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

Eocene Epoch (54-33 mya)

Early in the Eocene, the global climate remains warm. As the continents move ever closer to their present-day positions, this plate activity alters ocean and air circulation patterns. By the end of the Eocene, temperatures cool considerably and a drying period commences. In subtropical latitudes, open woodlands covered with ferns and shrubby plants replace forests.

Land mammals, which move out of the forests and into the open spaces, become noticeably larger. Rhinoceroses, three-toed horses, and early relatives of pigs, camels, and hippopotamuses first appear. As the temperature cools, some groups go extinct, especially those in higher latitudes. Primates, which had thrived in the more tropical conditions, manage to survive the extinction. With the evolution of whales from wolf-like land animals to sea-going creatures, mammals now occupy land, air, and sea.

Animal migrations continue, especially between continents in the northern hemisphere. Asian animals invade Europe, replacing some native species (like European tapirs) with non-native ones (like Asian rhinos).

50 mya: Whales

Features apparent in early whale fossils -- specialized teeth, nostrils near the tip the nose, hind limbs, and a pelvis attached to the skeleton -- offer irrefutable evidence that whales evolve from land mammals. While they shed many mammalian characteristics, like hair, whales still bear live young, breathe air, and have large and complex brains as they adapt to aquatic life.

border

Read more

Return to the sea (50 mya)

Gradually, over 5-10 million years, whale ancestors abandon their terrestrial ties in favor of a sea-going existence. Hoofed, wolf-like, carnivorous mammals called mesonychids hunt for food along the seashore. They probably spend more and more time offshore because a more abundant food supply exists in the water. Those better suited for aquatic environments thrive and reproduce, and eventually full-time marine creatures evolve.

Fossil finds in Pakistan's Valley of the Whales, now a desert but once home to inland seas, have allowed scientists to trace this transformation stage by stage, as generations of species subtly change in appearance and habit. These intermediate species are known in evolutionary terms as "transitional forms." They help define the relationship between living things and their ancestors, thereby improving our understanding of evolution and its many paths.

By comparing the ear regions of ancestral mesonychids, early Eocene whales, and whales known from later in the period, for example, we can understand the structural changes that enabled the later whales to hear underwater and to regulate pressure when diving.

border

40 mya: Continents near present-day positions

Following North America and Greenland's split from Europe and Australia's separation from Antarctica, continents drift near their present-day positions.

40 mya: Drying and cooling trend begins

Plate movements change ocean and atmospheric circulation patterns, triggering drier, more seasonal weather and the return of ice to the poles.

Late Eocene extinction

Date:

33 mya

Intensity:

3

Affected:

As many as 50-90 percent of species in certain groups of marine invertebrates, mammals, and plants go extinct

Hypotheses:

Global cooling

Summary:

Two extinction pulses likely occur -- the first at 37 mya, and the second at about 33 mya -- affecting both marine and land faunas. In marine communities, tiny single-celled organisms with shells called foraminifera suffer gradual but severe losses, as do gastropods, bivalves, and echinoids (sea urchins). On land, plants, rodents, primitive primates, and ungulates (hoofed mammals) are affected. While impact craters have been identified from this time period, evidence in rock layers suggests that meteors do not trigger the extinction. Instead, global cooling is a more likely cause. This hypothesis is supported on land by the appearance of open woodlands where densely forested habitats previously existed, and the extinction of many animal groups that lived in warm, higher latitudes.

-> Go to the Oligocene 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