Evolution__3rd_Edition

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18The History of Life
T his chapter gives a brief history of life, from its origin to
the present. We have two main sources of evidence for
historic reconstruction: the molecular clock, which we looked
at in Chapters 7 and 15, and the fossil record. We begin here
by looking at how fossils are formed and how their age is
estimated. This section of the chapter has, for paleontology, a
similar purpose as did Chapter 2 for genetics. We then move
on to the historic narrative. The chapter looks at the origin
of life, of cells, and of multicellularity, at the Cambrian
explosion, the colonization of land by plants and animals,
the evolution of mammals from reptilian ancestors, and
human evolution. These historic events are all examples of
evolution on the large scale, or macroevolution. The chapter
ends with a conceptual section on the possible relations
between micro- and macroevolution, using the historic
case studies as examples.