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.