The Pleistocene Epoch, commonly called the “Ice Age,” spanned from approximately 2.6 million to 11,700 years ago. It is part of the Quaternary Period, following the Pliocene Epoch and preceding our current Holocene Epoch. The Pleistocene is defined by repeated episodes of widespread glaciation that shaped the planet’s geology and life.
The Glacial Climate and Landscape
The Pleistocene’s climate was cyclical, characterized by the advance and retreat of continental ice sheets. These cycles of glacials (cold periods) and interglacials (warmer periods) were driven by variations in Earth’s orbit that altered solar radiation. During peak glacial periods, ice covered an estimated 30% of the Earth’s surface, with glaciers over a mile thick in parts of North America, Europe, and Asia.
This buildup of ice locked away large amounts of water, causing global sea levels to drop by as much as 390 feet (120 meters). This drop exposed the continental shelf, creating land bridges that connected landmasses now separated by seas. The Bering Land Bridge, a plain up to 1,000 miles wide, connected Asia with North America, allowing for the movement of animals and people. Similarly, Doggerland connected Great Britain to mainland Europe.
These climatic conditions created biomes with no modern equivalent, the most widespread being the mammoth steppe. This cold, dry grassland ecosystem stretched across northern Eurasia and North America. The environment consisted of grasses, sedges, and low shrubs, and its fertile soils supported large herds of grazing animals.
The Reign of Megafauna
The Pleistocene landscape supported large animals known as megafauna, including the woolly mammoth (Mammuthus primigenius). A relative of modern elephants, mammoths were adapted to the cold, standing up to 12 feet tall and weighing over six tons. They had a thick coat of shaggy hair, insulating fat, small ears to prevent heat loss, and curved tusks for defense and foraging. Mammoths roamed the steppe in herds, feeding on grasses and sedges.
The American mastodon was a shorter, stoutly built proboscidean that browsed on trees and shrubs in woodland habitats. Predators included the saber-toothed cat (Smilodon fatalis), a robust hunter about the size of a lion, weighing up to 620 pounds. It used its eight-inch-long canine teeth to deliver fatal bites to large prey.
Another predator was the dire wolf (Aenocyon dirus), similar in size to large gray wolves but with a heavier build and a more powerful bite. Fossil evidence suggests they hunted in packs, tackling herbivores like horses and bison. The giant ground sloth (Megatherium) was an herbivore that could stand over 12 feet tall and weigh up to four tons. It used its size and claws to pull down tree branches for feeding. The giant short-faced bear (Arctodus simus) was the largest mammalian land carnivore of its time, standing over 11 feet tall on its hind legs and built for long-distance travel across open grasslands.
Human Evolution and Migration
Human evolution advanced during the Pleistocene with the development and spread of several hominin species. Early in the epoch, Homo erectus was the first of our ancestors to migrate from Africa, spreading across Asia and Europe starting around 1.9 million years ago. This species had a human-like body plan, a larger brain, and created hand-axes as part of the Acheulean stone tool industry. Homo erectus was also the first hominin known to control fire.
Later in the Pleistocene, other archaic human groups emerged, including Neanderthals (Homo neanderthalensis), who inhabited Europe and Western Asia from at least 200,000 years ago. They adapted to cold climates, were skilled hunters who created Mousterian tools, and lived in social groups. In Asia, the Denisovans were identified through genetic evidence. Genetic analysis shows Denisovans were a sister group to Neanderthals and interbred with modern humans.
Anatomically modern humans, Homo sapiens, emerged in Africa around 300,000 years ago. Between 70,000 and 100,000 years ago, they began migrating out of Africa, eventually populating the entire globe. With advanced toolkits and complex social structures, Homo sapiens expanded across Asia, reached Australia by 65,000 years ago, and entered Europe around 45,000 years ago. They entered the Americas via the previously mentioned Bering Land Bridge, following herds of game.
The End-Pleistocene Extinction
The Pleistocene Epoch ended around 11,700 years ago with an extinction event that eliminated much of the world’s megafauna. In North America, about 73% of large herbivore species vanished, including the woolly mammoth, mastodon, saber-toothed cat, dire wolf, and giant ground sloth. The cause of this extinction is debated, with two primary hypotheses.
The first explanation centers on climate change. As the last glacial period ended, rising global temperatures melted the ice sheets. This warming altered landscapes, causing the mammoth steppe to be replaced by forests and wetlands. This shift removed the primary food sources for large grazers and disrupted the ecosystems the megafauna depended on.
The second theory is the “overkill hypothesis,” which posits that hunting by newly arrived human populations was the primary driver of extinction. This view points to the correlation between the arrival of humans in regions like North and South America and the disappearance of animals that had not faced such an effective predator. However, many scientists conclude that the extinction was not caused by a single factor, but a combination of climate-induced environmental stress and human hunting pressure.