Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
  High-Bandwidth Version
Search Evolution  
 
Click to return to the Evolution Home Page
darwin change extinction survival sex humans religion
Darwin    
   
Deep Time

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

Cenozoic Era: (248 mya-present)

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

border

border

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 America.

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 traveling.

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.

border

Did you know?

Ice ages occur in cycles that seem to correspond to variations in the shape of Earth's orbit and the orientation of its spin axis. While these factors operate independently from one another, on occasion they combine to trigger great temperature swings. The cold or hot weather determines whether glaciers expand or retreat.

border

-> Go to the Pleistocene Epoch

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

  related web activities  
   
Origins of Humankind
See the humanlike species that came before us.
 
 
A Modern Mass Extinction?
Are we in the midst of one? And if so, did we trigger it?
 
 
Life's Grand Design
Are nature's complex forms evidence of "intelligent design?"
 
   
  related topics  
   
  Deep Time/History of Life  
   
  Evolution of Diversity  
   
  Evidence for Evolution  
 
 
       
         
 
Videos Web Activities Site Guide About the Project FAQ Glossary Site Map Feedback Help Shop