Geologic time is measured through the radioactive decay process, determining the radiometric age of rocks in years. A geomagnetic time scale has been established from the number and duration of reversals in the magnetic field of the earth's past.

Correlation and the determination of the numerical ages of rocks and events have led to the development of a geologic time scale. The major blocks of time on this calendar are called eons. The Hadean eon, 3.8 to 4.5 billion years ago, is the period of time before the earth's surface solidified. The Archean eon, 2.5 to 3.8 billion years ago, is a time when the atmosphere lacked oxygen and only certain kinds of prokaryotic organisms existed. The Proterozoic eon, 2.5 billion to 540 million years ago, saw the development of an oxidizing atmosphere, the first eukaryotic cells, and the first multicellular organisms. The time encompassed by the Hadean, Archaean, and Proterozoic is also referred to as the Precambrian. The Phanerozoic eon, 540 million years ago to the present, is a period of time that has seen the elaboration of the various kinds of living things we see today as well as many groups that have gone extinct. The Phanerozoic eon is divided into smaller units of time known as eras. The eras are the (1) Cenozoic, the time of recent life; (2) Mesozoic, the time of middle life; and (3) Paleozoic, the time of ancient life. The eras are divided into smaller blocks of time called periods, and the periods are further subdivided into epochs.

There are many different kinds of fossils—evidence of former living things. They are formed when organisms are covered over and prevented from being destroyed. Most are modified chemically but still give information about past living things. The fossil record is seen to change over geologic time with certain kinds of fossils being associated with certain periods of time. There have been several great extinctions of living things. Some of the extinctions eliminated more than 80 percent of the organisms alive at the time. These extinctions appear to be related to changes in climate that may have been initiated by changes in the location and arrangement of continents, volcanic activity, or meteorite impact.

The early evolution of humans has been difficult to piece together because of the fragmentary evidence. Beginning about 4.4 million years ago the earliest forms of Australopithecus and Paranthropus showed upright posture and other humanlike characteristics. The structure of the jaw and teeth indicates that the various kinds of australopiths were herbivores. Homo habilis had a larger brain and appears to have been a scavenger. Several other species of the genus Homo arose in Africa. These forms appear to have been carnivores. Some of these migrated to Europe and Asia. The origin of Homo sapiens is in dispute. It may have arisen in Africa and migrated throughout the world or evolved from earlier ancestors found throughout Africa, Asia, and Europe.

To facilitate accurate communication, biologists assign a specificname to each species that is cataloged. The various species are cataloged into larger groups on the basis of similar traits. Taxonomy is the science of classifying and naming organisms. Phylogeny is the science of trying to figure out the evolutionary history of a particular organism. The taxonomic ranking of organisms reflects their evolutionary relationships.Fossil evidence, comparative anatomy, developmentalstages, and biochemical evidence are employed in the sciences of taxonomy and phylogeny.

The first organisms thought to have evolved were single-celled, prokaryotic organisms. These simple organisms gave rise to two different prokaryotic domains: the Archaea and the Eubacteria. These subsequently gave rise to the eukaryotic organisms in the domain Eucarya and established the kingdoms Protista, Fungi, Plantae, and Animalia.