Instead of years, these scientists usually measure deep time in terms of eons, eras and periods with exotic names such as Proterozoic, Tertiary, or Cretaceous. Recently scientists have begun to extend this nomenclature into the time before the great flowering of multicelled life that began about 550 million years ago.

To a non-specialist, such time divisions are unfamiliar, often making it difficult to understand a technical article or lecture on the ancient Earth and its biological history. But there are historical reasons for using names rather than numbers to define geological time. Radioactive dating techniques for measuring the ages of rocks, expressed in years, did not exist before the twentieth century, and only in the past few decades have they become precise. Nineteenth- and early twentieth-century efforts to classify and order ancient rocks were thus invented without reference to absolute ages. Instead, each time-span was identified in terms of the kinds of rock formed and the similarity of fossils present. Relative ages – determination of the sequences of the rocks – were inferred from stratigraphy, that is, by noting which rocks types lay on top of others in undisturbed layers or strata. Much of the nineteenth-century classification was done by European (especially British) scientists, explaining the prevalence of European names, such as Devonian for Devon England, Jurassic for the Jura Mountains, or Permian for Perm Russia.

The identification of fossils (mostly marine fossils) was an important part of the definition of each eon or era. Having a common set of fossils indicated that two deposits were of similar age even if the composition of the rocks (which depended on local circumstances) were different. Nineteenth-century scientists agreed on these definitions and established a standard or type locality for each geological division, before twentieth-century scientists were able to assign absolute or chronometric ages. Since most rocks are relatively young (less than 500 million years old) and the presence of fossils was important for classification, this effort was focused on the most recent eon, called the Phanerozoic Eon (approximately the past 560 million years). The Phanerozoic Eon is in turn divided into the Paleozoic Era (Old Life), the Mesozoic Era (Middle Life), and the Cenozoic Era (Recent Life). The boundaries between these three eras of the Phanerozoic are sharply defined by the two best-known mass extinctions: the PT extinction 251 million years ago, and the KT extinction 65 million years ago.

The huge eon that precedes the Phanerozoic is called the Proterozoic (Earlier or Developing Life). This period is of great interest to astrobiologists, since it encompasses several critical evolutionary developments, including multicellularity, as well as key geological events such as the rise of an oxygen atmosphere and the glacial periods called “Snowball Earth”. One of the research programs being coordinated by the NASA Astrobiology Institute involves drilling, and making available to research teams, extracted rock cores that cover much of Proterozoic time.

Writing in Science for 30 July 2004, four authors led by astrobiologists Andy Knoll of Harvard and Malcolm Walter of the Australian Centre for Astrobiology report on recent progress in understanding the last part of the Proterozoic Eon, which is called the Ediacaran Era. It is named for the fossils of soft-bodied but relatively large life forms that may have been among the first successful multi-celled creatures. In accord with the eras of the more familiar Phanerozoic Eon, the beginning of the Ediacaran Era has just been officially defined in terms of a type locality in the Flinders Ranges of South Australia. It is the first statigraphically defined new geological time period to be approved since 1891.

What is the time-span (in years) of the Ediacaran Era? It is about 100 million years, but the dates have not yet been determined precisely. The starting time is variously estimated at between 610 and 635 million years ago. The ending is more precise, since the Ediacaran ends where the Paleozoic Era of the Phanerozoic Eon begins, 542 million years ago. The era that precedes the Ediacaran, sometimes called the Cryogenian because of Earth’s extensive glaciation, is not yet well defined, but undoubtedly that will come too as scientists sharpen their focus on earlier chapters in Earth’s biological history.

Further reading about the early Earth and the development of the geological time scale:

Michael J. Benton – When Life Nearly Died: The Greatest Mass Extinction of All Time (Thames & Hudson, 2003)

Andrew Knoll – Life on a Young Planet: The First Three Billion Years on Earth (Princeton, 2003)

James Lawrence Powell – Mysteries of Terra Firma: The Age and Evolution of the Earth (Free Press, 2001)