DNA), the hereditary material in all living cells. DNA is made up of strings of four different subunits called nucleotide bases. Anatomists sometimes study the arrangement or sequence of these nucleotide bases in the DNA from different animals, looking for similarities and differences that provide clues to evolutionary family trees.

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II

The Animal Kingdom

Comparative anatomy is used in the study of all animal groups, but more work has been carried out on some animals than on others. Among invertebrates, or animals that lack a backbone, anatomists focus on a few major groups. Arthropods, which include crustaceans and insects, draw the attention of anatomists who are interested in finding out how the same basic body plan of a segmented body and jointed legs could give rise to such a stunning array of variations. The mollusks, a group of invertebrates that includes snails, clams, squids, and octopuses, have also been thoroughly studied. Squids and octopuses are of particular interest to scientists because they have the most highly developed nervous systems of all invertebrates and large eyes that work very much like those of humans. The anatomy of flatworms and roundworms has been thoroughly investigated because these two groups include many parasitic species, including some that infect humans (see Parasite).

Although invertebrates make up over 95 percent of animal species on Earth, work on their anatomy is still dwarfed by the studies carried out on vertebrates, animals that have internal bony skeletons. This is partly because these bony skeletons have left a fossil record of unparalleled richness, which anatomists draw on when comparing one species with another. In addition, vertebrates are the group to which humans belong, so anatomists are interested in studying them to find out how humans evolved.

Anatomical studies of vertebrates show how the same underlying body systems have adapted to life in water, on land, and in the air. In laboratory studies, a handful of animals—including the dogfish (a type of small shark), frog, pigeon, and rat—are used as standard examples of vertebrate anatomy. Anatomists also have studied thousands of other species in detail in an effort to piece together exactly how vertebrates have evolved.

Some of that history has been pieced together by studying tunicates and lancelets, sea-dwelling invertebrates that are closely related to vertebrates. Despite being brainless and boneless, tunicates and lancelets clearly show some of the physical characteristics that were key to vertebrate success. For example, tunicates use a series of slits for feeding. These slits developed into gills in fish, resulting in an efficient mechanism for extracting oxygen from the water. Lancelets have a stiff structure called a notochord, which enables them to swim efficiently. The vertebral column, which has replaced the notochord in vertebrates, is even more efficient. Such characteristics allowed vertebrates to diversify rapidly and become the most complex animals on Earth.

III

Principles of Comparative Anatomy

Despite the variety and complexity of animal life, several key anatomical features divide up the animal world. One of these features is symmetry, meaning that an animal’s body parts are the same in size, shape, and position on either side of a dividing line or central axis. Several groups of marine animals—including the cnidarians (jellyfish, sea anemones, and corals), comb jellies, and echinoderms (sea stars, sea urchins, and their relatives)—are radially symmetrical. Their body parts are arranged around a central axis like spokes in a wheel. Almost all other animals, including vertebrates, are bilaterally symmetrical, with two halves arranged on either side of a central dividing line.

Bilateral symmetry is often not quite as perfect as it seems. The human body looks more or less symmetrical from outside, but many internal organs are arranged in an asymmetrical way. For example, the liver lies mostly on the right side of the body’s dividing line, while the stomach is mostly on the left. In some animals, asymmetry goes much further. A sperm whale has a single blowhole on the left side of its head, while a fish known as a winter flounder has both eyes on the right side. Male fiddler crabs have one small pincer, which is used for feeding, and one giant one, which is used for signaling during courtship. This giant pincer can be either on the right or the left, and it often weighs as much as the rest of the body put together.

Some bilateral animals, notably annelid worms (such as earthworms) and arthropods, show a characteristic known as segmentation. Segments, known to biologists as metameres, repeat from front to back of the animal’s body. The segments are all built on the same plan: Each one of an earthworm’s segments contains nerves, blood vessels, and excretory organs called nephridia arranged in the same pattern.

Many bilaterally symmetrical animals also show a feature known as cephalization, a trend toward 'front-end' development. Some animals with only rudimentary cephalization simply have a distinct front end that leads the way when the animal moves. But in other animals, the front region, or head, has become the part of the body that houses the brain and most of the sense organs. Particularly noticeable in arthropods and vertebrates, cephalization gives active animals the earliest possible information about food, danger, and other aspects of the environment ahead.

In comparing two species, anatomists have to be careful to differentiate between homologous structures, which are ones that have evolved from a shared ancestor, and analogous structures, which have developed from different origins. Homologous structures are built on the same underlying plan. A human arm, a bat’s wing, and a whale’s flipper look quite different from the outside, but the bones inside reveal that these limbs all have the same basic structure. Analogous structures, by contrast, often look similar, but their similarities are only skin deep. A fish's tail fin and a whale's flukes are analogous structures—they look similar from the outside and perform similar functions, but their underlying structures are quite different. Homologous structures are evidence that two species have a shared ancestry. However, analogous structures most often indicate that two unrelated species evolved in a similar environment where both developed structures to perform the same function.

IV

Animal Body Systems

A complete anatomical study probes more than a dozen different body systems, from the skeletal and muscular systems, which support and move the body, to the nervous and sensory systems, which enable an animal to interact with its surroundings. To anatomists interested in evolutionary relationships, the underlying structure of each system is often more significant than the exact size or shape of its parts. Evolution changes the individual parts of a system more rapidly than the underlying pattern of how a system or animal is put together. Thus, such underlying patterns often remain intact, providing clues to how species are related.