Evolution

Evolution is the process by which living things slowly change over many generations. It is the reason the world is full of so many different kinds of life: every species you see has descended from earlier species that were a little bit different. Evolution does not happen because a creature wants to change; it happens because tiny random differences between individuals (some helpful, some harmful) are sorted by survival and reproduction. Over millions of years, those tiny differences add up to entirely new species.

• Life began on Earthapprox. 3.7 billion years agoAs single-celled microbes
• First multicellular lifeapprox. 600 million years agoThe Ediacaran fauna
• Mass extinctions5 big onesPlus a 6th, human-caused, happening now
• Species ever livedapprox. 5 billionOver 99% are extinct
• Living species todayapprox. 10 millionEstimated; most still undescribed
• Darwin's book1859"On the Origin of Species"

Charles Darwin and natural selection

Charles Darwin (1809-1882) was an English naturalist who, in 1859, published a book called On the Origin of Species. In it he laid out a powerful idea now called natural selection: every population has small natural differences between individuals; the individuals best suited to their environment are more likely to survive and have babies; those babies inherit the helpful features; over many generations, those features become common in the whole population.

Natural selection happens to every living thing all the time. It is sometimes called the "survival of the fittest", although fittest does not mean "strongest"; it 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. For a peacock, fittest means most attractive tail to a peahen.

The fossil record

One of the strongest pieces of evidence for evolution is the fossil record: the long chain of preserved bones, shells and tracks left in the rocks. Older rocks contain simpler life. Younger rocks contain newer kinds. Fossils show clear gradual changes from one kind of creature into another over millions of years, including famous examples like the gradual evolution of horses from small dog-sized forest animals into the large grass-eaters of today.

Mass extinctions

The history of life has been punctuated by a handful of catastrophic events called mass extinctions, when a huge percentage of the world's species died off in a short time. There have been five big ones over the last 540 million years:

• End-Ordovician (445 million years ago)
• Late Devonian (375 to 360 million years ago)
• End-Permian (252 million years ago, the "Great Dying": 95% of species lost)
• End-Triassic (201 million years ago)
• End-Cretaceous (66 million years ago, killed the non-bird dinosaurs)

Many scientists think we are in the early stages of a sixth mass extinction right now, this time driven by humans changing the climate, cutting down forests and over-fishing the oceans.

Fact Birds are dinosaurs. The chicken pecking around in your back garden is the closest living relative of Tyrannosaurus rex. When the famous asteroid hit Earth 66 million years ago, all the dinosaurs went extinct except for one small group of feathered theropods. Those survivors evolved into the 11,000 species of bird alive today. Look at a chicken's foot or a parrot's eye and you are looking at a 66-million-year-old dinosaur in modern clothing.

Pick a topic below to explore evolution in more depth.