Children are a little different from their parents. Some differences help a living thing survive and have babies, so those differences get passed on more often. Over many, many generations, small changes add up to big ones. That's evolution.
Evolution is as well supported as any idea in science. Fossils, DNA, anatomy, the way species are spread across the planet, and experiments all point to the same family tree of life. It doesn't say life came from nothing, and it doesn't say humans came from today's monkeys: we share ancestors with them.
"It's just a theory." "There are no transitional fossils." "If we came from monkeys, why are there still monkeys?" "Nobody has ever seen evolution happen." Each of these has a clear answer below.
1. How it works, in four steps
No two living things are exactly alike. Each baby has a few new DNA changes (mutations), and genes get shuffled.
Parents pass their DNA on, so the differences can be passed down.
More are born than can survive. Those whose differences fit their surroundings better leave more offspring.
Repeat for thousands or millions of generations, and the small shifts add up to new features and new species.
Nobody is "choosing". Nature just filters. That's why it's called natural selection.
2. Run it yourself: beetles and birds
These beetles come in many colours. Birds eat the beetles they can spot easily. Press Run and watch what the population looks like after a few generations, then change the ground and watch it shift back.
Generation 0. Beetles of every colour.
That's the whole idea in miniature: no beetle tried to change colour. The ones that blended in simply survived more often. The same thing has been seen in real life, from moths that darkened during the Industrial Revolution to lizards and finches measured over decades.
3. Evidence in the rocks
If evolution is true, fossils should appear in a particular order, with in-between forms in the right layers. That's exactly what we find, and scientists have used that to make predictions that came true.
| Fossil | Age | Why it matters |
|---|---|---|
| Tiktaalik | about 375 million years | A fish with a neck, wrists and lungs. Researchers predicted the age of rock where a fish-to-land animal should be, searched rocks of that age in Arctic Canada, and found it (2004).1 |
| Archaeopteryx | about 150 million years | Feathers and wings, but also teeth, claws and a bony tail like a small dinosaur. |
| Early whales (Pakicetus, Ambulocetus, Basilosaurus) | about 50 to 40 million years | A series from four-legged land mammals to fully marine whales. Modern whales still have tiny leftover hip bones. |
| "Lucy" (Australopithecus afarensis) | about 3.2 million years | Walked upright, but with an ape-sized brain. |
| Homo sapiens | about 300,000 years | The oldest fossils of our own species, from Jebel Irhoud, Morocco. |
Just as important is what we never find: no rabbits next to trilobites, no humans with dinosaurs. One fossil in the wrong layer could break the theory. In over 150 years of digging, it hasn't happened.
4. Evidence in our DNA
- Chromosome 2. Other great apes have 24 pairs of chromosomes; we have 23. Our chromosome 2 carries the marks of two ape chromosomes fused end to end: chromosome-end sequences in the middle, and a leftover second centre.2 Common ancestry predicted a fusion like this; it was found.
- Viral scars. About 8% of our DNA is made of old virus genes (endogenous retroviruses). We share many of them with chimpanzees in exactly the same places, which is what you'd expect if a shared ancestor was infected.
- Broken genes. We carry a broken copy of the gene other mammals use to make vitamin C. Chimps and other apes have the same break.
5. Evolution we've watched happen
- Bacteria in a flask. Since 1988, Richard Lenski's lab has grown 12 populations of E. coli. After about 31,500 generations, one population evolved the ability to eat citrate, something its species normally can't do.3
- Lizards on an island. In 1971, ten Italian wall lizards were moved to the island of Pod Mrčaru. About 36 years later their descendants had bigger heads, stronger bites, a plant-heavy diet, and new valves in their guts.4
- Germs that beat our medicines. Antibiotic-resistant bacteria are evolution happening in real time, which is why doctors tell you to finish the course.
6. Common misunderstandings
| People say | What's actually true |
|---|---|
| "It's just a theory." | In science a theory is a well-tested explanation, like the theory of gravity or germ theory. It's not a guess. |
| "Why are there still monkeys?" | We didn't come from today's monkeys. We share common ancestors with them, the way you and your cousins share grandparents. |
| "There are no transitional fossils." | There are thousands. Every fossil is a link between what came before and what came after. See the table above. |
| "Evolution says life came from nothing." | Evolution explains how life changes once it exists. How life began is a separate question: see Where did life come from? |
| "Something as complex as an eye couldn't evolve." | Animals alive today show every stage, from light-sensitive patches to cups to pinholes to lenses. Each stage is useful by itself. |
7. Watch it explained
Living things share DNA, anatomy and a fossil record that all fit one branching family tree. Evolution has been observed directly in labs and in the wild.
Each kind of animal was created separately, in its present form, a few thousand years ago.
Intelligent-design supporters like biochemist Michael Behe argue that some cell machines, like the bacterial flagellum, are "irreducibly complex": take away one part and they stop working, so they couldn't have been built step by step. Many Christians also accept evolution and see it as God's method. The scientific reply: parts of the flagellum work on their own for other jobs (one subset forms a pump that injects proteins into other cells), so "useless until finished" isn't true. In 2005 a U.S. federal judge, after hearing from both sides in Kitzmiller v. Dover, ruled that intelligent design is not science.5
Sources
- Daeschler, Shubin & Jenkins, "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan", Nature 440 (2006).
- IJdo et al., "Origin of human chromosome 2: an ancestral telomere-telomere fusion", PNAS 88 (1991); see also Fan et al., Genome Research (2002).
- Blount, Borland & Lenski, "Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli", PNAS 105 (2008); Carl Zimmer, "A new step in evolution", National Geographic.
- Herrel et al., "Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource", PNAS 105 (2008).
- Kitzmiller v. Dover Area School District, 400 F. Supp. 2d 707 (M.D. Pa. 2005).