The mammalian era covers a
period of fifty thousand years, starting from the times of the origin
of placental mammals to the end of the ice age. Mammals have several
survival advantages over other forms of life. They possess an ability
to adapt to their environment, superior intelligence, agility, delivery
of relatively mature offspring, and a tendency to nourish and protect
their young.
Fifty million years ago
placental mammals suddenly appeared in North America, descendants of
a small, highly active, springing-type of carnivorous dinosaur.
Forty-five million years
ago mammalian life was evolving rapidly. A small egg‑laying
type of mammal flourished. The ancestors of kangaroos existed in Australia.
Small versions of horses, rhinoceroses, tapirs, primitive pigs, squirrels,
lemurs, opossums and monkey-like creatures appeared. A ten-foot-tall
ostrich‑like bird developed which became the ancestor of the passenger
birds of later times.
These early mammals lived on
land, under the water, in the air and among the treetops. They had mammary
glands, were covered with considerable hair, developed two successive
sets of teeth, and had large brains. At this time North America was
connected to every continent except Australia; the world was overrun
by primitive mammals.
Thirty-five million years
ago began the age of domination by placental mammals. The dinosaur
species were on the decline. Various groups of mammals took origin in
an amphibious animal, now extinct, that was a cross between a cat and
a seal. The ancestor of dogs evolved in Europe. Rodents appeared.
Thirty million years ago
modern types of mammals began to develop. Hoofed types of grazing animals
appeared. Horses and rhinoceroses continued to evolve. A small hog-like
creature developed which became the ancestor of swine, peccaries, and
hippos. Camels, llamas, and ancient lemurs originated. A group of mammals
took to the seas and became ancestors to whales, dolphins, seals and
sea lions. Most modern bird types existed.
Twenty million years ago
the Bering land bridge was above water. Deer, oxen, camels, bison and
rhinoceroses migrated to North America from Asia. Fifty species of elephant
overran the entire world except Australia. Giant pigs became extinct.
Fifteen million years ago
enormous herds of horses roamed western North America. Primitive monkeys
and gorillas evolved in Asia. Wolves and foxes came into existence;
panthers and saber‑tooth tigers represented the cat family.
Ten million years ago
the last great world‑wide animal migration began. Camels entered
China; sloths, armadillos, antelopes and bears moved to North America.
The giraffe evolved in Africa. Mastodons migrated into every continent
except Australia. The ocean currents shifted and seasonal winds changed
direction. Parts of Europe and North America rose up to 30,000 feet
and more; the thick depths of snow that began to form on the highlands
eventually became masses of solid creeping ice.
Two million years ago
marks the beginning of the ice ages. Throughout the ice ages, glaciers
advanced and retreated across the continents, carving the landscape
as they went. Many animal species were destroyed and others were radically
changed by the rigors of migration. In North America, the last glacier
movement wiped out horses, tapirs, llamas, and saber‑tooth tigers.
The ice ages ended thirty-five thousand years ago.