What Is Evolution?

Evolution is the slow change in living things over many generations. Every plant, animal and microbe alive today is descended from earlier creatures that were slightly different, going all the way back to the first single-celled life over 3.7 billion years ago. Evolution is not random; it is shaped by a powerful process called natural selection, which gradually fits each species to its environment over thousands or millions of generations.

  • Age of life on Earthapprox. 3.7 billion yearsFirst single-celled microbes
  • First multicellular lifeapprox. 600 million years agoEdiacaran fauna
  • First fishapprox. 530 million years agoBeginning of vertebrates
  • First land plantsapprox. 470 million years agoMoss-like
  • Dinosaursapprox. 230 to 66 million years ago164 million years on Earth
  • Modern humansapprox. 300,000 years agoIn Africa

The basic idea

Evolution rests on a simple chain of observations.

  • In any population of living things, individuals are slightly different from each other (you and your siblings, for example, all look a bit different).
  • Some of those differences are passed on from parents to children through genes.
  • Individuals best suited to their environment are more likely to survive and have babies.
  • So the helpful features become more common in the next generation, and the unhelpful ones become less common.
  • Repeat over many generations and the whole population gradually changes.

That process is called natural selection, and it is the engine of evolution. Over millions of years, small changes add up to enormous ones. Whales evolved from small four-legged land mammals. Birds evolved from dinosaurs. Humans evolved from earlier apes. Every species you see today is the result of billions of these tiny adjustments stretching back to the dawn of life.

The tree of life

One of the most powerful ideas in evolution is the tree of life. Every species alive today shares a common ancestor with every other, somewhere back in time. The further back you go, the broader the common ancestor.

  • You and your siblings share a common ancestor about 0 to 100 years ago: your parents.
  • You and your second cousins share a common ancestor (your great-grandparents).
  • You and a chimpanzee share a common ancestor that lived about 6 to 7 million years ago.
  • You and a mouse share a common ancestor that lived about 80 million years ago.
  • You and a frog share a common ancestor that lived about 350 million years ago.
  • You and a daffodil share a common ancestor that lived around 1.6 billion years ago.

This is one of the most beautiful ideas in biology: every living thing on Earth is related to every other, with no exceptions. Trace any two species back far enough and you will eventually find a common great-great-...-grandparent.

The evidence

The evidence for evolution comes from many independent areas of science.

  • Fossils: dig in old rocks and you find simpler life. Dig in younger rocks and you find more recent forms. The order is exactly what evolution predicts.
  • Comparative anatomy: a human arm, a bat wing, a whale flipper and a cat's leg all share the same underlying bone structure, evidence of a common ancestor.
  • DNA: closely related species share more DNA than distantly related ones. The pattern fits a single tree of life.
  • Vestigial features: leftover body parts that no longer serve a purpose (the human tailbone, whale leg bones, the wings of flightless birds) are easy to explain as relics of evolved-away ancestors.
  • Directly observed evolution: scientists have watched bacteria evolve resistance to antibiotics in the lab, watched insects evolve resistance to pesticides in fields, and watched Galapagos finch beaks change shape from year to year as the weather changes their food supply.

Common misunderstandings

Evolution is one of the most misunderstood ideas in science. A few things to get right:

  • "Survival of the fittest" does not mean strongest. "Fittest" means "best suited to the local environment". For a polar bear, fittest means warmest fur. For a desert plant, fittest means most water-saving leaves.
  • Individuals do not evolve. Evolution happens to populations over many generations. A single animal does not change into a different species during its lifetime.
  • Evolution has no goal. It is not aiming at humans or any other "advanced" form. Bacteria are just as evolved as we are.
  • Humans did not evolve from chimps. Humans and chimps share a common ancestor that lived 6 to 7 million years ago. The common ancestor was not the same as a modern chimp.
  • Evolution is a theory in the scientific sense, the same kind of theory as the theory of gravity or atomic theory. In science, "theory" does not mean a guess; it means a well-tested explanation supported by huge amounts of evidence.
Fact Every modern bird is, technically, a dinosaur. When the famous asteroid hit Earth 66 million years ago, almost all the dinosaurs went extinct. But one small group of feathered, two-legged theropod dinosaurs survived. Their descendants spread, diversified and evolved into the over 11,000 bird species alive today. The chicken pecking around in your back garden really is the closest living relative of Tyrannosaurus rex.

How fast does evolution happen?

Most evolutionary change is slow on a human timescale: thousands or millions of years for major new features to appear. But evolution can also be fast in the right circumstances.

  • Bacteria evolve resistance to antibiotics in years.
  • Pesticide-resistant insects can appear within a few growing seasons.
  • Galapagos finches have measurably changed beak shape over just one or two generations during droughts.
  • Italian wall lizards moved to a small Mediterranean island in 1971 evolved a noticeably different head shape and diet in just 36 years.
Did you know? Charles Darwin was not the first person to suggest the idea of evolution. The basic idea had been floating around for over 50 years before his 1859 book. What Darwin did was provide a convincing mechanism (natural selection) and the careful evidence to back it up. He spent over 20 years gathering data before he was ready to publish.
Deeper dive: why evolution explains the diversity of life

One of the most powerful predictions of evolution is the pattern of diversity in living things. If species really did all descend from a few common ancestors over billions of years, then we should see a tree-like pattern of relatedness, with species sharing more features as you trace them back to closer branches. And that is exactly what we see, in feature after feature, gene after gene.

Compare any group of related animals (say, the mammals). All mammals share several features in common: warm blood, hair or fur, mammary glands. Within mammals, some subgroups share more specific features: hooved animals all have hoof-shaped feet; cats and dogs share retractable claws (well, cats do, dogs sort of do); rodents all have continuously-growing front teeth. The closer in the tree, the more shared features.

The same pattern appears in DNA. Compare the genomes of any group of animals and you can build a "genetic tree" purely from the DNA sequences. The genetic tree matches the anatomical tree almost perfectly, which is a powerful confirmation that both reflect a real underlying history. This is one of the strongest pieces of evidence for evolution: completely different lines of evidence point at the same tree of life.

This is also why evolution is sometimes called the "central idea" of biology. Without it, the patterns of similarities and differences between species would be a complete mystery. With it, almost everything in the living world starts to make sense.

For the engine of evolution, see natural selection and Charles Darwin. For how species form, see species and speciation.