Primate Origin

determine which of the above theories is correct. These issues reveal exciting research
and career opportunities for students just like you.
Ecosystem and Temporal Aspects of Primate Origins
Dinosaurs were the dominant vertebrates from about 230 to 65.5 million years ago
(MYA), at which point the fossil record indicates a mass extinction of dinosaurs, reptiles,
and plants in both terrestrial and aquatic environments. Although the causes of
this mass extinction are hotly debated, the prevailing theory is that a massive asteroid
impact threw up a long-lasting, worldwide layer of debris, thereby blotting out sunlight
and disrupting plant growth. The loss of plant species caused an extinction cascade of
herbivores and their predators in many parts of the planet. For reasons that are poorly
understood, some mammal and bird lineages survived this mass extinction event. This
information dispels the common misconception that mammals evolved after the extinction
of dinosaurs. In fact, mammals evolved from cynodonts (mammal-like dinosaurs)
about 220 MYA. Thus, mammals were already an ancient lineage by the time the dinosaurs
went extinct. The Cenozoic Era, which began about 65.5 million years ago and
continues through today, marks the end of the “Age of the Dinosaurs” and the beginning
of the “Age of the Mammals.” An era is one of 11 geologically determined and
clearly defined periods of time. This geological period is, therefore, particularly relevant
to primate origins and evolution. There were considerable geological and climatic
changes during the Cenozoic (Figure 5.6). The Cenozoic is subdivided into the following
epochs: Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene
(Table 5.1). An epoch is a span of time smaller than an era. We start with the
Paleocene, and then work our way forward in time to the Pliocene. We’ll cover the
Pleistocene and Holocene in the next chapter on hominin origins.
Era ■ One of approximately
11 units of
geological time that
cover the ca. 4.6billion-year
age of
Earth (e.g., Cenozoic
Era from 65.5 millions
of years ago to present).
Epoch ■ A unit of
geological time that is
a subdivision of an
era.
Figure 5.6
Changes in temperature
during
the Cenozoic era.