Figure 12.20: The horse has evolved over the last 60 million years. Horses today are much
larger than earlier horses. ( 3 )
Phanerozoic Eon as the Cambrian Explosion—meaning a time when the Earth “exploded”
with incredible numbers of new complex life forms.
Phanerozoic Eon
The Phanerozoic Eon is divided into three chunks of time called eras—the Paleozoic, the
Mesozoic, and the Cenozoic (Table(12.1). They span from about 540 million years ago to
the present. We live now in the Cenozoic Era. The table below shows how life has changed
during the long span of the Phanerozoic Eon. Notice that different types of organisms
developed at different times. However, all organisms evolved from a common ancestor. Life
gradually became more diverse and new species branched out from that common ancestor.
Most modern organisms evolved from species that are now extinct. To get an idea of how an
organism can change from a single common ancestor to many different types, think about
all the different types of dogs. All dogs evolved from a common wolf ancestor. Today there
are hundreds of varieties of dogs that all look very different.
Table 12.1: Development of Life During the Phanerozoic EonEra Millions of Years Ago Major Forms of Life
Cenozoic 0.2 (200,000 years ago) First humans
35 First grasses; grasslands be-
gin to dominate the land
Mesozoic 130 First plants with flowers
150 First birds on Earth