How do we know how the physical changes described in the previous section affected the evolution of life? To reconstruct life’s history, scientists rely heavily on the fossil record. As we have seen, geologists divided Earth’s history into eons, eras, and periods based on distinct fossil assemblages (see Table 24.1). Biologists refer to the assemblage of all organisms of all kinds living at a particular time or place as a biota. All of the plants living at a particular time or place are its flora; all of the animals are its fauna.

About 300,000 species of fossil organisms have been described and named, and the number steadily grows. The number of named species, however, is only a tiny fraction of the species that have ever lived. We do not know how many species lived in the past, but we have ways of making reasonable estimates. Of the present-day biota, about 1.8 million species have been named. The actual number of living species is estimated to be at least 10 million, and possibly much higher, because many species have not yet been discovered and described by biologists. So the number of described fossil species is only about 3 percent of the estimated minimum number of living species. Life has existed on Earth for about 3.8 billion years. Many species last only a few million years before undergoing speciation or going extinct. From this we know that Earth’s biota must have turned over many times during geological history. So the total number of species that have lived over evolutionary time must vastly exceed the number living today. Why have only about 300,000 of these tens of millions of species been described from fossils to date?