People throughout the world have developed systems of classification that help them understand things: earth and sky; seen and unseen; animal, vegetable, and mineral; past, present, and future. Among these systems of classification was one invented in eighteenth-century Europe that divided all living things on earth into groups. Each of these divisions — such as that between plants and animals — is further subdivided into smaller and smaller groups, such as class, order, family, and genus. The final important division is the species, which is generally defined as a group of organisms that can interbreed with one another and produce fertile offspring of both sexes.

In their natural state, members of a species resemble one another, but over time they can become increasingly dissimilar. (Think of Chihuahuas and Great Danes, both members of the same species.) Ever since humans began shaping the world around them, this process has often been the result of human action. But in the long era before humans, the increasing dissimilarity resulted, in the opinion of most scientists, from the process of natural selection. Small variations within individuals in one species enabled them to acquire more food and better living conditions and made them more successful in breeding, thus allowing them to pass their genetic material on to the next generation. When a number of individuals within a species became distinct enough that they could no longer interbreed successfully with others, they became a new species. Species also become extinct, particularly during periods of mass extinctions such as the one that killed the dinosaurs about 65 million years ago. Natural processes of species formation and extinction continue, although today changes in the biosphere — the living matter in the world — result far more from human action than from natural selection.

The scientists who developed this system of organizing the world placed humans within it, using the same means of classification that they used for all other living things. Humans were in the animal kingdom, the order of Primates, the family Hominidae, and the genus Homo. Like all classifications, this was originally based on externally visible phenomena: humans were placed in the Primates order because, like other primates, they have hands that can grasp, eyes facing forward to allow better depth perception, and relatively large brains; they were placed in the hominid family along with chimpanzees, gorillas, and orangutans because they shared even more features with these great apes. Over 98 percent of human DNA is the same as that of chimpanzees, which indicates to most scientists that humans and chimpanzees share a common ancestor. That common ancestor probably lived between 5 million and 7 million years ago.

Physical remains were the earliest type of evidence studied to learn about the distant human past, and scholars used them to develop another system of classification, one that distinguished between periods of time rather than types of living creatures. (Constructing models of time is called “periodization.”) They gave labels to eras according to the primary materials out of which tools that survived were made. Thus the earliest human era became the Stone Age, the next era the Bronze Age, and the next the Iron Age. They further divided the Stone Age into the Old Stone Age, or Paleolithic era, during which people used stone, bone, and other natural products to make tools and gained food largely by foraging — that is, by gathering plant products, trapping or catching small animals and birds, and hunting larger prey. This was followed by the New Stone Age, or Neolithic era, which saw the beginning of agricultural and animal domestication. People around the world adopted agriculture at various times, and some never did, but the transition between the Paleolithic and the Neolithic is usually set at about 9000 B.C.E., the point at which agriculture was first developed.

Geologists refer to the last twelve thousand years as the Holocene (meaning very recent) epoch. The entire history of the human species fits well within the Holocene and the previous geologic epoch, the Pleistocene (PLIGH-stuh-seen), which began about 2.5 million years ago.

The Pleistocene was marked by repeated advances in glaciers and continental ice sheets. Glaciers tied up huge quantities of the earth’s water, leading to lower sea levels, making it possible for animals and eventually humans to walk between places that were separated by oceans during interglacial times. Animals and humans were also prevented from migrating to other places by the ice sheets themselves, however, and the colder climate made large areas unfit to live in. Climate thus dramatically shaped human cultures.