The Pleistocene epoch is a geological time period that includes the last ice age, when glaciers covered huge parts of the globe. Also called the Pleistocene era, or simply the Pleistocene, this epoch began about 2.6 million years ago and ended 11,700 years ago, according to the International Commission on Stratigraphy (opens in new tab).
Modern humans, or Homo sapiens, evolved during the Pleistocene and spread across most of Earth before the period ended, according to the University of California Museum of Paleontology (opens in new tab). The epoch also featured ice age giants, such as woolly mammoths (Mammuthus primigenius) and saber-toothed cats, many of which disappeared at the end of the Pleistocene in a major extinction event.
The Pleistocene was preceded by the Pliocene epoch and followed by the Holocene epoch, which we still live in today, and is part of a larger time period called the Quaternary period (2.6 million years ago to present). The name "Pleistocene" is the Latin combination of two Greek words: "pleistos" (meaning "most") and "cene," which comes from "kainos" (meaning "new" or "recent"), according to the Collins Dictionary (opens in new tab).
What caused the Pleistocene ice ages?
Scientists are still learning about how ice ages occur, but we know they are driven by a series of factors, such as fluctuating carbon dioxide levels, Earth's position in the solar system and how much heat our planet receives from the sun, Live Science previously reported. For example, the shape of Earth's orbit varies on a 96,000-year cycle, and the planet is cooler when it is pulled by Jupiter's gravity farther from the sun.
Earth has been experiencing a trend of cooling for about the past 50 million years. About 4.5 million years ago, the Isthmus of Panama land bridge formed between North America and South America, which may have triggered the last ice age. The Atlantic and Pacific oceans could no longer exchange tropical water, forcing warm water northward and increasing precipitation in the Northern Hemisphere, which fell as snow. The snowfall created glaciers and ice sheets, thus deflecting sunlight and continuing Earth's cooling trend, according to the National Oceanic and Atmospheric Administration's Climate.gov (opens in new tab) website.
The glaciers advanced during colder periods of the Pleistocene, called glacials, and retreated during warmer periods, called interglacials. Scientists have identified four stages, or ages, within the Pleistocene epoch: the Gelasian (2.6 million to 1.8 million years ago) and Calabrian (1.8 million to 781,000 years ago), representing the lower or early Pleistocene; the Chibanian (781,000 to 126,000 years ago), representing the middle Pleistocene; and the late Pleistocene (126,000 to 11,700 years ago), representing the upper or late Pleistocene, according to the International Commission on Stratigraphy.
Ice age glaciers mostly retreated and melted away as the planet warmed after the Pleistocene ended, but some ice cover has stood the test of time. For example, glaciers in the Antarctic Peninsula may date back to the earlier Pleistocene, according to the U.S. Geological Survey (opens in new tab).
How cold was the Pleistocene?
The ice age peaked during the Last Glacial Maximum about 20,000 years ago, when glaciers covered vast swathes of North America, Europe, South America and Asia. At that time, global temperatures were about 11 degrees Fahrenheit (6 degrees Celsius) lower than they are today, according to a 2020 study published in the journal Nature (opens in new tab).
Ice age conditions were also drier than today. Because most of the water on Earth's surface was ice, there was little precipitation; rainfall was about half of current levels. The sea level was much lower, and the shorelines were typically much farther out because glaciation trapped water in ice sheets, according to the American Museum of Natural History in New York City.
Life during the ice age(opens in new tab)
The last ice age is known for hosting many large mammals called megafauna. Mammoths, saber-toothed cats, giant ground sloths and mastodons roamed North America during this period, according to the Florida Museum of Natural History (opens in new tab). But it wasn't just giant mammals. An 11-foot-tall (3 meters), flightless bird that weighed almost as much as a polar bear inhabited Europe during the early Pleistocene, Live Science previously reported. Meanwhile, Megalania prisca, the largest known terrestrial lizard, lived in open forests, woodlands and other Pleistocene habitats across much of eastern Australia during the epoch, according to the Australian Museum (opens in new tab) in Sydney.
Although plenty of Pleistocene animals are now extinct, much of the wildlife would be familiar to humans today. For example, alongside mammoths in Alaska, there were the same brown bears (Ursus arctos), caribou (Rangifer tarandus) and wolves (Canis lupus) as there are today, according to the National Park Service (opens in new tab). On the other side of the world, in Western Australia, the remains of an ancient campfire discovered in 2018 suggest Aboriginal people held a kangaroo feast at the peak of the Pleistocene ice age about 20,000 years ago. Of course, there weren't any nonavian dinosaurs, as they became extinct at the end of the Cretaceous period, more than 60 million years before the Pleistocene epoch began.
Most of the megafauna went extinct toward the end of the Pleistocene epoch. In North America, about 38 groups of mammals disappeared, and most of those were over about 99 pounds (45 kilograms), according to a 2020 study in the journal Proceedings of the National Academy of Sciences (opens in new tab). Short-faced bears, armadillo-like Glyptotherium and helmeted muskox (Bootherium bombifrons) were among the extinction casualties, as well as mastodons, saber-toothed cats and most mammoths.
Scientists have debated for many decades what caused the Pleistocene extinctions. The main argument put forward is that either natural climate change or human activity, including overhunting, primarily drove the extinctions, according to the Sam Noble Museum (opens in new tab) at the University of Oklahoma. Another, more controversial theory is that the explosions caused by the breakup of a large comet as it entered the atmosphere 12,900 years ago led to forest fires in North America and climate change, which, in turn, played a major role in the extinctions.
Earth was warming as it exited the Pleistocene epoch about 11,700 years ago. The glaciers retreated, and humans started farming at the dawn of a new era: the Holocene epoch, also called the "age of man."
See some of the Pleistocene finds from the La Brea Tar Pits on the La Brea Tar Pits and Museum (opens in new tab) website. To learn about attempts to resurrect a Pleistocene ecosystem and bring back mammoths from extinction to combat climate change, watch this short YouTube video by BBC Reel (opens in new tab). For a more detailed look at Pleistocene megafauna, check out "Vanished Giants: The Lost World of the Ice Age (opens in new tab)" (University of Chicago Press, 2021).
Anne Mussuer, The Australian Museum, "Megalania prisca," Aug. 17, 2020. https://australian.museum/learn/australia-over-time/extinct-animals/megalania-prisca/ (opens in new tab)
Cohen et al., International Commission on Stratigraphy, "The ICS International Chronostratigraphic Chart," 2022. https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf (opens in new tab)
Collins English Dictionary, "Definition of Pleistocene." https://www.collinsdictionary.com/dictionary/english/pleistocene (opens in new tab)
David Polly, University of California Museum of Paleontology, "The Pleistocene," April 30, 1994. https://ucmp.berkeley.edu/quaternary/ple.html (opens in new tab)
Florida Museum of Natural History, "The Pleistocene Epoch," Aug. 12, 2021. https://www.floridamuseum.ufl.edu/fossil-horses/time-scales/pleistocene/ (opens in new tab)
Laura Geggel, Live Science, "Aboriginal People Held a Kangaroo Feast Around a Campfire 20,000 Years Ago," May 29, 2018. https://www.livescience.com/62685-ancient-aboriginal-kangaroo-feast.html (opens in new tab)
Megan Gannon, Live Science, "Why Do Ice Ages Happen?" Sep. 1, 2019. https://www.livescience.com/what-causes-ice-ages.html (opens in new tab)
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Mindy Weisberger, Live Science, "Extinct 11-Foot 'Super-Ostrich' Was As Massive As a Polar Bear," June 27, 2019. https://www.livescience.com/65807-extinct-super-ostrich.html (opens in new tab)
Norris et al., American Museum of Natural History, "Plio-Pleistocene." https://research.amnh.org/paleontology/perissodactyl/concepts/deep-time/plio-pleistocene (opens in new tab)
Pamela Groves, National Park Service, "Pleistocene Megafauna in Beringia," Feb. 7, 2019. https://www.nps.gov/articles/aps-17-1-4.htm (opens in new tab)
Sam Noble Museum, the University of Oklahoma, "Pleistocene Extinctions." https://samnoblemuseum.ou.edu/understanding-extinction/extinctions-in-the-recent-past-and-the-present-day/pleistocene-extinctions/ (opens in new tab)
Tierney et al. "Glacial cooling and climate sensitivity revisited," Nature, Volume 584, Aug. 26, 2020. https://doi.org/10.1038/s41586-020-2617-x (opens in new tab)
U.S. Geological Survey (USGS), "Are today's glaciers leftovers from the Pleistocene ice age?" https://www.usgs.gov/faqs/are-todays-glaciers-leftovers-pleistocene-ice-age (opens in new tab)
Ben Biggs contributed to this article