It has now been 166 years since Charles Darwin’s On the Origin of Species was published, an event that permanently altered the course of science and biology while sending shockwaves through religious communities around the world. Evolution is an established scientific fact, yet creationism persists in significant numbers, particularly among those who hold to Abrahamic accounts of Adam and Eve. Most of us have encountered creationists at some point, often conservative Christians and Orthodox Jews in an American context. What follows is a plain-language case for why evolutionary theory is correct, regardless of whether you or not you believe in it.

Something many people do not realize, even those who fully embrace science, is that Darwin did not arrive at the concept of evolution in isolation. His own grandfather, Erasmus Darwin, had written about similar ideas when the subject was still called transmutation. Jean-Baptiste Lamarck had already proposed his own formal theory of biological change when Darwin was still a child, a framework known as Lamarckism.

Lamarckism held that organisms evolved in response to their own behavior and effort, and that these acquired characteristics could be inherited by offspring. A blacksmith who built exceptional strength through years of labor might, under this thinking, pass that strength along to his children through some biological mechanism tied to his exertion. It was a reasonable attempt for its time, but it was eventually shown to be entirely wrong. Strangely, versions of Lamarckist thinking survived well into the twentieth century in some places. In the early Soviet Union, favoring natural selection over Lamarckism was not merely an intellectual position but a politically dangerous one.

When Darwin reached the Galapagos Islands, the diversity of life he observed began to reveal the actual mechanism at work. Natural selection holds that the environment itself determines which organisms survive long enough to reproduce and pass on their characteristics. The traits that persist in a population do so because they improve survival odds, while those that do not are gradually lost. These traits shift over time through mutations, which are changes in the sequence of letters making up an organism’s DNA.

One of the most striking human examples of natural selection involves smallpox. When the disease first emerged in the Eastern Hemisphere, it was catastrophically lethal to nearly everyone it touched. Over thousands of years, however, the individuals who happened to carry genetic resistance survived in greater numbers and passed those genes along. Then European contact with the Americas introduced the disease to populations that had never encountered it. The result was devastating on a scale that is difficult to fully comprehend. Millions of Indigenous people died from exposure to a pathogen their immune systems had never had any reason to develop defenses against.

Among animals, polar bears offer a clear illustration of the same process. Their distant ancestors living in cold northern environments did not originally have white fur. But over generations, individual bears whose coloring allowed them to blend into snow and ice were better hunters and better survivors. They reproduced more successfully, and white fur became the norm. Human skin color tells a familiar story. People living for millennia in regions of intense solar radiation developed higher concentrations of melanin as protection against UV damage, long before anyone had invented sunscreen. In regions with less sunlight, lighter skin proved more efficient at absorbing vitamin D, and those populations shifted accordingly.

The fossil record and the living world are filled with examples of natural selection in action, far more than any single piece of writing could address. Some organisms reach a point of such effective adaptation that they essentially stop changing, because they can survive almost anything the environment throws at them. Crocodilians are a reasonable example. Horseshoe crabs are a better one, having remained fundamentally unchanged for roughly 450 million years, predating the dinosaurs by a wide margin. My personal favorite example is the tardigrade, sometimes called the water bear, a microscopic animal of extraordinary resilience. Tardigrades have been tested in the vacuum of space and survived.

Natural selection has been operating for over 3+ billion years, but it is not the only mechanism of evolutionary change. Artificial selection, or selective breeding, has been guided by human hands across countless generations of plants and animals. Dogs are the classic example. Beginning with wolves that were tolerant enough of human presence to initiate contact, selective breeding over thousands of years produced the entire range of domestic dogs we know today, from Chihuahuas to Great Danes, from gentle retrievers to aggressive fighting breeds. All of these animals are the same species. Depending on how you draw the line, they are simply highly modified wolves, and in a broad taxonomic sense could be classified as gray wolves. One meaningful difference between dogs and wolves, though, is dietary. Wolves are (obligate) carnivorous and cannot sustain themselves for long on plant-based food. Dogs evolved alongside humans who ate more varied diets, and they adapted to digest starchy foods as well, making them technically omnivores, even if very few of them would voluntarily choose a lifetime of only fruits, grains, and vegetables.

Walk through a typical American grocery store and you will see hundreds of varieties of fruits and vegetables. Now ask yourself whether you have ever come across any of them growing wild in a forest. You have not, because virtually everything in that produce section was shaped by selective breeding from less palatable ancestors, just as dogs were shaped from wolves. Bananas in their wild form are smaller, visually unappealing, and not particularly edible by human standards. Wild cabbage is one of my favorite examples precisely because it looks like a rough cabbage, yet it gave rise to an entire family of distinct vegetables: regular cabbage, cauliflower, Brussels sprouts, kohlrabi, kale, and broccoli all trace their lineage to that single plant.

Dolphins and whales provide one of the most fascinating evolutionary stories in the animal kingdom. Despite being mammals, their flippers bear a superficial resemblance to a shark’s fin from the outside. Look inside those flippers, however, and you find a skeletal structure that is recognizably hand-like, with bones corresponding to fingers and a wrist. The scientific explanation is that life on Earth began in the water, eventually colonized land, and dolphins and whales are the descendants of land-dwelling mammals that returned to the sea. Their anatomy carries the obvious evidence of that journey.

The relationship between humans and other primates is the point where misunderstanding is most common. We are not descended from chimpanzees or bonobos; rather, we share a common ancestor with them. Chimps and bonobos are our closest living relatives, and the overlap in biology is significant enough that we share many of the same diseases. Gorillas are the next closest. Beyond the great apes, our connections extend to other primates. Somewhere deep in evolutionary history, all of us trace back to a common ancestor that would have appeared decidedly more monkey-like than any of us do today. Our branch of the family tree simply diverged in a direction that moved furthest from that shared starting point.

One final thought. Christianity is the dominant religion across most of the Americas and Europe. Islam holds that position across much of the Middle East and parts of Africa. Buddhism is the most widespread faith across large portions of Asia outside India, which has a strong Hindu majority. Among all of these traditions, there are enormous numbers of believers who accept evolutionary theory without any conflict between their faith and the science. Every credible educational institution across all of these societies, religious or otherwise, teaches evolution as established fact. Perhaps the most remarkable thing about Charles Darwin is that he worked out the foundational logic of natural selection without any knowledge of genes or genomes. The subsequent development of genetics and our modern understanding of DNA has not just confirmed what Darwin intuited; it has made the creationist position essentially impossible to paint evolution as “just a theory.” Also we never use the term “evolutionists,” we are simply humans living in reality.