chapter 1

We are all one with creeping things;
And apes and men
Blood-brethren.

From ‘Drinking Song’ by Thomas Hardy

The consensus among the scientific community is that the Earth is a planet orbiting a fairly typical star, one of many billions of stars in a galaxy among billions of galaxies in an expanding universe of enormous size, which originated about 14 billion years ago. The Earth itself formed as the result of a process of gravitational condensation of dust and gas, which also generated the Sun and other planets of the solar system, about 4.6 billion years ago. All present-day living organisms are the descendants of self-replicating molecules that were formed by purely chemical means, more than 3.5 billion years ago. The successive forms of life have been produced by the process of ‘descent with modification’, as Darwin called it, and are related to each other by a branching genealogy, the tree of life. We human beings are most closely related to chimpanzees and gorillas, with whom we shared a common ancestor 6 to 7 million years ago. The mammals, the group to which we belong, shared a common ancestor with living species of reptiles about 300 million years ago. All vertebrates (mammals, birds, reptiles, amphibia, fishes) trace their ancestry back to a small fish-like creature that lacked a backbone, which lived over 500 million years ago. Further back in time, it becomes increasingly difficult to discern the relationships between the major groups of animals, plants, and microbes, but, as we shall see, there are clear signs in their genetic material of common ancestry.

Less than 450 years ago, all European scholars believed that the Earth was the centre of a universe of at most a few million miles in extent, and that the planets, Sun, and stars all rotated around this centre. Less than 250 years ago, they believed that the universe was created in essentially its present state about 6,000 years ago, although by then the Earth was known to orbit the Sun like other planets, and a much larger size of the universe was widely accepted. Less than 150 years ago, the view that the present state of the Earth is the product of at least tens of millions of years of geological change was prevalent among scientists, but the special creation by God of living species was still the dominant belief.

The relentless application of the scientific method of inference from experiment and observation, without reference to religious or governmental authority, has completely transformed our view of our origins and relation to the universe, in less than 500 years. In addition to the intrinsic fascination of the view of the world opened up by science, this has had an enormous impact on philosophy and religion. The findings of science imply that human beings are the product of impersonal forces, and that the habitable world forms a minute part of a universe of immense size and duration. Whatever the religious or philosophical beliefs of individual scientists, the whole programme of scientific research is founded on the assumption that the universe can be understood on such a basis.

Few would dispute that this programme has been spectacularly successful, particularly in the 20th century, which saw such terrible events in human affairs. The influence of science may have indirectly contributed to these events, partly through the social changes triggered by the rise of industrial mass societies, and partly through the undermining of traditional belief systems. Nonetheless, it can be argued that much misery throughout human history could have been avoided by the application of reason, and that the disasters of the 20th century resulted from a failure to be rational rather than a failure of rationality. The wise application of scientific understanding of the world in which we live is the only hope for the future of mankind.

The study of evolution has revealed our intimate connections with the other species that inhabit the Earth; if global catastrophe is to be avoided, these connections must be respected. The purpose of this book is to introduce the general reader to some of the most important basic findings, concepts, and procedures of evolutionary biology, as it has developed since the first publications of Darwin and Wallace on the subject, over 140 years ago. Evolution provides a set of unifying principles for the whole of biology; it also illuminates the relation of human beings to the universe and to each other. In addition, many aspects of evolution have practical importance; for instance, pressing medical problems are posed by the rapid evolution of resistance by bacteria to antibiotics and of HIV to antiviral drugs.

In this book, we shall first introduce the main causal processes of evolution (Chapter 2). Chapter 3 provides some of the basic biological background, and shows how the similarities between living creatures can be understood in terms of evolution. Chapter 4 describes the evidence for evolution derived from Earth history, and from the patterns of geographical distribution of living species. Chapter 5 is concerned with the evolution of adaptations by natural selection, and Chapter 6 with the evolution of new species and of differences between species. In Chapter 7, we discuss some seemingly difficult problems for the theory of evolution. Chapter 8 provides a brief summary.