Chapter I
A Theory in a Notebook
The idea that species change was not new in the nineteenth century. Lamarck had proposed it in 1809, and Darwin's own grandfather had speculated about it. What was missing was a mechanism that could produce the exquisite fit between organisms and their ways of life, the fit that natural theologians like William Paley took as proof of a designer.
Charles Darwin found one after returning from the five-year voyage of the Beagle. In 1838, reading Malthus on population, he saw that if more individuals are born than can survive, and if they vary in heritable ways, then favourable variations will accumulate. Breeders did this deliberately with pigeons and dogs. Nature would do it blindly, over far longer times. He wrote it up privately in 1844 and then spent fourteen years gathering evidence.
Chapter II
Wallace's Letter
In June 1858 a letter arrived from Alfred Russel Wallace, collecting beetles and birds in the Malay Archipelago. It contained the same theory. Darwin's friends arranged for both men's papers to be read at the Linnean Society, and Darwin rushed out an "abstract" of his planned big book. That was On the Origin of Species (1859). Booksellers ordered more than its whole first printing of 1,250 copies before it was even published.
The argument had two parts. The first, that species share common ancestors and change over time, persuaded most naturalists within about fifteen years, helped by fossils like Archaeopteryx, described by Richard Owen in 1863. The second, that natural selection is the main cause, did not. Darwin could not explain inheritance. The theory of his day, that offspring blend their parents' traits, would dilute any favourable variation to nothing within a few generations. Around 1900 many biologists doubted selection altogether, a period later called the "eclipse of Darwinism".
Chapter III
A Closer Look: Time Enough for an Eye
Paley's strongest example of design was the eye, and Darwin admitted that it seemed absurd to suppose it formed by natural selection. In 1994 Dan-Eric Nilsson and Susanne Pelger estimated how long it would actually take. They modelled a flat patch of light-sensitive cells, with a transparent layer above and dark pigment below, and let it change in small steps, each improving vision slightly: the patch cups, deepens, its opening narrows, and a lens forms with a gradient of density.
They made each step a change of 1% in some dimension of the structure, and counted the steps needed to turn a flat patch into a camera eye like a fish's. The answer was 1,829 steps. Compounded, that is an enormous total change:
an eighty-million-fold change in the relevant proportions. But each step is tiny.
How many generations does a 1% change take? From measurements of heritability and selection in natural populations, they deliberately chose pessimistic values: heritability of 0.5, and selection so weak that each generation changes the average by only 0.005% of the trait. That gives about 364,000 generations for the whole sequence. For small aquatic animals, a generation is typically a year. So a complete camera eye could evolve in fewer than 400,000 years.
The Cambrian explosion, when eyes first appear in the fossil record, lasted around twenty million years. By this estimate, eyes could have evolved from scratch dozens of times over in that span. And eyes have evolved independently many times, in molluscs, vertebrates and arthropods. The calculation does not show how eyes did evolve, which depends on genes and development. It shows that the time available is not a problem.
Chapter IV
The Synthesis and After
The rescue came from Mendel's genetics. Inheritance works through discrete units that do not blend, so variation is preserved. In the 1920s and 30s, the two were joined mathematically in population genetics, which made natural selection the core of modern biology.
The tree of life itself kept surprising. Lynn Margulis argued in 1967 that the mitochondria in our cells were once free-living bacteria, an idea rejected by about fifteen journals before it was published and later confirmed by their DNA. In 1977 Carl Woese, reading the sequence of a molecule every cell shares, found that one group of "bacteria" formed an entirely separate domain of life, the archaea. Evolution was now being read from molecules, and that line of work leads into genomics.
Two old questions remain at the edge of the map: how life began, and why most complex creatures reproduce sexually when it looks so costly.