Introduction

The Phanerozoic eon is a major division in the fossil record that dates radioisotopically at a bit younger than 600 million years before present. Its geologic marker is the appearance of numerous complex multicellular organisms in the fossil record. This eon has no counterpart on any other planet, even if Mars harbored simple life-forms within the first billion years of its history. On Phanerozoic Earth, life began to occupy just about every conceivable niche on land, sea, and air. Geologically, Earth was more or less modern in form as the eon opened: the total continental mass was comparable to that today, modernstyle plate tectonics were operating, and oxygen levels in the atmosphere were approaching present-day values.

The Phanerozoic eon is divided into eras, eras into periods, and periods into epochs. The boundaries between most of the periods are defined by extinction episodes in which a number (sometimes very large) of species disappear and are replaced in the sedimentary fossil record above that point by new species. Although the resulting story of complex multicellular organisms is too large to tell in detail in this book, some of the highlights are shown in Figure 18.1.

The presence of multicellular organisms per se was not new. Multicellular bacterial colonies had existed since the Archean; multicellular algae (for example, green seaweed) made their appearance shortly after the first unicellular eukaryotes in the fossil record. In each of these, and many other cases, there is little or no specialization among cells, and only limited communication.