Like the dinosaurs before them, a few of the Tertiary mammals were so
long on brawn and short on brains that they evolved into grotesque
monsters and overspecialized themselves into early extinction.
Fortunately, however, most of the mammals evolved more slowly and
moderately into the forms we find today.
One group—the anthropoid primates—began to think, so they developed
their brains rather than their brawn, particularly the Tertiary
ancestors of man. Few remains of these ancestors have been found in
Tertiary rocks, but many more have been discovered in rocks of the next
geologic period—the Quaternary. Thus, this period may properly be
regarded as the age of man, for man then began to dominate the Earth for
better or for worse.
The Quaternary—latest and shortest of the geologic periods—is divided
into the Pleistocene and Holocene (recent) Epochs (fig. 61).