Evolution

In 1835 a young naturalist on a surveying ship spent five weeks on the Galápagos Islands, collecting birds. Back in England he noticed that the finches from different islands had different beaks: thick ones for cracking seeds here, slender ones for catching insects there. All looked like variations on a single mainland species. Charles Darwin spent the next twenty years working out what that meant. The answer, published in 1859, changed biology for ever: species are not fixed. They change, and they share ancestors.

The idea

Evolution by natural selection rests on three observations that anyone can check. First, individuals in a population vary; no two finches, or people, are quite alike. Second, some of that variation is inherited; offspring resemble their parents. Third, more offspring are born than can survive, and which ones survive is not random. Individuals whose traits suit their circumstances slightly better, a beak a little stronger where seeds are hard, leave slightly more descendants, and so the trait becomes more common. Repeat for thousands of generations and the population changes. Repeat for millions, in populations separated from one another, and you get new species.

Darwin did not know where variation came from. We do now: it comes from mutations, small random changes in DNA. Selection is not random at all. It is the environment, generation after generation, keeping what works. Nothing plans it, and nothing aims at it. Evolution has no goal and no ladder; it simply tracks what survives.

Evidence in stone

The fossil record shows life changing through time. The oldest rocks hold only microbes. Then, about 540 million years ago, animals with shells appear; then fish; then, around 370 million years ago, fish with sturdy fins and necks, like Tiktaalik, half way to walking; then amphibians, reptiles, mammals and birds, in that order, each group appearing after and never before the one it descended from. Fossils of whales with legs, birds with teeth and clawed fingers, and humans with small brains and ape-like bodies fill in the transitions that Darwin's critics said could not exist.

Evidence in bodies

Living things carry their history in their anatomy. Your arm, a bat's wing, a whale's flipper and a horse's leg contain the same bones in the same order, stretched or shrunk to different jobs, because they were inherited from a common ancestor. Whales have remnant hip bones though they have no legs. Humans have a useless tail at the base of the spine, an appendix, and goosebumps that once fluffed up fur we no longer have. Embryos of fish, chickens and people look remarkably alike in their early weeks, with gill-like pouches in all three.

Evidence in DNA

The most powerful evidence is the newest. Every species' DNA is a record of its ancestry, and the more recently two species shared an ancestor, the more alike their DNA is. Humans and chimpanzees differ by about 1.2 percent; humans and mice by far more; humans and yeast by more still, yet thousands of our genes are recognisably the same as theirs. The family tree built from DNA matches the one built from fossils and anatomy almost perfectly. Nobody designing species from scratch would have arranged it that way; descent from common ancestors explains it immediately.

Evolution you can watch

Evolution is usually slow, but not always. Bacteria reproduce every twenty minutes, and when a new antibiotic is introduced, resistant strains appear within years; that is natural selection in a hospital. Viruses such as influenza evolve so fast that vaccines must be updated annually. Peppered moths in industrial England turned dark within decades as soot blackened the trees they rested on, and turned pale again when the air was cleaned. In the Galápagos, Darwin's finches have been watched evolving beak size in response to droughts within a single human lifetime.

Humans

Humans are part of the tree, not apart from it. We share an ancestor with chimpanzees that lived about six or seven million years ago in Africa. Fossils trace our lineage through creatures that walked upright but had small brains, through larger-brained toolmakers, to our own species, Homo sapiens, which appeared about 300,000 years ago. For most of that time we shared the planet with other human species, including Neanderthals, and DNA shows we interbred with them; most people outside Africa carry a percent or two of Neanderthal ancestry.

Why it matters

Evolution is the central idea of biology, the reason the living world makes sense. It explains why there are so many species and why they are so clearly related, why bodies are built the way they are, why diseases change and how to fight them, and where we come from. It also sets us in perspective: a young species on a very old tree, sharing deep roots with every other living thing.