Evolution by natural selection is not a process by which objects called “species” change over time under selective pressures. There are no such objects. It is a process by which organisms that are related in salient ways, by which they are specific we might say (as opposed to being members of a species), are replaced by others. There is more than one way in which they can be specific. One is related compatibility, which some organisms, the xs, exhibit just when they have common ancestry, when they are reproductively compatible, and when every organism so related to one of the xs is among the xs. Given this way of understanding specificity, we current humans, the Homo sapiens presently inhabiting the planet, could give way to future humans that resemble us only insofar as they are reproductively compatible with us. What is more, our humanity is not essential to us. Neither is our origin, in that we might have originated in a different time and place. So in theory – given technology that is currently out of sight – we could change ourselves into creatures that are very unlike us.

Mass extinctions decimate the planet’s biodiversity, and in doing so, they can change the composition of the planet’s biota.The biota that goes into a mass extinction is not the same as the one that emerges. The actual extinctions are over very quickly – 20,000 years in the case of the end-Permian. But the recovery takes much longer. It takes time for new species to evolve and the biosphere to recover – and the Earth System will not operate properly until both processes are complete. Detailed analysis of the fossil content of sediments deposited following the end-Cretaceous extinction event reveal a long-term ecological recovery that parallels the short-term ecological succession that follows modern environmental disasters such as fires and floods. But the succession that follows a mass extinction occurs on a global scale and over a much longer time frame – often millions of years. This new post-mass-extinction recovery phase has been dubbed the Earth System succession.

The book sets today's defaunation event into its historical setting. The best source of information available to do that is the fossil record. Unfortunately, the fossil record has some inbuilt limitations that affect any analysis that uses it. This chapter starts by outlining these limitations and what we can do limit them. I then look at how the extinction and origination (evolutionary appearance) of species are recorded in the fossil record. This is important as the pattern of extinction and origination of species over deep time forms the basis of the geological time scale that is used throughout the book. Finally, I examine the beginnings of modern extinction studies and the recognition of the Big Five mass extinctions.