appear to have evolved by about 25 million years ago. Grasses were the most
important angiosperms, providing food for ungulates throughout the Ceno-
zoic.The grazing habits of many large mammals likely evolved in response to
the widespread availability of grasslands.
The first primates lived some 60 million years ago and were about the
size of a mouse. Then, the primate family tree split into two branches. Mon-
keys were on one limb.The great apes, including the hominoids—our human-
like ancestors, were on the other. Beginning about 37 million years ago, the
New World monkeys unexplainably migrated from Africa to South America
when those continents had already drifted far apart. About 30 million years
ago, the precursors of apes lived in the dense tropical rain forests of Egypt,
which is now mostly desert. These apelike ancestors migrated out of Africa
and entered Europe and Asia between about 25 and 10 million years ago.
Between 12 and 9 million years ago, the forests of Europe were home to
a tree-living, fruit-eating ape called Dryopithecus,which is thought to have
evolved into Ramapithecus,an early Asian hominoid that had more advanced
characteristics than earlier species.Between 9 and 4 million years ago, the fos-
sil record jumps from the hominid-like but mainly ape form of Ramapithecus
to the true hominids and the human line of ascension. During this time, much
of Africa entered a period of cooler, drier climate, which caused the forests to
retreat, offering many evolutionary challenges to our human ancestors.
MARINE MAMMALS
Some 70 species of marine mammals known as cetaceans were among the
most adaptable animals and included dolphins, porpoises, and whales, which
evolved during the middle Cenozoic. The dolphins had reached the level of
intelligence comparable to living species by 20 million years ago probably due
to the stability of their ocean environment.Sea otters, seals, walruses, and man-
atees are not fully adapted to a continuous life at sea and have retained many
of their terrestrial characteristics.The manatees, which have inhabited Florida
waters for 45 million years, are rapidly becoming an endangered species.
Pinnipeds, meaning “fin-footed,” are a group of marine mammals with
four flippers. The three surviving forms include seals, sea lions, and walruses.
The true seals without ears are thought to have evolved from weasel-like or
otterlike forms.Sea lions and walruses, however, are believed to have devel-
oped from bearlike forms. This type of dual development, called diphyletic
evolution, makes originally dissimilar animals resemble each other simply
because they have adapted to a life in the same environment,in this case water.
The similarity in their flippers, however, suggests that all pinnipeds evolved
from a single land-based mammal that entered the sea millions of years ago.
TERTIARY MAMMALS