If we could look at Earth 3,500,000,000 years ago, soon after life’s beginning,
we would find only bacterial cells. Among these cells would be our most remote
ancestors, utterly different from ourselves. And if we then time-traveled through
life’s history toward the present, we would see played out before us a drama
grander and more splendid than we can imagine: a planetary stage of many
scenes, on which emerge and play—and then, most likely, die—millions and mil-
lions of species with features and roles more astonishing than any writer could
conceive. In this chapter, we take that journey through time.
The history of evolution can be inferred in two ways that complement each
other. As we have seen, phylogenetic inferences from living organisms can tell us
a lot about when some lineages evolved and about patterns of change in their
characters. But fossils provide direct evidence of some events that living organ-
isms cannot reveal. Fossils tell us of the existence of innumerable creatures that
have left no living descendants, of great episodes of extinction and diversifica-
tion, and of the movements of continents and organisms that explain their pres-
ent distributions. Without this record, we could not calibrate the speed of DNA
divergence, for it is only from the fossil record that we can obtain an absolute
time scale for evolutionary events, or evidence of the environmental conditions
in which they transpired. The fossil record also provides many details of which
we would otherwise have no knowledge. For instance, although we can infer
some evolutionary changes in the human lineage from comparisons with other
primates, fossils provide other information—such as the sequence of particular
anatomical changes—that comparisons with other living species cannot provide.A sagittal section through the chambered shell of Cleoniceras, an early Cretaceous
ammonite. Larger chambers were formed as the animal’s body grew in size. Am-
monites—cephalopods related to squids—were extremely diverse in the Mesozoic,
but became entirely extinct at the era’s end.The History
Mesozoic Life
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