Origin of life and Darwin's idea

Evolution is the story of how life changed over time. It begins with the universe itself, about 13.8 billion years old by the Big Bang theory. The earth formed about 4.5 billion years ago, and life appeared about four billion years ago.

Where did the first life come from? People once believed living things could arise from rotting matter: spontaneous generation. Louis Pasteur disproved it. In flasks that had been sterilised and sealed, killed yeast gave no new life; in flasks open to the air, new organisms appeared. Life comes only from pre-existing life.

But the very first life must have come from non-living matter. Oparin and Haldane proposed that it formed from simple chemicals: chemical evolution came first. The early earth's atmosphere had methane, ammonia, hydrogen and water vapour, energised by lightning and UV light. In 1953, S.L. Miller tested this: he sealed CH₄, H₂, NH₃ and water vapour in a flask at 800 °C and passed electric sparks through it. After a week, amino acids had formed. Later experiments produced sugars, nitrogen bases, pigments and fats.

Darwin's theory. Charles Darwin sailed round the world on the ship H.M.S. Beagle, studying plants, animals and fossils. He concluded:

• Members of a population vary, and many variations are inherited.
• More offspring are born than can survive, so they compete for limited resources. (Darwin may have been influenced by the economist Thomas Malthus, who wrote about populations outgrowing their food.)
• Those with variations that suit their surroundings survive and leave more offspring. This is natural selection.
• Over many generations, such changes add up, and new kinds of organisms arise.

For Darwin, fitness finally means reproductive fitness: the fit are those who leave more offspring. (The phrase 'survival of the fittest' was actually coined by Herbert Spencer.) Alfred Wallace, working in the Malay Archipelago, reached the same idea at about the same time.

An earlier idea, from Lamarck, was that organs change by use and disuse, and that the changes are passed on: the giraffe got its long neck by stretching for leaves, generation after generation. This idea of inherited acquired characters is no longer accepted: a change in your body during your life does not change your genes.