This argument relies on a misunderstanding of how the word "theory" is used in science. In everyday language, "theory" often means a guess or speculation.1 In science, it means something very different.
Scientific theories
The National Academy of Sciences defines a scientific theory as "a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence."1 Scientific theories explain how and why natural phenomena occur, make testable predictions, and could in principle be proven wrong by new evidence.2
Several foundational scientific theories illustrate what "theory" means in practice. Before the 1860s, many believed diseases arose spontaneously or from "bad air" (miasma). Louis Pasteur and Robert Koch established the germ theory of disease—that microorganisms cause infectious diseases.3 This theory revolutionized medicine, leading to antiseptic surgery, vaccination, and antibiotics.3
The atomic theory that all matter is composed of atoms has been confirmed by countless experiments, from Brownian motion observations to scanning tunneling microscopy that images individual atoms.4 Einstein's theory of general relativity describes gravity as the curvature of spacetime and has been confirmed by gravitational lensing, the precession of Mercury's orbit, gravitational waves detected by LIGO, and the functioning of GPS satellites, which must account for relativistic time dilation to maintain accuracy.5
Cell theory holds that all living organisms are composed of cells, and that cells arise from pre-existing cells, forming the foundation of biology.6 Plate tectonic theory explains that Earth's lithosphere is divided into plates that move, collide, and separate, accounting for earthquakes, volcanic activity, mountain formation, and the distribution of fossils across continents.7 Plate movements are now directly measured by GPS to millimeter precision.18
None of these theories will be "promoted" to facts. They are explanatory frameworks supported by overwhelming evidence, and that is exactly what makes them theories.2
Fact and theory
The word "evolution" actually refers to two related but distinct concepts.16 The fact of evolution is that species change over time, which is directly observable and thoroughly documented.8 The fossil record documents the history of life over 3.5 billion years, showing progressive changes in organisms and transitional forms between major groups.8 Transitional fossils like Tiktaalik (fish to tetrapod) and Archaeopteryx (dinosaur to bird) document evolutionary transitions between major groups.19
Bacteria develop antibiotic resistance through mutation and natural selection—a process so well-documented that the CDC tracks it as a major public health concern, with 2.9 million Americans infected by antibiotic-resistant pathogens annually.9 Scientists have observed new species emerge in real time. Darwin's finches on the Galápagos Islands have been documented diverging into new species over just a few decades.10 European flounders have speciated within approximately 3,000 generations.11 Laboratory experiments with fruit flies and bacteria have produced reproductive isolation—the hallmark of speciation.12
DNA comparisons reveal the same nested hierarchy predicted by common descent. Humans share 98.8% of their DNA with chimpanzees, and genetic similarities decrease predictably with evolutionary distance.13 Endogenous retroviruses—ancient viral sequences embedded in genomes—appear in the same chromosomal locations across related species, a pattern explicable only by common ancestry.14
DNA sequence similarity to humans13
| Species | Similarity | Divergence |
|---|---|---|
| Chimpanzee | 98.8% | ~6 mya |
| Gorilla | 98.4% | ~9 mya |
| Orangutan | 96.9% | ~14 mya |
| Mouse | 85% | ~90 mya |
The theory of evolution explains the mechanisms by which evolution occurs—natural selection, genetic drift, gene flow, and mutation.1 It explains how and why species change. Like all scientific theories, it is subject to refinement as new evidence emerges, but its core principles are extraordinarily well-supported. As of 2014, approximately 98% of the scientific community accepts evolution.15
Scientific acceptance of evolution (2014)15
The hierarchy myth
A common misconception is that scientific ideas progress from hypothesis to theory to fact, as if theories are promoted to facts once they're proven. This is incorrect.16 In science, these terms have distinct meanings.2 A fact is an observation about the world that has been repeatedly confirmed. A hypothesis is a tentative, testable explanation for a specific phenomenon. A theory is a well-substantiated explanation that unifies a broad range of observations and has withstood repeated testing. A law is a descriptive statement about how nature behaves under certain conditions, often expressed mathematically.2
A theory doesn't become a fact any more than an explanation becomes an observation. They serve different purposes.16 Gravity is a fact—objects fall. The theory of gravity explains why. Similarly, evolution is a fact—species change. Evolutionary theory explains how.
The evidence
Evolutionary theory has been tested and confirmed by evidence from multiple independent fields.8 The fossil record shows a progression from simple to complex organisms, with transitional forms connecting major groups.8 Comparative anatomy reveals homologous structures—like the forelimbs of humans, whales, bats, and dogs—that share common ancestry despite serving different functions.20 Vertebrate embryos show similar developmental stages, reflecting shared evolutionary history.20 DNA and protein sequences confirm evolutionary relationships predicted by anatomical and fossil evidence.13 The geographic distribution of species reflects evolutionary history and continental drift.8
These independent lines of evidence all converge on the same conclusion.8 In 1922, the American Association for the Advancement of Science stated that "no scientific generalization is more strongly supported by thoroughly tested evidences than is that of organic evolution."17 A century of additional evidence has only strengthened this conclusion.