Evolution: How It Works main content.

Evolution: How It Works

Part of Hall of Human Origins.

Billions of years ago, the only living things on Earth were single-celled organisms similar to bacteria.

All forms of life evolved from these primitive ancestors. Over time, countless changes in the shape, size and function of various parts of the body gave rise to new forms of life—from ants to amoebas, hummingbirds to humans. These evolutionary changes have resulted from changes in DNA and the information it contains.

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© AMNH ExhibitionsSkulls of extinct ape Ouranopithecus macedoniensis and modern gorilla

Examine the Evidence: Similar Skulls

Fossils show that long ago, the species roaming the Earth were different from those we see today. Fossils also can show us how species change over time.

Compare the fossil skull of an extinct primate that lived around nine million years ago with the modern gorilla skull. Can you see how they are similar?

Similarities between the skulls suggest that a close relative of this extinct ape probably evolved into the modern gorilla.

Variation

In evolution, the fate of an individual depends on which features it has. Each human, for example, inherits a unique combination of thousands of different features, such as eye color, blood type and sensitivity to certain tastes. Some variants, or versions of a feature, may help an organism to survive. Others may be harmful. Still other variants are neutral—neither helpful nor harmful.

Seeing Spots

In Bicyclus anynana butterflies, a single gene controls the sizes and positions of the spots on the undersides of their wings. Which spot patterns will help a butterfly to survive?

Where Does Variation Come From?

Mutation

Mutations are random changes in DNA. Some mutations affect a visible feature of the individual, such as the size of a butterfly's wing spots. Others can create resistance to a virus, such as HIV. The same mutation can be harmful, helpful or neutral, depending on the environment.

Recombination

DNA changes when chromosomes shuffle, or recombine. This happens normally in the process of making a sperm or an egg. As a result, every new child or offspring inherits a unique assortment of DNA from each parent.

Although both children inherited their chromosomes from the same parents, each child got different versions.

Selection

The fate of any trait in evolution depends on survival. In the process of natural selection, organisms with an advantageous trait survive and reproduce, passing that trait on to future generations. Those with a harmful trait may die before they reproduce. Which traits are advantageous? It depends on the environment.

Chance Takes Over

When a trait is completely neutral—neither helpful nor harmful—an unpredictable process called drift causes random changes in the frequency of a trait over many generations. If wing spots were neutral, both butterfly types would be equally likely to get eaten. But eventually, one type would become more common than the other, simply by chance.

Slightly Different Genes

Drift explai