Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Cenozoic Era: (248 mya-present)

Paleocene | Eocene | Oligocene | Miocene | Pliocene | Pleistocene | Holocene

Oligocene Epoch (33-24 mya)

By history's standards, the Oligocene is relatively uneventful, both geologically and biologically. Following the rapid cooling that ends the Eocene, the climate remains cooler and more seasonal. Plants adapted to dry and cool climates, including herbaceous plants and grasses, begin to diversify, but marine plankton populations, the foundation of the food chain, diminish. Antarctica's split from Australia has opened a deep-water pathway that completely surrounds the polar continent when it completes its split from South America.

Animal life on land evolves more or less in place, with little migration between the continents. Early horses and other herbivores feed primarily on woodland plants and, toward the end of the epoch, spreading grasses. Members of the camel family, which first appear in North America, look more like deer, with the males of some species growing horns. Members of the bear family, too, resemble another group of contemporary creatures: foxes. The largest land-dwelling mammal ever found -- a rhinoceros standing nearly 20 feet tall at the shoulders and weighing in at 30,000 pounds -- feeds on trees in southeast Asia.

25 mya: Grasses

Grasses, a product of the cooler, drier climate, become one of the most important groups of organisms on the planet. As they spread extensively over several million years, they feed herds of grazing mammals, shelter smaller animals and birds, and stabilize soil, which in turn reduces erosion. They are high-fiber, low-protein plants and must be eaten in large quantities to provide adequate nutrition. Because they contain tiny silica fragments, though, they are tough to chew and wear down animal teeth. Grasses, which grow throughout the blade, are adapted to recover quickly after their tips are grazed. Unlike many other flowering plants, grasses do not display colorful petals, and they rely on wind for pollination rather than insects or birds.

25 mya: Ape/human line evolves

The hominoid primates include early human ancestors as well as the great apes (chimps, gorillas, and orangutans) and lesser apes (gibbons and siamangs). Hominoids display prominent jaws, are tailless, and have large brains. Fossil specimens found beyond Africa and into southeast Asia and southern Europe indicate an extensive range. The oldest hominoid fossil -- an ape jaw -- was discovered in northern Kenya.

-> Go to the Miocene Epoch

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era