History of evolutionary thought

Evolutionary thought, the idea that species change over time, has roots going back to ancient times. As scientists began to group living things in the late 1600s, two ideas shaped how people saw nature. Some believed each species looked the same forever, while others noticed that species could look different.

In 1858, Charles Darwin and Alfred Russel Wallace shared a new theory about how species change. Darwin wrote about this in his famous book, On the Origin of Species, in 1859. He suggested that all living things come from common ancestors and change through a process called natural selection. At first, many scientists were unsure about this idea, but it later became important in biology. Today, evolutionary biology helps us understand how life on Earth has developed and continues to change.

Antiquity

Indigenous cultures

Some cultures long ago had ideas about how humans might come from other animals. For example, the Malagasy people thought humans came from indri, the Aboriginal Tasmanians believed humans came from kangaroos, and some Native American cultures, like the Navajo, told stories of humans changing from animal-like creatures.

Greeks

Long ago, Greek thinkers wondered if animals could change over time. Anaximander thought the first animals lived in water and that early humans might have started in water too. Empedocles had ideas about how animals might combine in different ways to survive. But many Greeks, like Plato and Aristotle, believed that each type of animal was fixed and unchanging by a divine plan.

Plato strongly believed that each living thing had a perfect, unchanging form. Aristotle, who studied animals carefully, also thought each species was fixed and had a purpose. Other Greek thinkers agreed that nature seemed designed for a purpose.

Chinese

Some ancient Chinese thinkers, like Zhuang Zhou, thought that species could change in response to their environment. They saw nature as always changing, unlike many Western thinkers of the time.

Roman Empire

The Roman poet Lucretius explained the world through natural processes without any divine help. This was different from other Roman thinkers, who believed nature was designed for a purpose.

Early Church Fathers

Origen of Alexandria

The Christian thinker Origen said the creation story in the Book of Genesis should not be taken literally. He thought it was an allegory, or a story with deeper meaning.

Gregory of Nyssa

Gregory of Nyssa suggested that God created the world in stages, with simpler life forms coming before more complex ones, including humans.

Augustine of Hippo

Augustine of Hippo also believed the Genesis story should not be taken literally. He thought God's creation unfolded over time in stages, with simpler forms of life appearing before more complex ones. He believed that science and faith could work together.

Middle Ages

Islamic philosophy and the struggle for existence

See also: Early Islamic philosophy and Science in the medieval Islamic world

Long ago, after the Roman Empire fell, people in the Islamic world kept thinking about how life changes. During a time called the Islamic Golden Age, from the 8th to the 13th century, many thinkers talked about how living things might change over time. They thought about how life could start from simple things and become more complex, like going from rocks to plants, then to animals, and even to humans.

One important writer, al-Jāḥiẓ, wrote about how stronger animals eat weaker ones, showing a struggle for survival. Another thinker, Ibn Khaldūn, wrote about how humans might have come from simpler forms of life, like monkeys, suggesting that species can change over time.

Christian philosophy

Thomas Aquinas on creation and natural processes

While many Christian teachers believed that God made the world exactly as it is and it never changes, some thought that God might have let nature develop over time. Thomas Aquinas believed that God created the world so that natural processes could work within it. He thought that God's plan allowed the world to develop step by step, showing God's goodness through the order and beauty of nature. Aquinas did not agree with ideas that the world could develop without a purpose, believing that everything was part of God's design.

Scientific Revolution and Enlightenment

Main article: Evolutionary ideas of the Scientific Revolution and Enlightenment

During the 1600s and 1700s, many clever thinkers began to ask questions about how nature and living things change over time. Some thought the world worked like a machine, while others believed life had a spiritual side. One scientist, Pierre Louis Maupertuis, suggested that tiny changes during the birth of new animals could add up over many years, creating new kinds of animals.

Another scientist, John Ray, believed each animal type had special traits that never changed and could never become another type. But a famous French scientist, Georges-Louis Leclerc, Comte de Buffon, had different ideas. He thought the animals we see today might all come from just a few original types that changed because of where they lived. For example, he wondered if lions, tigers, and house cats might share a common ancestor. Other thinkers also imagined that new animals might appear and disappear over time, depending on what worked best in their environment.

Early 19th century

In the early 1800s, scientists started to learn how species change over time. Georges Cuvier studied living elephants and fossils and found that some animals, like mammoths, were very different from any living today. This showed that species could disappear. James Hutton suggested that Earth's features were shaped by slow, steady processes over a very long time.

Jean-Baptiste Lamarck thought animals could change based on how they used their bodies, and these changes could be passed to their young. But many scientists disagreed. Charles Lyell argued that Earth's features were formed by gradual changes, not sudden events, which later helped shape ideas about how species evolve.

Paleontology and geology

See also: History of paleontology

In 1796, Georges Cuvier studied elephants and fossil record differences and found mammoths and mastodons were unique species. This ended debates about extinction. James Hutton in 1788 described gradual geological changes over deep time. William Smith in the 1790s started ordering rock strata using fossils, while Cuvier and Alexandre Brongniart in 1811 published studies on Paris's geologic past using stratigraphic layers. These works showed Earth's great age. Cuvier supported catastrophism to explain extinctions and faunal succession.

By the 1840s, the geologic timescale became clearer, with John Phillips naming three eras: Paleozoic with marine invertebrates and fish, Mesozoic as the age of reptiles, and Cenozoic with mammals. Even careful scientists like Adam Sedgwick and William Buckland agreed but thought new creatures appeared after big disasters. Charles Lyell in Principles of Geology from 1830 to 1833 suggested uniformitarian gradual changes, influencing later thinkers like Charles Darwin.

Transmutation of species

Main article: Transmutation of species

Jean-Baptiste Lamarck in 1809 proposed in Philosophie zoologique that species could change over time. He thought a natural life force made life more complex and that animals changed based on how they used their bodies, passing these changes to their young, a concept called Lamarckism. This idea influenced others.

A group of British scientists, including Robert Edmond Grant, supported these ideas and studied how animals might share common beginnings. Étienne Geoffroy Saint-Hilaire compared animal body plans, sparking debates. Grant, influenced by Lamarck and Erasmus Darwin, explored these changes and suggested plants and animals might share origins. Young Charles Darwin worked with Grant on marine life.

In 1844, Robert Chambers published Vestiges of the Natural History of Creation, suggesting an evolutionary path for the Solar System and life on Earth, causing much debate. These ideas faced opposition from those who believed species were fixed.

Conservative thinkers, like Cuvier, argued species could not change, pointing to fossils and ancient drawings showing no change. In Great Britain, William Paley’s Natural Theology used a watchmaker analogy against these ideas. Geologists like Buckland and Adam Sedgwick attacked evolutionary thoughts. Idealists like Louis Agassiz and Richard Owen believed each species was set by a creator’s design. Owen’s studies of body patterns influenced Darwin, though Grant faced criticism.

Anticipations of natural selection

Some early thinkers hinted at ideas later used in evolution, though they did not fully develop them. When Darwin developed his theory, he studied animal breeding and was influenced by Thomas Robert Malthus’s ideas on population struggles. Writers before Darwin touched on similar ideas, but it was Darwin who brought them together in a powerful way.

William Charles Wells in 1813 talked about human evolution and natural selection, though Darwin and Alfred Russel Wallace were unaware of this. Patrick Matthew in 1831 described how species could change over time, which Darwin later acknowledged.

Natural selection

Main articles: Inception of Darwin's theory, Development of Darwin's theory, Publication of Darwin's theory, and Natural selection

Charles Darwin’s work in places like the Galápagos Islands during the second voyage of HMS Beagle made him question if species were unchanging. By 1837, he began secret notes on how species might change. Reading Thomas Robert Malthus about population struggles helped Darwin form his theory of natural selection. He shared his ideas with friends but waited 20 years to publish, gathering evidence.

Alfred Russel Wallace, inspired by Vestiges, also came to similar ideas about species changing. In 1858, Wallace sent Darwin an essay on evolution, leading to a joint publication of their ideas. Darwin then prepared his book, On the Origin of Species, published in 1859.

1859–1930s: Darwin and his legacy

See also: Reactions to On the Origin of Species

By the 1850s, many scientists were talking about whether species could change over time. Charles Darwin's book On the Origin of Species changed this forever. Darwin said that species change through natural selection. This is where nature helps traits that help an organism survive and have babies.

Thomas Henry Huxley, a British scientist, was one of the first to support Darwin. Huxley used new fossils, like Archaeopteryx in Europe and early bird fossils in North America, to show that birds came from reptiles. He also helped trace the evolution of the horse from small, five-toed ancestors. Many scientists in English-speaking countries accepted evolution by the 1870s, but it took longer for scientists in France, southern Europe, and Latin America to agree. In Germany, scientists like August Weismann and Ernst Haeckel strongly supported evolution.

Although Darwin's theory was important, it couldn’t explain everything about how traits change or how they are passed down. Darwin had an idea called pangenesis, but it wasn’t very helpful for other scientists.

Application to humans

Darwin didn’t talk about human evolution in his book On the Origin of Species, but it became very important after the book came out. For a long time, scientists thought humans appeared suddenly, but discoveries of stone tools with ancient animal bones showed that humans lived much earlier than people thought.

In 1871, Darwin published The Descent of Man, and Selection in Relation to Sex. He talked about human evolution in this book. He believed that differences between humans and apes were differences in degree, not kind. He thought changes in how our ancestors lived—from trees to plains—and sexual selection explained human evolution. The debate about what makes humans special continued for many years.

Alternatives to natural selection

Main articles: Alternatives to evolution by natural selection and The eclipse of Darwinism

Many scientists accepted evolution but disagreed about natural selection. Some believed that a higher power guided evolution, a view called theistic evolution. Others thought that traits gained during life could be passed to children, known as neo-Lamarckism. Some believed life had a built-in drive toward perfection, called orthogenesis, while others thought new species appeared suddenly through big changes, called saltationism.

Mendelian genetics, biometrics, and mutation

Main article: Mutationism

In 1900, scientists found Gregor Mendel’s work on inheritance again. This led to a debate between Mendelians, who studied specific traits, and biometricians, who studied changes in groups. Thomas Hunt Morgan’s experiments with fruit flies showed that Mendelian genetics was correct. They found that small changes, like eye color, were more common than creating whole new species at once.

1920s–1940s

The 1920s and 1940s were important years for learning how animals and plants change over time. Scientists found that tiny changes in genes can add up to big differences in groups of animals. They also studied real examples, like how some moths turned darker in places with lots of factories.

Population genetics

Main article: Population genetics

Scientists combined ideas about genes and how they change in groups of animals. Ronald Fisher showed how many small genes working together could create big changes. J. B. S. Haldane used math to show how these changes happened quickly. Sewall Wright looked at how genes change in small groups of animals, especially when they can't mix with larger groups. Their work helped create the new science of population genetics.

Modern synthesis

Main article: Modern synthesis (20th century)

In the early 1900s, many scientists thought there were other ways animals changed besides genes. But as science learned more about genes, these ideas changed. Theodosius Dobzhansky wrote a book in 1937 that helped connect what scientists knew about small changes in genes with what they saw in big groups of animals in nature. E. B. Ford showed how the environment could affect which genes stayed in a group of animals. Ernst Mayr wrote about how new species form when groups of animals are separated for a long time. George Gaylord Simpson studied fossils and found they matched what scientists expected from their new ideas about how animals change. G. Ledyard Stebbins did the same for plants. Together, these scientists created what we call the modern synthesis, which brought many parts of biology together to better understand how evolution works.

1940s–1960s: Molecular biology and evolution

Main article: History of molecular evolution

Further information: Neutral theory of molecular evolution and Molecular clock

During the middle of the 20th century, scientists learned more about how life works at very small levels. They found out that genes are made of DNA. This DNA helps make proteins, which are important for living things. New tools allowed scientists to study these proteins closely.

Two scientists, Linus Pauling and Emile Zuckerkandl, suggested that by studying differences in proteins from different species, we could estimate when those species last had a common ancestor. Later, Motoo Kimura and others explained why these differences occur. They showed that many changes in genes do not help or harm the organism and happen by chance. This idea led to discussions among scientists about how these random changes affect evolution compared to natural selection.

Late 20th century

Gene-centered view

Main article: Gene-centered view of evolution

See also: Evolution of sexual reproduction

In the mid-1960s, George C. Williams argued against ideas that focused on the whole group or species. Instead, he and others like W. D. Hamilton, George R. Price, and John Maynard Smith suggested that changes happen mainly because of how genes act. This idea was popularized in a 1976 book called The Selfish Gene by Richard Dawkins.

In 1973, Leigh Van Valen introduced the idea of the "Red Queen," named after a character from a book by Lewis Carroll. He used it to describe how species must keep changing to stay ahead of others they interact with.

Sociobiology

Main article: Sociobiology

The work of W. D. Hamilton on how family ties affect behavior helped start the field of sociobiology. In 1975, E. O. Wilson wrote a book called Sociobiology: The New Synthesis, suggesting that evolution could explain many behaviors in animals and humans. Some scientists thought this went too far, saying it ignored how environment and culture also shape behavior. Still, the study of how evolution affects behavior continued to grow.

Evolutionary paths and processes

See also: Speciation and History of speciation

During the 1970s, Niles Eldredge and Stephen Jay Gould suggested that species often stay the same for long periods and only change quickly when new species form. Better tools for studying genes let scientists test these ideas using lots of genetic data. This led to big changes in how we understand evolution.

Advances in computers helped scientists create and test new models of how evolution works. Discoveries in technology now allow scientists to change whole sets of genes, opening new ways to study evolution.

Microbiology, horizontal gene transfer, and endosymbiosis

Main article: Horizontal gene transfer

Early ideas about evolution paid less attention to tiny organisms because they are hard to classify. But with better tools, scientists now study these tiny life forms and find lots of diversity.

In 1959, scientists in Japan found that bacteria can share genes with other species. More recently, scientists think this sharing of genes has been important for all life.

The idea that certain parts inside our cells came from tiny organisms living together long ago was suggested in the late 1800s but got more support in the 1960s and 1970s, especially through the work of Lynn Margulis.

From spandrels to evolutionary developmental biology

Main article: Evolutionary developmental biology

In the 1980s and 1990s, scientists began looking more closely at how ideas about the shape and form of living things affect evolution. They studied how development — the process of growing from a tiny cell into a complex organism — can guide evolutionary changes.

During this time, scientists discovered that many of the same basic proteins control development in all animals. Changes in how these proteins are used explain the wide variety of shapes and forms in the animal world. This led to the creation of a new field called evolutionary developmental biology, or "evo-devo."

21st century

Further information: Modern synthesis (20th century) § Later syntheses

Macroevolution and microevolution

Main articles: Macroevolution and Microevolution

One big idea in biology is that large changes in species over long times come from small changes in groups of the same species. In the late 20th century, some scientists wondered if other things, like sudden changes or effects on many species, also help shape how species change. Some thought that how an organism grows and its environment might work together to make new body plans. Others believed that small genetic changes in groups were enough to explain big changes in species. Some think both genetic changes and physical processes are needed to understand major changes in species.

Epigenetic inheritance

Main article: Epigenetics

Epigenetics studies changes in how genes work or how cells look that are passed down, but not because of changes in the DNA itself. By the early 2000s, scientists agreed these epigenetic ways were important for how cells become different types in a body. While these patterns usually reset when animals have babies, some observations show that in a few cases, these non-genetic changes can be passed to the next generation. This might help species adapt to their environment and affect evolution. Some people have suggested this could mean a type of evolution similar to ideas from a scientist named Lamarck.

Extended evolutionary syntheses

Further information: Modern synthesis (20th century) § Later syntheses

The idea of an extended evolutionary synthesis builds on ideas from the 20th century but adds new concepts. These include how selection can work on many levels, how traits can be passed down in ways other than just DNA, how organisms change their own environments, and how ready a species is to change. However, scientists don’t all agree on what should be included in this extended synthesis.

Unconventional evolutionary theory

Omega Point

Further information: Omega Point and Orthogenesis

Pierre Teilhard de Chardin wrote about a theory called the Omega Point in his book The Phenomenon of Man (1955). He thought the universe grew from small particles to human society, which he believed was the final and most important stage.

Gaia hypothesis

Main article: Gaia hypothesis

James Lovelock created the Gaia hypothesis. It suggests that Earth's living and nonliving parts work together like one living being. Scientists like Lynn Margulis see this idea as coming from thoughts about how tiny living things live inside others and work together.

Self-organization

Main article: Structuralism (biology)

Mathematician Stuart Kauffman thought that self-organization could be important in evolution. He believed it could affect how groups of living things change, how tiny parts of living things evolve, and how living forms develop shapes.