Introduction to Evolution


Credits: Most text by Andrielle Swaby and Warren D. Allmon, Paleontological Research Institution, Ithaca, New York. Incorporated into Earth@Home by Jonathan R. Hendricks 2022-2023.

Image above: Hummingbird clearwing moth on milkweed, Tompkins County, New York. Photograph by Jonathan R. Hendricks.

Last updated December 13, 2022


Nothing in biology makes sense except in the light of evolution.

-Theodosius Dobzhansky

Evolution explains how all living things on Earth—people, plants, bacteria, turtles, amoebas, and everything else—came to exist in all their variation and complexity. All organisms are connected by genealogy, and have changed through time as a result of descent with modification. The theory of evolution is the fundamental underpinning that unites all aspects and subfields of biology. Playing a similar role to atomic theory in physics and chemistry, or plate tectonics in geology, evolutionary theory is the basic modern scientific description of why living things are the way they are.

Is evolution a fact or theory?

Science is a method for understanding the natural world. It uses observations about the world and the rules of logic to test hypotheses that explain natural phenomena.

Hypotheses are ideas about natural phenomena that might or might not be true. Hypotheses are testable—this involves making predictions from them and then comparing these predictions to observations from the physical world. Hypotheses that pass such tests are accepted, although they can be overturned by enough credible contrary evidence.

Although a “theory” in common English can mean a mere guess or supposition, in science a theory is the basic unit of our understanding of reality. A scientific theory is an idea or set of ideas and hypotheses that have been rigorously tested. Theories are supported by, connect, and explain a very large number of observations. In science, a theory is about as good as it gets.

Evolution is as close to being a fact as any other widely accepted scientific theory, including the theory of gravitation, atomic theory, the theory of special relativity, and the germ theory of disease.

What does evolution explain?

Evolution explains several key observations about organisms, their features, history, and distribution.

Life's order

Living things are frequently adapted to their environments—they have a close “fit” or “suitability” to do what they do. In addition, many living things also possess features that are not especially suitable or useful. Evolution by natural selection explains all of these observations by suggesting that organisms are related to each other in genealogical patterns (“family trees”) and have evolved to fit as well as possible—given the limitations in the genetic and anatomical structures they inherit—into their place in nature.


Life's history

The fossil record shows us that life has a long history of dramatic change. Success and failure are both important aspects of this history—the vast majority of all species that have ever lived on Earth are now extinct. Evolution by natural selection explains life’s history by suggesting that all life shares a distant common ancestor, which gave rise to all the organisms that followed. The changes among living things over the past 3.5 billion years are a result of the struggles of individual organisms to survive and reproduce in a spectrum of changing environments.


Internal sutures of a Jurassic ammonite fossil from Russia. Photograph by Jonathan R. Hendricks.

Life's diversity

The history of life is not just the history of change in form. Life on Earth, now and in the past, also shows astonishing variety. Evolution by natural selection argues that life’s incredible diversity is the result of genetic variation. This helps organisms survive changing environmental pressures and intense competition.


Modern conch shells (Strombidae). Collections of the Paleontological Research Institution. Photograph by Jonathan R. Hendricks.

Brief history of evolutionary biology

The diversity of life—the presence of a huge number of different living things—is one of the most conspicuous and puzzling aspects of biology. Since the 1700s, scientists have used the word species to refer to different types of organisms. In the early 19th century, scientists began to pay close attention to observed patterns in nature, including the distribution of the fossil record and the diversity of life. They wondered whether these patterns could be explained by natural laws, much as Isaac Newton had done so successfully for physical objects and phenomena.

Previously, most approaches to understanding the natural world were based on theological explanations. Charles Darwin’s observations during his five-year journey around the world as naturalist aboard the H.M.S. Beagle prompted him to develop a nontheological explanation for what he had seen in places like South America and the Galapagos Islands. In his 1859 book On the Origin of Species, Charles Darwin argued for two separate but related ideas: that evolution (descent with modification) occurs, and that it is mostly caused by natural selection.


While Darwin did not invent the idea of evolution, he was the first to assemble enough evidence to convince a large number of people. Evolution as a process has been accepted by the scientific community since approximately 1880. However, Darwin’s proposed mechanism for evolution—natural selection—was not widely accepted by scientists until more than 60 years after his death.

Darwin and the other evolutionary scientists who worked in the half-century after 1859 were hindered by their lack of understanding of genetics, the science of heredity. The widespread acceptance of natural selection in the 1940s resulted from a combination of scientific insights, known as the Modern Evolutionary Synthesis. Aspects of this view have been challenged, modified, and expanded since the 1960s, especially by work in paleontology, molecular biology, and developmental biology. However, most evolutionary scientists still think that natural selection is the most important, though not the only, cause of evolutionary change. Nothing discovered since Darwin’s time has caused scientists to doubt whether evolution occurs, even while active scientific research and controversy continue about the mechanisms by which evolution occurs.