Introduction to Biology

Biology, the science that deals with living things. The earth is home to a diverse range of living organisms. An estimated 10 million species live on the planet. All living things, despite their diversity, have some characteristics in common. Biology is concerned with all these forms of life—from humans to microbes, from whales and trees to insects and molds. Biologists study the makeup of living things, how they live, and the physical and chemical changes that occur within them.

Biology is important because people themselves are animals and live in a world filled with an enormous diversity of other living things. Medicine, agriculture, and engineering are just three of the many fields that benefit from the knowledge gained from biological studies. Through the study of biology, people have learned how to adapt nature to their own needs—by breeding superior cattle, for example, and by using fertilizers to improve crops. Advanced research in biology has led to the discovery of better ways to prevent and treat diseases. Wildlife conservation has been made possible through research in ecology.

Biology is traditionally classified into botany and zoology. Apart from botany and zoology, biology can also be divided into ecology, physiology, and systematics. Ecology is the study of the relationship between the living things and their environment. Physiology is the study of various biological functions of the living thing, such as respiration, digestion and reproduction. Systematics deals with classification of living things. Systematics is also called taxonomy.

The study of living things involves other sciences as well. To understand various biological functions and processes, biologists use data from physics, chemistry, and statistics.

This article is about the specialized areas biologists work in, the history of biology, and the various advances in the field of biology.

Major fields of biology
Anatomy deals with the structure of living things.
Bacteriology is the study of bacteria.
Biochemistry examines the chemical processes and substances that occur in living things.
Biophysics applies the tools and techniques of physics to the study of living things.
Botany is the study of plants.
Cryobiology analyzes how extremely low temperatures affect living things.
Cytology studies the structure, composition, and functions of cells.
Ecology concerns the relationships living things have with one another and their environment.
Embryology deals with the formation and development of plants and animals from fertilization until they become independent organisms.
Entomology is the study of insects.
Ethology concerns animal behavior under natural conditions.
Evolutionary biology is the study of the evidence supporting the theory of evolution.
Genetics is the study of heredity.
Ichthyology is the study of fishes.
Immunology concerns the body's defenses against disease and foreign substances.
Limnology studies bodies of fresh water and the organisms that live in them.
Marine biology investigates life in the ocean.
Medicine is the science and art of treating and healing.
Microbiology deals with microscopic organisms.
Molecular biology analyzes molecular processes in cells.
Neurobiology deals with the nervous system of animals.
Ornithology is the study of birds.
Paleontology is the study of prehistoric life.
Pathology examines the changes in the body that can cause disease or are caused by disease.
Physiology deals with the functions of living things.
Sociobiology focuses on the biological basis for social behavior in human beings and other animals.
Systematics, also called taxonomy, is the scientific classification of organisms.
Virology concerns viruses and virus diseases.
Zoology is the study of animals.

What Are Living Things?

A living thing is called an organism. Life on earth is represented by a great variety of different kinds of organisms. Organisms are often described as being simple or complex. Such organisms as amoebas and bacteria are called simple because they are small, and when compared with such complex organisms as trees or elephants, they are seemingly simple. However, no living organism, no matter how small, is in reality simple. The descriptions of simple and complex are useful only for making comparisons. The larger, complex organisms are distinguished by having specialized parts, such as hearts or leaves, that perform special jobs.

Simple organisms are probably similar to primitive living things that appeared early on the earth. During millions of years, the larger, complex organisms are thought to have developed from simple organisms through the process of evolution. The theory of evolution is one of the basic concepts of biology: it explains why there are so many different kinds of organisms.

Living things are divided into five groups, or kingdoms—the monera (bacteria and blue-green algae), protist (protozoans, slime molds, and algae), fungus (mushrooms, molds, and lichens), plant (mosses, ferns, cone-bearing plants, and flowering plants), and animal kingdoms. Organisms within each kingdom have certain basic similarities. Living things formerly were divided into two kingdoms—the plant kingdom and the animal kingdom.

Despite the vast differences between the simple and complex forms, all living organisms have certain qualities or properties in common. The basic unit that is fundamental to all living matter is the cell. Cells may differ in details of structure and function, but there are fundamental similarities in their composition and in the chemical reactions they perform.

Cells contain genes—molecules of DNA (deoxyribonucleic acid). It is DNA that determines the characteristics of an organism and directs all the chemical activities carried out by the cell, and ultimately by the whole organism. For example, its genes control what kinds of proteins a cell makes. Many proteins serve as enzymes—large molecules without which the chemical reactions could not take place.

Chemical activities, collectively called metabolism, take place in all living things, and all organisms need a source of energy to carry out their activities. Living organisms obtain energy from the chemical breakdown of food substances. Organisms such as plants and algae manufacture their own food, using the energy from sunlight, through the process of photosynthesis. Organisms that cannot make their own food, such as animals, must obtain it from their surroundings. The energy obtained from the breakdown of foods may be used immediately or may be stored in such chemical compounds as carbohydrates or fats for later use. Involved in using foodstuffs are such life processes as digestion, absorption, assimilation, and excretion.

Living things grow (increase in size) in an organized pattern. They also go through a process of development, during which the various parts of the organisms take shape, mature, and assume their adult functions. The simple forms of life, such as bacteria and amoebas, show comparatively little development. In more complex organisms, development is a complicated process that may take many years.

Living matter has the ability to reproduce itself so that the characteristics of each type of organism are carried on from one generation to the next. It is one of the fundamental principles of biology that life comes only from living things. Scientists believe that the original source of life was a mixture of organic compounds present on the earth billions of years ago.

Another characteristic of life is that living things are able to react or respond to conditions outside themselves. The stems of most plants, for example, turn toward a source of light. The ability to react to outside conditions is even more developed in complex animals than in plants. Living things also have the power of movement. In many organisms, however, this ability is quite limited.

Except possibly for some types of bacteria, every organism relies in some way upon some other form of life. Some plants—red clover, for example—can produce seeds only if their pollen is carried from one flower to another by insects. In some cases, a special type of relationship, called symbiosis, has developed between two entirely different kinds of organisms.

Protozoans, animals, and other organisms that cannot manufacture food must get their food from food-making organisms such as algae and plants. For example, an animal gets its food by eating either a plant or a plant-eating animal. When living things die, their tissues are broken down by decay through the action of such organisms as bacteria and fungi. The decaying organisms add elements to the soil that are needed by plants to grow.

Living things compete with each other. For example, plants vie for space, sunlight, and moisture; animals compete with each other for food and often prey on each other. This competition prevents the earth from becoming overcrowded with any one type of organism. The living things that cannot compete successfully become extinct, and disappear from the earth.

The Work of the Biologist

Biologists specialize in a particular area of biology. They study biological processes, living things that live in particular environments, or a specific kind of living thing. Cytology is the study of the cells of the living organism. Marine biology deals with living organisms that live in the ocean. Ornithology involves the study of birds.

Biologists need to use a variety of tools and methods for their work. The biologist, like other scientists, reaches conclusions as a result of observing and experimenting. One important biological technique is dissection—cutting into a specimen to study its internal structure. The biologist's most important tool is probably the microscope. It is used not only to study microorganisms but also to examine cross sections of tissue from larger organisms.

Other sciences have contributed a number of tools and techniques to biology. The biologist's most important tool is probably the microscope. It is used not only to study microorganisms but also to examine cross sections of tissue from larger organisms.X rays are used to photograph and analyze organisms and their internal structures. The development of vacuum tubes made possible electron and field ion microscopes, many times more powerful than optical microscopes. The scanning electron microscope can be used to greatly magnify cells or small organisms and produce three-dimensional images.

The use of radioactive isotopes enables biologists to identify specific compounds and to determine their activity within the organism. The radiation emitted can be detected either with a Geiger-Müller counter or by autoradiography (a photographic technique that uses special film to locate radioactive material in cells or tissue). High-speed centrifuges, called ultracentrifuges, are used to prepare materials for electron microscopy and to separate various cellular components.

Other research methods include spectroscopy and electrophoresis. All these methods, by using different factors (such as light or electrical charge), aid in identifying and separating organic compounds. Another method, achieving similar results, is chromatography. In chromatography, closely related organic compounds can be separated by allowing a solution of them to seep through an absorbing medium. As the solution passes through the medium the constituent compounds are gradually separated into a series of discrete zones.

To interpret the results of their observations and experiments, biologists must keep accurate records. The branch of mathematics called statistics is important in their work because it allows them to draw meaningful conclusions from a great mass of data. Computers are often used to organize data in such a way that meaningful interpretations can be made. The application of statistics to the study of biology is called biometry.

Like other fields of study, biology consists of a number of specialties, each dealing with certain limited parts of the subject. Many of these specialties overlap other specialties or other sciences, such as chemistry, physics, archeology, or agriculture. All these divisions are interrelated, and it is impossible to study one without considering some of the others.

Biology can be divided into several broad fields, including zoology, the study of animals; botany, the study of plants; and microbiology, the study of such living things as bacteria and protozoans, which are so small that they cannot be seen without the aid of a microscope. Each of these fields can be broken down into further specialties. For example, a zoologist may concentrate on the study of insects, a field called entomology; a botanist may specialize in the study of trees, a field called dendrology; or a microbiologist may concentrate on the study of bacteria, a field called bacteriology.

Other fields of biology are concerned with problems common to many forms of life. These fields include:

Economic Biology

This is the study of organisms that are useful to humans, or that affect human activities. The various phases of economic biology are often taught in agricultural colleges with emphasis on such practical matters as production and selling techniques. Horticulture is the study of all ornamental or food plants except field crops. The study of field crops—cotton, corn, wheat, etc—and of their relationship to the soil is called agronomy. Animal husbandry is the study of farm animals.


Ecology is the study of living things in relation to each other and in relation to their surroundings.


Genetics is the study of the physiological mechanisms by which living things pass traits from generation to generation. Biologists concerned with evolution study the development of complex living things from simpler forms of life. Paleontology is the general science of fossils. Paleobotany is the study of fossil plants; paleozoology is the study of fossil animals. Eugenics is concerned with improving the human race through heredity.

Molecular Biology

Molecular biology is the branch of biology that uses the techniques of biochemistry and biophysics in the study of the molecular structure of living organisms. Scientists in this field generally use simple organisms, such as bacteria, in their studies, but the principles they discover generally apply to all living things. Research has been concentrated on heredity, cell development, and evolution.


Morphology is the study of form, usually visible form, in living things. The branch called anatomy is the study of structures, chiefly internal.


Pathology is the study of diseases. It is both a biological and a medical subject.


Physiology is the study of the functions of living cells, tissues, organs, or systems.


Taxonomy is the classification of organisms into orderly groups according to their physical structure and other traits.

Some biologists concentrate on the use of certain tools or techniques in their study of living things. The radiobiologist, who studies the effects of radioactivity on living organisms, is such a specialist. Another is the naturalist, who "hunts" wild animals with a camera or tape recorder.

Some biologists are concerned with basic research, seeking to increase our knowledge of living things regardless of whether this knowledge has any immediate use. Others are engaged in applied research, trying to solve specific problems, such as finding the cure for a certain disease. Still others are concerned mainly with teaching or with applying biology to other fields, such as agriculture.

To become a professional biologist requires a college education and usually graduate study as well. A biologist can work in government organizations, research, and industry. Those with a bachelor's degree in biology can teach in schools. A graduate or higher degree enables you to work as a researcher. Those who love nature and wildlife can work in sanctuaries, national parks, and zoos. Certain phases of biology, however, can be fascinating and rewarding hobbies for amateurs, including children. These hobbies include birdwatching, the collecting of butterflies and other insects, and microscope study.


The word biology was first used about 1800. Before then the various biological sciences—such as zoology and anatomy—had been grouped together with geology and called "natural history."

The history of biology is the history of many fields, including medicine, botany, and zoology. The following is a brief account of some of the developments that apply to biology as a whole. For more details, consult the articles listed in the cross-references under the headings Specialties and Related Fields and Some Noted Biologists.

Early Biology

The first knowledge of biology grew out of primitive hunters' observations of animals and out of food-gathering and cultivation. Progress was slow, however, because nature was often considered a goddess and disease an evil spirit—and few persons dared to tamper with either. This attitude still exists among certain primitive peoples.

The earliest studies of biology were probably made by ancient physicians and embalmers. People of ancient India, China, and the Middle East had a vast knowledge of various medicinal plants. The Babylonians and Egyptians had some knowledge of human anatomy. The first man known to approach disease as a natural, rather than a supernatural, process was Hippocrates of Cos (460?-377? B.C.), a Greek who became known as the Father of Medicine.

The greatest student of biology in the ancient world was the Greek philosopher Aristotle (384-322 B.C.). His writings include encyclopedialike works on birth, death, the nature of life, and all phases of animal life. He influenced scholars for nearly 2,000 years. Pliny the Elder (23-79 A.D.), a Roman, compiled an interesting but inaccurate work on nature called Natural History. This text was widely studied for 1,500 years. Galen (130?-201? A.D.), a Greek living in Rome, studied anatomy by dissecting animals. His works were used in medical schools throughout the Middle Ages.

Although many of the ideas of the ancient writers were at least partly correct, many were also misleading or wrong. One of their mistaken ideas that lasted well into modern times was that a living body is made up of four juices, or humors—blood, phlegm, black bile, and yellow bile. Another was the theory of spontaneous generation, according to which certain living things, such as maggots, came not from other living things but from nonliving matter.

The Middle Ages

The Middle Ages saw hardly any contributions to biology in Europe. But the works of the ancient Greeks and Romans were studied by the Arabs.Scholars during the Middle Ages did not investigate nature firsthand, but confined their study to ancient books. Not only were the original writers often wrong, but bad translations caused even more errors. Virtually the only careful studies of the physical characteristics of plants and animals were those made for artistic purposes by craftsmen and artists. Some new knowledge of plants resulted from the work of the herbalists, who collected and studied herbs for use in medicine.

Later Advances

During the 14th to 17th centuries, there was a surge in the study of biology. Traditional ideas formulated by ancient Greeks and Romans were challenged. Observation and scientific study was emphasized. Scholars again turned to actual observation of plants and animals as a means of gaining information. The first really accurate textbooks on botany and zoology were written in the early 14th century. Leonardo da Vinci, the great Italian artist, dissected dead human bodies and made drawings of the human anatomy. In the same period Andreas Vesalius (1514-64), a Belgian, gave the first accurate and complete description of the human body. He was one of the first scientists since ancient times to dissect a human body.

The importance of experiment in the study of biology was shown by the Englishman William Harvey (1578-1657). He proved his theory that blood circulates in the veins and arteries by cutting into animals to show how it happens. Another important step forward was the development of the microscope in the early 17th century. Marcello Malphigi, the Italian anatomist, used the microscope to study blood circulation. Robert Hooke observed the structure of many organisms through the microscope and reported his observations in Micrographia, published in 1665. Five years later, Anton van Leeuwenhoek chanced upon organisms that could only be seen through the microscope.

By the 18th century, biologists had come to the conclusion that life could be explained in terms of biological processes that took place within the living organism. They rejected the idea of supernatural or divine intervention in biological processes. Their views formed part of materialistic physiology. In the 18th century, Antoine Lavoisier demonstrated that respiration involved the use of oxygen and the release of carbon dioxide and heat. Almost a century later, Claude Bernard pointed out that animals and plants had internal systems to ensure that conditions needed to maintain life were available. One such mechanism was regulation of body temperature in mammals.

The classification of organisms into groups began as early as Aristotle. But a really logical system was not devised until the 18th century. Carolus Linnaeus (1707-78) of Sweden refined earlier systems and developed the method of classification that (with certain changes) is still used today. His system provided a logical approach to the study of living things, and gave biologists a uniform method of description. Orderly classification also made it possible to see more clearly the relationships between various kinds of life. Linnaeus had used the similarities in structures of plants and animals to group them. This led to another field of biology—comparative anatomy. Here, different plants and animals were compared for similarities and differences. In the 18th century, Baron Cuvier proposed a system of classification where organisms were classified based on their body type.

Biologists broadened their knowledge of nature through voyages of exploration. The first important scientific expeditions were those commanded by Captain James Cook in the 1760's and 1770's. Later expeditions were made by the British ships Beagle (1831-36), on which Charles Darwin was naturalist, and Challenger (1872-76).

Important developments were made in many fields during the 19th century as biologists began to apply scientific methods to their work. Discoveries were made not only about specific organisms, but also about the nature of life in general. Early in the century it was discovered that all living things are made up of cells. In Origin of Species (1859) Charles Darwin (1809-82) gave evidence to show that complex forms of life generally evolve from simpler forms by means of natural selection. In 1865 Gregor Mendel (1822-84), an Austrian abbot, presented his findings on the principles of heredity—the first scientific studies of the subject.

Biology Today

During the 20th century, more emphasis was placed on experimental knowledge and less on theory. Systematically conducted experiments and use of statistical tools helped biologists understand the various biological processes. As biology became more scientific in its methods, it also became more useful in its practical applications. The discovery by Louis Pasteur (1882-95) of how infection is produced by bacteria, and the development of penicillin and other “wonder drugs,” contributed greatly to the control of disease.Genes have been shown to have a relationship with the kind of antibody, or disease resisting protein, produced by the body. This has helped treat deadly diseases like AIDS.

Gregor Mendel had found that physical traits are transmitted from generation to generation through units from the parent to offspring. These essential units, known as genes, are located on chromosomes within the cells, as postulated by Thomas Hunt Morgan in 1910. In 1953, James Watson and Francis Crick discovered the structure of DNA that makes up genes. This led to greater understanding of the mechanism of heredity and the process of evolution.

Research in genetics led to many new breeds of plants and farm animals, thus increasing the supply of food. Better understanding of the physical structure and habits of simple forms of life made it possible to control many types of harmful insects and other pests.

Pollution of air and water and increased use of land areas for housing and commercial development have created survival problems for various kinds of organisms. These problems have increased interest in ecology and conservation. New tools to study the complex relationships between organisms and their environment led to the development of ecology as a separate field of biology in the 1960's. There is also great interest among biologists in ethology, the study of animal behavior, especially aspects related to survival.

Space exploration has intensified interest in the possibility of life existing elsewhere in the universe. Scientists who study these possibilities, and who attempt to devise methods of detecting extraterrestrial life, are called exobiologists.

Advances in neurobiology, the study of the nervous system, has increased our understanding of how the brain and nerve cells function. Immunology, the study of the disease resistance mechanisms of the body, also benefited from the new methods and tools used in biological studies.

In basic research, the study of life processes continues, particularly in genetics and other areas of biochemistry. Since the 1950's, there has been much research involving the composition and functions of nucleic acid molecules. In the 1970's, the first successful experiments in genetic engineering were made. Genetic engineering involves the transfer of genes from one organism to another. This is very useful in agriculture and medicine, where desirable traits of certain organisms can be transferred to other organisms. Genetic engineering has raised concerns related to ethics and the potential negative effects of genetically altered organisms on the environment. This concern was particularly evident in 1996, when Ian Wilmut created the world's first cloned animal, Dolly the sheep. In the 1990's, scientists determined the complete genetic codes of a number of simple organisms, including certain types of bacteria. Gene sequencing, or mapping of all the genes in the human body, was started in the form of the Human Genome Project during the decade. Since the 1980's, advanced medical imaging techniques have been used to study many biochemical processes, including those involved in memory, language, drug addiction, mental disorders, and aging.

The American Institute of Biological Sciences (AIBS) is a professional federation of biological associations and industrial research laboratories. Its objectives are to unify and coordinate the efforts of persons engaged in biological research and teaching and to further the relationships of the biological sciences to other sciences, the arts, and industry. Publications include BioScience, a monthly. AIBS headquarters are in Washington, D.C.

Important dates in biology
c. 400 B.C. Hippocrates established the principles of modern medical practice based on the idea that diseases have only natural causes.
A.D. 100's Galen greatly extended knowledge of anatomy and physiology through his treatment of injured gladiators and dissections of apes and pigs.
1543 Andreas Vesalius's On the Fabric of the Human Body, the first scientific text on human anatomy, was published.
1628 William Harvey published his discovery of how blood circulates through the body.
1665 The first drawings of cells appeared in Robert Hooke's book Micrographia.
Mid-1670's Anton van Leeuwenhoek discovered microscopic forms of life.
1735 Carolus Linnaeus classified organisms according to their structural similarities, laying the foundation for modern scientific classification.
Late 1700's Antoine Lavoisier conducted chemical studies of such physiological processes as respiration and the conversion of food to energy.
c. 1800 Baron Cuvier made major contributions in comparative anatomy (the comparison of the structures of different species) and paleontology (the study of prehistoric life).
1838-1839 Matthias Schleiden and Theodor Schwann proposed that the cell is the basic unit of life.
Mid-1800's Gregor Mendel discovered the basic laws of heredity.
1859 Charles Darwin set forth his theory of evolution in The Origin of Species.
Middle and late 1800's Louis Pasteur and Robert Koch firmly established the germ theory of disease.
1953 James D. Watson and Francis H. C. Crick proposed a model of the molecular structure of deoxyribonucleic acid (DNA), the hereditary material in chromosomes.
Late 1970's Researchers used genetically engineered bacteria to produce insulin--a hormone for treating diabetes.
1983 Researchers used genetic engineering to transfer human growth hormone genes into mice, causing the mice to grow to about twice their normal size.
1996 Scientists led by Ian Wilmut achieved the first successful cloning of a mammal from the cells of an adult animal. They produced a clone of a sheep.
Early 2000’s The Human Genome Project and Celera Genomics Corporation, a private firm, completed the sequencing of essentially the entire human genome.