The Pleistocene epoch represented the culmination of 200 million years of mammalian evolution, as bears, lions, armadillos, and even wombats grew to bizarrely large sizes--and then went extinct due to climate change and human predation. The Pleistocene is the last named epoch of the Cenozoic Era (65 million years ago to the present) and is the first epoch of the Quaternary period, which continues to this day. (Up to the year 2009, when paleontologists agreed on a change, the Pleistocene officially began 1.8 million rather than 2.6 million years ago.)
Climate and Geography
The end of the Pleistocene epoch (20,000 to 12,000 years ago) was marked by a global ice age, which led to the extinction of many megafauna mammals. What most people don't know is that this capitalized "Ice Age" was the last of no less than 11 Pleistocene ice ages, interspersed with more temperate intervals called "interglacials.' During these periods, much of North America and Eurasia was covered by ice, and ocean levels plummeted by hundreds of feet (due to the freezing of available water at and near the poles).
Mammals: The dozen or so ice ages of the Pleistocene epoch wreaked havoc on megafauna mammals, the largest examples of which were simply unable to find enough food to sustain their populations. Conditions were especially severe in North and South America and Eurasia, where the late Pleistocene witnessed the extinction of Smilodon (the Saber-Toothed Tiger), the Woolly Mammoth, the Giant Short-Faced Bear, Glyptodon (the Giant Armadillo), and Megatherium (the Giant Sloth). Camels disappeared from North America, as did horses, which were only reintroduced to this continent during historical times, by Spanish settlers.
From the perspective of modern humans, the most important development of the Pleistocene epoch was the continuing evolution of hominid apes. At the start of the Pleistocene, Paranthropus and Australopithecus were still extant; a population of the latter most likely spawned Homo erectus, which itself competed with Neanderthals (Homo neanderthalensis) in Europe and Asia. By the end of the Pleistocene, Homo sapiens had appeared and spread around the globe, helping to hasten the extinction of the megafauna mammals that these early humans either hunted for food or eliminated for their own safety.
Birds: During the Pleistocene epoch, bird species continued to flourish around the globe, inhabiting various ecological niches. Sadly, the giant, flightless birds of Australia and New Zealand, such as Dinornis (the Giant Moa) and Dromornis (the Thunder Bird), quickly succumbed to predation by human settlers. Some Pleistocene birds, like the Dodo and the Passenger Pigeon, managed to survive well into historical times.
Reptiles: As with birds, the big reptile story of the Pleistocene epoch was the extinction of oversized species in Australia and New Zealand, most notably the giant monitor lizard Megalania (which weighed up to two tons) and the giant turtle Meiolania (which "only" weighed half a ton). Like their cousins around the globe, these giant reptiles were doomed by a combination of climate change and predation by early humans.
The Pleistocene epoch witnessed the final extinction of the giant shark Megalodon, which had been the top predator of the oceans for millions of years; otherwise, though, this was a relatively uneventful time in the evolution of fish, sharks, and marine mammals. One notable pinniped that appeared on the scene during the Pleistocene was Hydrodamalis (aka Steller's Sea Cow), a 10-ton behemoth that only went extinct 200 years ago.
There were no major plant innovations during the Pleistocene epoch; rather, during these two million years, grasses and trees were at the mercy of intermittently plunging and rising temperatures. As during preceding epochs, tropical jungles and rainforests were confined to the equator, with deciduous forests and barren tundra and grasslands dominating northern and southern regions.