Pliocene Epoch (5.3-1.8 mya)
The cooling and drying trend that began in the Miocene epoch continues
in the Pliocene. A shift of the Caribbean tectonic plate eastward creates a land bridge
that connects North America with South America. This enables land animals to freely move
between the continents and divides the surrounding ocean into two -- the Atlantic and
Pacific -- setting segregated marine life on different evolutionary paths.
In the higher latitudes, cool-weather plants evolve. Hardy plants that
can tolerate a short growing season, such as sedges, mosses, and lichens, inhabit the
almost permanently frozen tundra. In slightly warmer regions, taiga forests consist mostly
of evergreens. In lower latitudes, grasslands are marked by fewer and fewer trees. These
habitats offer limited food sources for animals and support less diversity.
During the Pliocene, humanlike primates evolve in eastern Africa. These
hominids descend from tree-dwelling creatures and probably live at the edges of forests.
Elephants, horses, cattle, and other large herbivores prosper, likely because they are
capable of surviving seasonal food shortages and digesting coarser vegetation. But they
face fierce predators, including saber-toothed cats, pack-hunting dogs, and bears. Many
Pliocene animals resemble those alive today, but some are considerably larger. Giant
armadillos (glyptodons) and large ground sloths (Megatherium) both live in South
5.2 mya: Hominids
The first known hominids, or humanlike primates, evolve in eastern
Africa. Hominids feature prominent jaws, and most species have large brains relative to
those of apes. Most hominids probably live in groups either in or near forests, and
some later species make and use tools. The oldest fossils -- a jawbone, teeth, and a
toe bone found in Ethiopia -- date to 5.2 mya. A younger, near-complete hominid skeleton,
named Lucy by its discoverers, and a set of remarkably preserved footprints in Tanzania
reveal more about their appearance and one of their most distinctive traits: They walk
upright on two legs. This adaptation affords certain advantages, such as the ability to
see over the top of high vegetation and to easily carry food or tools and weapons while
2 mya: Global ice ages begin
The gradual cooling that began in the Eocene continues through the
Pliocene. By about 2 mya, both poles are covered by ice sheets. Several times over the
next 2 million years, glaciers spread and retreat over large areas.