Evolution is a process of change over time. The word evolution may refer to various types of change. For example, scientists generally describe the formation of the universe as having occurred through evolution. Astronomers think that the stars and planets evolved from a huge cloud of hot gases. Anthropologists study the evolution of human culture from hunting and gathering societies to complex, industrialized societies.
Most commonly, however, evolution refers to the formation and development of life on Earth. The idea that all living things evolved from simple organisms and changed through the ages to produce millions of species is known as the theory of organic evolution. Most people call it simply the theory of evolution.
The French naturalist Chevalier de Lamarck proposed a theory of evolution in 1809. But evolution did not receive widespread scientific consideration until the late 1850’s, when British naturalist Charles R. Darwin presented his theory of evolution. Since then, advances in various scientific fields have resulted in refinements of the theory. The main ideas of evolution, however, have remained largely unchanged.
Main ideas of evolutionary theory
The theory of evolution consists of a set of several interrelated ideas. The basic idea states that species undergo changes in their inherited characteristics over time. The most important mechanism of evolution is natural selection, a process by which the individuals better suited to their environment tend to leave more descendants. All living organisms must compete for a limited supply of food, water, space, mates, and other things they need to successfully reproduce. Scientists use the term fitness to refer to an individual’s overall ability to reproduce.
There are two main types of change in organic evolution: anagenesis and cladogenesis. Anagenesis refers to changes that occur within a species over time. Because of anagenetic change, the forms and traits of many species today differ from the forms and traits of their ancestors. Cladogenesis refers to the splitting of one species into two or more descendant species. This branching process, also called speciation, can be repeated to create many species. Current evolutionary theory holds that all species evolved from a single form of life which lived more than 31/2 billion years ago. Over time, repeated speciation events and anagenetic changes have produced the more than 10 million species inhabiting Earth today.
Related to speciation is the idea of common ancestry. Because all organisms evolved from one basic life form, any two species once had a common ancestor. Closely related species share a more recent common ancestor, but distantly related species must trace their ancestry far into the past to find a common ancestor. For example, human beings, chimpanzees, and gorillas evolved from a common ancestor that lived between 4 million and 10 million years ago, while the common ancestor of human beings and reptiles lived about 300 million years ago.
Other ideas relate to the tempo of evolutionary change. Gradualism is the idea that some evolutionary changes occur continuously over long stretches of time. Punctuated equilibrium is the idea that some evolutionary changes take place in relatively short periods, from tens to hundreds of generations, followed by longer periods of little change called stasis. Both gradual and punctuated patterns can occur for different traits. The theory of evolution holds that evolution continues today at rates comparable to those of the past.
Although evolution is called a “theory,” this term does not mean that evolutionary biology is guesswork or is not supported by evidence. In science, a theory is a set of ideas based on observations about nature that explains many related facts. The theory of evolution is supported by evidence from many scientific fields. When a theory is supported by so much evidence, it becomes accepted as a scientific fact. Almost all scientists consider the theory of evolution to be a scientific fact.
Many people, however, reject the theory of evolution because of their religious beliefs. They believe the theory conflicts with the Biblical account of the Creation, which they interpret to mean that all forms of life were created essentially as they exist today.
Causes of evolutionary change
Much evolutionary change results from the interaction of two processes: (1) mutation and (2) natural selection. Mutation produces random (chance) variation in the biological makeup of a species or a population—that is, individuals of the same species living in the same area. Natural selection sorts out these random changes according to their value in enhancing the individual’s reproduction and survival. Such selection ensures that variations that make individuals better adapted to their environment will be passed on to future generations. At the same time, natural selection eliminates variations that make individuals less able to survive. A third process called genetic drift also helps create evolutionary change.
Mutation
is a permanent change in the hereditary material of an organism. Mutations may produce changes in the inherited characteristics of an organism. To understand how mutations produce these changes, one must understand how characteristics are inherited.
How characteristics are inherited.
Hereditary characteristics of organisms are carried by threadlike structures called chromosomes in cells. Chromosomes carry large numbers of genes, the basic units of heredity. Genes consist of a substance called DNA (deoxyribonucleic acid). DNA contains the coded information that influences hereditary characteristics.
Among most animals and plants, each body cell has a full set of paired chromosomes. Human body cells, for example, have 46 chromosomes arranged in 23 pairs. Offspring inherit half a set of chromosomes from each parent. Parents pass on their chromosomes to their offspring during sexual reproduction. Egg cells and sperm cells form in a special process called meiosis that gives them one chromosome at random from each pair of the parent’s set. As a result, egg and sperm cells have half the number of chromosomes found in all other cells in the body. During reproduction, a sperm and an egg unite in the process called fertilization, and the fertilized egg then has the full number of chromosomes.
Sometimes, the genes from one of a pair of chromosomes change places with genes on the other pair as a sperm or egg cell forms. This change in the arrangement of genes, called recombination, can result in new combinations of inherited traits.
As the fertilized egg cell begins to grow, each chromosome in the nucleus of the cell duplicates itself. The chromosome and its duplicate lie next to each other in pairs. During normal cell division, called mitosis, one of each pair of chromosomes goes into each of two new cells. Thus, the new cells contain chromosomes that are identical with those in the original cell. This process of growth through cell division continues until it has produced all the cells that make up an organism.
How mutations change a species.
Mutations may be caused by environmental factors, such as chemicals and radiation, which alter the DNA in genes, or by errors in the copying of DNA during cell division. After a gene has changed, it duplicates itself in its changed form. If these mutant genes are present in the egg or sperm cells of an organism, they may alter some inherited characteristics. Only such mutations can introduce new hereditary characteristics. For this reason, mutations are the building blocks of evolutionary change and of the development of new species.
Mutations occur regularly but are usually infrequent, and most of them produce unfavorable traits. Albinism is one such mutation. Albino animals have mutant genes that lack the ability to produce normal skin pigment. These animals do not survive and reproduce as well as normal animals. In most cases, such mutant genes are eliminated by natural selection because most individuals that inherit them die before producing any offspring.
Some mutations, however, help organisms adapt better to their environment. A plant in a dry area might have a mutant gene that causes it to grow longer roots. The plant would have a better chance of survival than others of its species because its roots could reach deeper for water. This type of beneficial mutation provides the raw material for evolutionary change.
Natural selection
can involve any feature that affects an individual’s ability to leave offspring. These features include appearance, body chemistry, and physiology (how an organism functions), as well as behavior.
For natural selection to operate, two biological conditions must be met. First, the individuals of a population must differ in their hereditary characteristics. Human beings, for example, vary in almost every aspect of their appearance, including height, weight, and eye color. People also differ in less obvious ways, such as brain size, thickness of bones, and amount of fat in the blood. Many of these differences have some genetic basis.
The second requirement for natural selection is that some inherited differences must affect chances for survival and reproduction. When this occurs, the individuals with higher fitness will pass on more copies of their genes to future generations than will other individuals. Over time, a species accumulates genes that increase its ability to survive and reproduce in its environment.
Natural selection is a group process. It causes the evolution of a population or a species as a whole—not the evolution of an individual—by gradually shifting the average characteristics of the group over time.
There are several types of natural selection. They include (1) directional selection, (2) stabilizing selection, (3) balancing or diversifying selection, and (4) sexual selection.
Directional selection
favors changes in traits that help a species adapt to its environment. Lizards called anoles provide an example of directional selection. These lizards live on many Caribbean islands. In the late 1900’s, scientists moved populations of one anole species from their native habitat to several uninhabited islands. Some of these islands had little vegetation, and the lizards had to perch on twigs and small branches. Other islands had large trees. After just 10 to 14 years, the anoles on islands without large trees had evolved smaller hindlimbs, which gave them needed agility on their small perches. The anoles on islands with large trees developed longer hindlimbs, giving them the speed they needed to catch prey and avoid predators in their new habitat. These changes showed that anoles with shorter-than-average hindlimbs had left more offspring than average on islands without large trees. At the same time, anoles with larger-than-average hindlimbs had left more offspring than average on forested islands. Such natural selection helps species adapt to new environments by favoring more extreme traits. Over time, continued mutation and directional selection can produce species that differ significantly from their ancestors.
Stabilizing selection
occurs if a species is already well adapted to its environment. In such cases, the individuals with average characteristics leave the most offspring, and individuals that differ most from the average leave fewest. One example of stabilizing selection is the survival rate of human babies according to birth weight. Babies of average weight tend to survive better than those who are either heavier or lighter. Unlike directional selection, stabilizing selection eliminates extreme traits.
Balancing or diversifying selection
occurs when natural selection maintains two or more alternative traits in the population. The gene that causes sickle cell anemia, a blood disease, provides an example of balancing selection. An individual who inherits the sickle cell gene from both parents may develop fatal anemia. But people who inherit the gene from only one parent do not get anemia and become more resistant to malaria, a common tropical disease. Thus, in areas threatened by malaria, the beneficial effects provided by a single copy of the sickle cell gene balance the harmful effects of inheriting two copies of the gene. Because of this balance, natural selection maintains the sickle cell gene in the population, along with the traits of both anemia and malaria resistance.
Sexual selection
occurs primarily among animals. Adults of many species prefer mates who display certain behaviors or have certain external features. Over time, this process can lead to the evolution of complicated courtship rituals, bright coloring to attract mates, and other features. Sexual selection explains, for example, why males of many bird species have more-colorful feathers than do the females.
Genetic drift
is a random change in the frequencies of genes in populations. It is caused by the random way that egg and sperm cells receive some chromosomes from each parent as they form. Because these reproductive cells contain only half a set of chromosomes, only half of a parent’s genes exist in an egg or sperm. If the parents produce a limited number of offspring, some of their genes may not be passed on. Genetic drift alone does not enable species to adapt to their environment. However, just as random mutations may lead to evolutionary change, so may the random changes caused by genetic drift.
In small populations, genetic drift can quickly bring about large evolutionary changes. An interesting form of genetic drift occurs when a new population is established from a small founding population. For example, in Lancaster County, Pennsylvania, members of the Protestant Old Order Amish group have a high occurrence of Ellis-Van Creveld syndrome, a rare inherited disorder that causes people to become dwarfs (unusually small adults) with malformed hearts. These people are descendants of no more than 200 immigrants from Europe in the 1700’s. A few of the immigrants probably carried the harmful trait. Biologists attribute the high frequency of this syndrome to genetic drift in the founding Amish population.
Evolution of new species
A species is a group of organisms that has become a distinct evolutionary lineage—that is, its members share critical adaptations required for successful reproduction. Various devices in nature maintain the distinct evolutionary lineages of species. In species that reproduce sexually, reproductive isolating factors play important roles. They include factors that prevent different species living in the same area from mating. For example, many species of birds have unique courtship rituals, and females of one species will not respond to the courtship of males from other species. If mating does occur between species, such matings may produce offspring that cannot survive or reproduce. A mule, for example, is the offspring of a female horse and a male donkey, and it is sterile.
Many species remain distinct even though they can interbreed and produce fertile offspring. In northwestern California, the Western sword fern has adapted to moist, shady habitats, while the narrowleaf sword fern has adapted to sites with drier soil and higher levels of light. Ferns reproduce by means of tiny cells called spores, which can blow in the wind for long distances. These California sword fern species live close enough to each other to interbreed and produce hybrids. Although the hybrids are fertile, they do not successfully reproduce in the Western sword fern’s habitat or in the narrowleaf sword fern’s habitat. Thus, natural selection favors keeping only one type of fern in each habitat, preserving the distinctiveness of each species.
Many biologists think speciation, or the evolution of a new species, often begins when a species is separated into two or more geographically isolated groups. The geographic isolation of land species may result from the movement of continents over millions of years or from the division of habitats by glaciers, rivers, and other features. The rise of land bridges, such as the Isthmus of Panama, may separate marine species.
Over time, isolated populations evolve in different ways because their environments differ and because genetic drift and different mutations occur in each population. If geographic isolation lasts long enough, the populations may grow so dissimilar that each becomes adapted to a completely different environment, or the populations develop reproductive isolating factors and lose the ability to breed with each other. In either case, the populations have thus become distinct species.
Speciation sometimes takes millions of years, but it often occurs more rapidly. Rapid speciation is especially likely after a species settles in a new habitat, such as an unpopulated island. The founding population often experiences genetic drift and becomes subject to strong new forces of natural selection, such as a different climate or food supply. Occasionally, a major increase in the number of chromosomes, called polyploidy, can give rise to a new species. One form of polyploidy called allopolyploidy commonly occurs in plants and can give rise to new species within two generations.
Evidence of evolution
Evidence for evolution comes from observations of sources that document or indicate the occurrence of evolution. These sources include the fossil record, the geographic distribution of species, embryology, vestigial organs, and comparative anatomy. Evidence for evolution also comes from directly observing evolving populations and from artificial selection.
The fossil record
provides some of the strongest evidence for evolution. Most organisms preserved as fossils were buried under layers of mud or sand that later turned into rock. Scientists determine the age of fossils by means of radiocarbon dating and other methods of dating. In radiocarbon dating, researchers estimate the age of materials that were once part of a living organism by measuring the amount of radioactive carbon the materials contain.
Loading the player...Fossil formation
The fossil record has many gaps because relatively few species were preserved. Nevertheless, paleontologists (scientists who study prehistoric life) have found enough fossils to form a fairly complete record that documents much of the history of life on Earth.
The fossil record shows a progression from the earliest types of one-celled life to the first simple, multicelled organisms, and from these organisms to the many simple and complex organisms living today. Fossils found in ancient layers of rock include the simplest forms of life. They differ greatly from many organisms that exist today. Fossils in recently formed layers of rock include complex as well as simple forms of life and are more similar to living plants and animals. Thus, the fossil record shows that many species became extinct, and that the species alive today have not always lived on Earth.
The fossil record also documents many examples of continuous evolutionary change and speciation. A famous example is the evolution of mammals from early synapsids. Certain synapsids were the ancestors of mammals, and they had some mammalian characteristics. However, in other respects they somewhat resembled reptiles. Synapsids first appeared about 300 million years ago, while the first mammals did not appear until about 200 million years ago. Between these two periods, scientists have found many remains of transitional forms, animals with characteristics of both mammals and early synapsids. The skeletons of the first synapsids show few mammalian characteristics. Later synapsid skeletons are nearly identical to those of mammals. The transition from early synapsids to mammals is so gradual that scientists cannot fix an exact point when synapsids gave rise to true mammals.
Geographic distribution of species,
also known as biogeography, provides important evidence for the theory of evolution. Certain island groups, called oceanic islands, arose from the sea floor and have never been connected to continents. The species found on oceanic islands are those that can travel easily, particularly over large stretches of water. These islands are rich in flying insects, bats, birds, and certain types of plants that floated to the islands as seeds. But oceanic islands lack many major types of animals and plants that live on continents. For example, the Galapagos Islands have no native amphibians or land mammals. These animals cannot easily migrate from continents to islands. 
In addition, the majority of species found on oceanic islands are most similar to those on the nearest mainland, even if the environment and climate are different. The Galapagos Islands, for instance, lie off the coast of mainland Ecuador in South America. The islands are much drier and rockier than the coast, which has a humid climate and lush tropical forests. But the plants and birds on the Galapagos Islands more closely resemble those of the wet, tropical coast than they do the plants and birds of other arid islands. This suggests that the first species to inhabit the Galapagos Islands came from South America rather than originating on the islands.
Oceanic islands contain a far smaller variety of species than do continents, and some island species are found nowhere else. The Galapagos Islands, for example, have 21 native species of land birds. Of these, 13 species are finches—a much higher proportion of finches than exists on any continent. The finches developed as different species partially because they ate different foods. They thus evolved specialized beaks and other adaptations for their different eating habits. These finch species live only on the Galapagos Islands. Therefore, the distribution of species supports the idea that a limited number of species came to the islands from the nearest mainland and then evolved into new species. 
Embryology
is the study of the way organisms develop during the earliest stages of life. The embryonic development of many organisms includes peculiar events that can be explained by the evolution of the organism from another species. For example, a mammal embryo forms three different pairs of kidneys in succession during its development. The first two pairs of kidneys perform no function and break down and disappear shortly after they form. The third kidney pair then takes shape and develops into the mature, functioning kidneys of the mammal. In an embryo of a fish, amphibian, or reptile, however, one of the first two types of kidney pairs becomes the animal’s mature kidneys. These events suggest that mammals have retained some developmental features of their evolutionary ancestors. Scientists call this phenomenon recapitulation.
Vestigial organs
are the useless remains of organs that were once useful in an evolutionary ancestor. For example, many species of animals that live in caves are blind but still have eyes. Some species have nearly complete eyes but lack an optic nerve, while others have tiny, malformed eyes. Some cave-dwelling crayfish have eyestalks but no eyes. They evolved from ancestors with functioning eyes. Because eyes are useless in a dark habitat, and may indeed be harmful if they are injured, mutations that damaged the vision of these species did not decrease their fitness. Thus these species lost their sight over time. Similarly, many whales have tiny vestigial leg bones, the useless evolutionary remains of their land-dwelling ancestors.
Comparative anatomy
is the comparison of anatomical structures in different organisms. Such comparisons often indicate how evolution occurred. For example, the forelimbs of amphibians, reptiles, birds, and mammals all have a similar basic bone structure. This similarity suggests that these animals evolved from a common ancestor.
Direct observation of evolution
is commonplace because much evolutionary change occurs rapidly. Scientists have observed both anagenesis (evolution within species over time) and speciation.
An important example of observed anagenesis involves the virus HIV-1. The virus gradually weakens the immune system in most infected individuals, eventually leading to AIDS, the final, life-threatening stage of HIV infection. The virus uses a protein to bind to human proteins and infect human cells. Two major types of the protein exist, called the nonsyncytium inducing (NSI) form and the syncytium inducing (SI) form. The SI form, which is more lethal to the cells it infects, can evolve from the NSI form through two simple mutations. Almost all new HIV-1 infections start with the NSI form, which is better adapted to healthy human immune systems. As the virus gradually weakens the patient’s immune system, however, natural selection favors the SI form. Once the SI form has become common in an infected patient, the person usually dies from an illness that takes advantage of the body’s weakened immune system. This frightening example of anagenetic change has occurred millions of times in AIDS patients.
Scientists have also directly observed speciation in the laboratory. One important study involved the anomalous sunflower, which grows in the southwestern United States. Researchers found that this species evolved from hybrids of two other sunflower species, the common sunflower and the prairie sunflower, after only a few generations. By crossbreeding the common and prairie sunflowers in the laboratory, researchers produced, on several different attempts, hybrid species genetically similar to the anomalous sunflower.
Artificial selection.
Animal and plant breeders use a method similar to natural selection to produce new varieties. Breeders commonly breed only the individuals in a species that show desired characteristics. This process, called artificial selection, eventually leads to large changes in a species. For example, the various breeds of dogs differ widely in size, appearance, and behavior. They probably descended, however, from one or a few dog species that were bred to develop various traits. Many of these traits helped the dog perform a specific job, such as hunting badgers or herding sheep.
Plant breeders developed most food crops from wild ancestors by the same process. For example, cabbage, broccoli, kohlrabi, cauliflower, and Brussels sprouts all belong to a single species that was selectively bred to evolve different characteristics.
Artificial selection differs from natural selection only because human beings—instead of the natural environment—determine which characteristics give individuals an advantage in reproduction. The ability of artificial selection to cause dramatic changes in a short time leaves little doubt that natural selection could cause larger changes over the vast spans of Earth’s history.
History of the theory of evolution
Early theories.
Two French naturalists, Comte de Buffon and Baron Cuvier, conducted some of the first scientific investigations of evolution in the 1700’s. They concluded from their studies of fossils and living animals that life on Earth had undergone a series of changes. But neither Buffon nor Cuvier had any idea how long ago these changes had occurred.
In 1809, the French naturalist Chevalier de Lamarck formulated the first comprehensive theory of evolution. He observed that an animal’s body parts could change during its lifetime, depending on the extent to which it used them. Organs and muscles that were frequently used became larger and stronger, but those that were rarely used tended to shrink. According to Lamarck, such acquired traits became hereditary. His theory of the inheritance of acquired characteristics influenced many scientists. Later discoveries in genetics disproved his theory.
Darwin’s theory.
In 1858, Charles R. Darwin presented a joint paper written by him and Alfred R. Wallace, another British naturalist, that proposed a theory of evolution. This theory, in modified form, is accepted by almost every scientist today. It states that all species evolved from a few common ancestors by means of natural selection. Darwin developed the theory more thoroughly in his book, The Origin of Species (1859). The book became a bestseller.
Darwin used three principal sources in developing his theory. These were (1) his personal observations, (2) the geological theory of the British scientist Sir Charles Lyell, and (3) the population theory of the British economist Thomas Robert Malthus. Darwin made many of his observations as a member of a scientific expedition aboard the H.M.S. from 1831 to 1836. The ship made stops along the coast of South America, and Darwin collected many specimens of plants and animals and wrote detailed notes.
Darwin was particularly impressed by the variety of species on the Galapagos Islands. He found striking differences not only between species on the islands and those on the mainland, but also among those on each island. Darwin’s findings led him to reject the idea that all living things were created as they currently existed. He then searched for another explanation for the origin of species.
The theories of Lyell and Malthus influenced Darwin’s ideas about Earth’s history and the relationship between living things and their environment. Lyell’s Principles of Geology, published in the early 1830’s, stated that Earth had been formed by natural processes over long periods. Darwin wondered whether life on Earth had also developed gradually as a result of natural processes. In 1798, Malthus wrote that the growth of the human population would someday exceed the food supply unless checked by such factors as war and disease. Darwin assumed that environmental factors also regulated the population of all other living things. He concluded that only the individuals most fit in their environment would tend to survive and pass on their characteristics to their offspring.
The synthetic theory
was formulated during the 1930’s and 1940’s by a number of scientists, including four American biologists—Sewall Wright, George G. Simpson, Russian-born Theodosius Dobzhansky, and German-born Ernst W. Mayr—and two British geneticists, Ronald A. Fisher and J. B. S. Haldane. Their theory synthesizes (combines) Darwin’s theory of natural selection with the principles of genetics and other sciences. Darwin had observed that the characteristics of organisms may change during the process of being passed on to offspring. However, he could not explain how or why these changes took place because the principles of genetics were not yet known.
The genetic principles of variation and mutation filled this gap in Darwin’s theory. Gregor Mendel, an Austrian monk, had discovered the principles of genetics in the 1860’s. Mendel’s findings remained unnoticed until the early 1900’s, when the science of genetics was established. About 1910, the American biologist Thomas Hunt Morgan discovered that chromosomes carry genes. Morgan also described the process of recombination, in which chromosomes exchange genes among one another, producing new combinations of hereditary traits. 
The creation of the synthetic theory did not lead to universal agreement on all details of evolution. For example, some scientists emphasized the importance of both genetic drift and natural selection in shaping adaptive evolution. But others felt that genetic drift did not play a meaningful role in adaptation. This and other controversies still exist within the synthetic theory.
DNA studies.
Discoveries in other fields have enabled biologists to study evolutionary processes in far greater detail. Some of the major contributions have come from molecular biology, which deals with the genetic processes that underlie all evolution. In the 1940’s, molecular biologists identified DNA as the substance in chromosomes that carries hereditary information. In the 1950’s and 1960’s, studies of DNA revealed much about its structure and its role in evolutionary changes. These studies have led scientists to conclude that, at the molecular level, evolution occurs through changes in the DNA.
In the 1960’s and 1970’s, molecular biologists developed methods to determine partial sequences of DNA molecules. These genetic surveys revealed that virtually all species have much genetic variation in their gene pools, the genes collectively shared by the entire species or population. Scientists thus learned that genetic variation, the raw material of all evolutionary change, occurred in abundance.
At the same time, molecular biologists developed techniques to compare genes in different species. Such comparisons enabled biologists to determine the evolutionary histories of species more precisely. For example, zoologists did not know whether the giant panda was more closely related to raccoons or to bears. But DNA analysis has led scientists to classify giant pandas as bears.
DNA comparisons also enabled scientists to discover links between evolution and time. Biologists now know that the amount by which certain species differ from each other genetically is roughly proportional to the time that has gone by since those species shared a common ancestor. For example, human beings are more closely related genetically to chimpanzees than they are to monkeys. Moreover, the common ancestor of human beings and chimpanzees lived on Earth more recently than did the common ancestor of human beings and monkeys. The difference in genetic variation between human beings and chimpanzees and human beings and monkeys is roughly proportional to the difference in time between when the common ancestor of human beings and chimpanzees lived on Earth and when the common ancestor of human beings and monkeys lived on Earth.
Such analysis has led scientists to revise their view of the evolutionary history of life on Earth. For example, the explosive growth of multicellular life thought to have occurred at the beginning of the Cambrian Period (539 million years ago) is now thought to be the result of evolutionary changes that began more than 1 billion years ago, during Precambrian time.
In the 1990’s, molecular biologists developed techniques for recovering small amounts of DNA from some fossils. These techniques enable scientists to directly examine prehistoric genes in the laboratory. Because of such advances, biologists can more accurately test their ideas about specific aspects of evolution. For example, scientists compared DNA from fossils of early human beings called Neandertals (also spelled Neanderthals) with modern human DNA. They found that, on average, the differences between Neandertal DNA and modern human DNA were considerably greater than the differences between the DNA of any two populations of modern human beings. Such research shows how closely all modern human beings are related to one another genetically.
Contributions from other fields.
Developmental biology, the science of how a fertilized egg develops into an individual organism, has also contributed to evolution. Starting in the 1990’s, developmental biologists found that a common set of genes controls basic aspects of development in different plants and animals. These aspects include the formation of body segments in animals as diverse as worms and human beings. Such findings imply that the abovementioned genes were present long ago in the common ancestors of most species, and have remained unchanged even while being used to construct diverse body plans. See Evolutionary developmental biology.
Finally, the area of paleontology (the study of the fossil record) continues to contribute to our knowledge of evolution. During the 1900’s, paleontologists discovered fossils that indicated life on Earth was about 3.5 billion years old, far older than previously thought. Paleontologists also used fossil evidence to show that Earth’s evolutionary history included several mass extinctions. For example, fossil evidence shows a collision with an asteroid led to the mass extinction that killed off the dinosaurs. This major extinction enabled mammals to become the world’s dominant large-bodied animals.
Acceptance of evolution
Today, the theory of evolution is considered the most important fundamental concept in the biological sciences. Virtually all biologists accept it. However, large numbers of people opposed the theory when it was introduced. Many people still do not accept it today.
In Darwin’s time, the theory of evolution was attacked by many scientists, religious leaders, and other groups. Biologists argued that the evolutionary concept of hereditary variations within species contradicted the theory of blending inheritance. According to this theory, which was popular during the 1800’s, hereditary characteristics became mixed and diluted as the blood carried them from one generation to another.
By the early 1900’s, discoveries in genetics and other fields had resolved virtually all the original scientific objections to evolution. But other philosophical or religious objections remained. Some Christian leaders denounced the idea of evolution because it conflicted with their interpretation of the Biblical account of the Creation and suggested that human beings had evolved from apelike ancestors.
In the United States, much of the controversy centered on whether evolution should be taught in schools. In the 1920’s, some states passed laws that banned such teaching in the public schools. In 1925, John T. Scopes, a Tennessee high-school teacher, was convicted in the famous “monkey trial” of teaching Darwin’s theory (see Scopes trial). Although Scopes’s conviction was later overturned because of a legal error, few public schools included evolution in the biology curriculum for many years after the trial.
In 1968, the Supreme Court of the United States ruled that laws banning the teaching of evolution were unconstitutional. The ruling stated that such laws made religious considerations part of the curriculum and thus violated the First Amendment to the Constitution. During the 1970’s and 1980’s, many religious groups proposed legislation that would require evolution to be taught along with an opposing view called creationism. Creationists believe that each species has remained relatively unchanged since the Creation and that no species has evolved from any other. Strict creationists accept the Bible’s account of the Creation as literal truth. They believe Earth is only thousands of years old. They also hold that all species were created simultaneously and that much of early life was destroyed by a global flood.
In 1981, Arkansas became the first state to enact a law requiring public schools to teach creationism whenever evolution is taught. However, a federal court declared this law unconstitutional before it went into effect. The court ruled that creationism constituted a religious and not a scientific explanation of life. Therefore, the court held that the Arkansas law violated the separation of church and state guaranteed by the First Amendment.
Because of these court decisions, opponents of evolution have largely abandoned their efforts to obtain laws banning its teaching. However, such opponents have turned their attentions toward influencing local school boards to reduce or eliminate the teaching of evolution in biology classes.
Since the late 1900’s, a number of people have tried to combine aspects of evolutionary theory and creationism. One of the most prominent theories to come out of this effort is the intelligent design theory. It states that Earth is in fact billions of years old, rejecting a literal interpretation of the Bible. But it also argues that an “intelligent designer,” similar or equal to the Biblical God, must have played a role in the development of life.
Many people—including some Christians, Muslims, and Orthodox Jews—still do not accept the theory of evolution because it conflicts with their religious beliefs. Many others, however, accept the basic principles of evolution within the framework of their religion. For example, some people interpret the story of the Creation as a symbolic and not a literal account of the origin of life. They find this symbolic interpretation compatible with evolutionary biology. For many, the idea that human beings evolved from other forms of life does not diminish the uniqueness of human capabilities and the achievements of human civilization.
