The first essence in the theory was that of evolution itself. Even though some of Darwin’s ideas on evolution had an older pedigree, the French and English naturalists that preceded him (among whom, figures such as Pierre-Louis Moreau de Maupertuis, Jean-Baptiste Lamarck, Robert Chambers, and Darwin’s own grandfather, Erasmus Darwin, stood out) failed to provide a convincing mechanism for evolution to take place. Here is how Darwin himself described evolution: “The view which most naturalists entertain, and which I formerly entertained—namely, that each species has been independently created—is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species.” In other words, the species that we encounter today did not always exist. Rather, these are the descendants of some earlier species that became extinct. Modern biologists tend to distinguish between microevolution and macroevolution. Microevolution encompasses small changes (such as those sometimes observed in bacteria) that are the results of the evolutionary process over relatively short periods of time, typically within local populations. Macroevolution refers to the results of evolution over long timescales, typically among species—and which could also involve mass extinction episodes, such as the one that snuffed out the dinosaurs. In the years since the publication of The Origin, the idea of evolution has become so much the guiding principle of all the research in the life sciences that in 1973 Theodosius Dobzhansky, one of the twentieth century’s most eminent evolutionary biologists, published an essay entitled “Nothing in Biology Makes Sense Except in the Light of Evolution.” At the end of this article, Dobzhansky noted that the twentieth-century French philosopher and Jesuit priest Pierre Teilhard de Chardin “was a creationist, but one who understood that the Creation is realized in this world by means of evolution.”

Darwin borrowed the idea embodied in his second pillar, that of gradualism, mainly from the works of two geologists. One was the eighteenth-century geologist James Hutton, and the other was Darwin’s contemporary and later close friend Charles Lyell. The geological record showed horizontal banding patterns covering large geographical areas. This, coupled with the uncovering of different fossils within these bands, suggested a progression of incremental change. Hutton and Lyle were largely responsible for the formulation of the modern theory of uniformitarianism: the notion that the rates at which processes such as erosion and sedimentation occur at present are similar to the rates in the past. (We shall return to this concept in chapter 4, when we’ll discuss Lord Kelvin.) Darwin argued that just as geological action shapes the Earth gradually but surely, evolutionary changes are the result of transformations that span hundreds of thousands of generations. One should not, therefore, expect to see significant alterations in less than tens of thousands of years, except perhaps in organisms that multiply very frequently, such as bacteria, which, as we know today, can develop resistance to antibiotics in extremely short times. Contrary to uniformitarianism, however, the rate of evolutionary changes is generally nonuniform in time for a given species, and it can vary further from one species to another. As we shall see later, it is the pressure exerted by natural selection that determines primarily how fast evolution manifests itself. Some “living fossils” such as the lamprey—a jawless marine vertebrate with a funnel-like mouth—appear to have hardly evolved in 360 million years. As a fascinating aside, I should note that the idea of gradual change was put forth in the seventeenth century by the empiricist philosopher John Locke, who wrote insightfully, “The boundaries of the species, whereby men sort them, are made by men.”

The next pillar in Darwin’s theory, the concept of a common ancestor, is what has become in its modern incarnation the primary motivator for all of the present-day searches for the origin of life. Darwin first argued that there is no doubt that all the members of any taxonomic class—such as all vertebrates—originated from a common ancestor. But his imagination carried him much further with this concept. Even though his theory predated any knowledge of the facts that all living organisms share such characteristics as the DNA molecule, a small number of amino acids, and the molecule that serves as the currency for energy production, Darwin was still bold enough to proclaim, “Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.” Then, after cautiously acknowledging that “analogy may be a deceitful guide,” he still concluded that “probably all the organic beings which have ever lived on the earth have descended from some one primordial form, into which life was first breathed.”

But, you may wonder, if all life on Earth originated from a single, common ancestor, how did the astonishing wealth of diversity arise? After all, this was the first hallmark of life that we have identified as one that requires an explanation. Darwin did not flinch, and took this challenge head-on—it was not an accident that the title of his book had the word “species” in it. Darwin’s solution to the diversity problem involved another original idea: that of branching, or speciation. Life starts from a common ancestor, just as a tree has a single trunk, Darwin reasoned. In the same way that the trunk develops branches, which then split into twigs, the “tree of life” evolves by many branching and ramification events, creating separate species at each splitting node. Many of these species become extinct, just like the dead and broken branches of a tree. However, since at each splitting the number of offspring species from a given ancestor doubles, the number of different species can increase dramatically. When does speciation actually occur? According to modern thinking, mainly when a group of members of a particular species becomes geographically separated. For instance, one group may wander to the rainy side of a mountain range, while the rest of the species stays on the dry slope. Over time, these rather different environments produce different evolutionary paths, eventually leading to two populations that can no longer interbreed—or in other words, different species. In rarer occasions, speciation could create new species that arise from interbreeding between two species. Such appears to have been the case of the Italian sparrow, which was shown in 2011 to be genetically intermediate between Spanish sparrows and house sparrows. Italian and Spanish sparrows behave like distinct species, but Italian and house sparrows do form hybrid zones, where the ranges of the two interbreeding species meet.

Amazingly, in 1945, author Vladimir Nabokov, of Lolita and Pale Fire fame, came up with a sweeping hypothesis for the evolution of a group of butterflies known as the Polyommatus blues. Nabokov, who had a lifelong interest in butterflies, speculated that the butterflies came to the New World from Asia in a series of waves lasting millions of years. To their surprise, a team of scientists using gene-sequencing technology confirmed Nabokov’s conjecture in 2011. They found that the New World species shared a common ancestor that lived about ten million years ago, but that many New World species were more closely related to Old World butterflies than to their neighbors.