Pleistocene

The Pleistocene is a time period in Earth's history, often called the Ice Age, that lasted from about 2.58 million to 11,700 years ago. During this time, Earth went through many cold periods called glacial periods and warmer periods called interglacials. The climate changed a lot, with temperatures dropping so low that huge sheets of ice covered much of North America and Asia.

Because of these cold periods, sea levels dropped by as much as 120 metres lower than today, which allowed people and animals to move between Asia and North America through a land bridge called Beringia. This time was also important for human history. Early humans, known as archaic humans, first appeared in Africa and later spread to many parts of the world.

Later in the Pleistocene, modern humans left Africa and moved to new places, including Australia and the Americas for the first time. This period also saw the disappearance of many large animals outside of Africa. Scientists think that changes in climate and the arrival of humans were likely reasons for these animals no longer being around.

Etymology

Charles Lyell created the word "Pleistocene" in 1839. He used it to describe layers of rock in Sicily where most of the sea creatures living there today were also found in those rocks. This made the Pleistocene different from the older Pliocene Epoch. Lyell made up the name "Pleistocene," which means "most new" or "newest," using words from the Greek language. This name helps us see how the Pleistocene is different from the Pliocene, which means "newer," and the Holocene epoch, which means "wholly new" and goes up to today.

Dating

The Pleistocene, also called the Ice Age, lasted from about 2.58 million years ago to around 11,700 years ago. Scientists use special methods, like studying tiny sea creatures and ice layers, to figure out the exact dates. During this time, Earth went through many cold periods when large glaciers covered parts of the land.

In 2009, scientists updated the start date of the Pleistocene from 1.806 million years ago to 2.58 million years ago. They now divide the Pleistocene into four main stages: the Gelasian, Calabrian, Chibanian, and Upper Pleistocene. These stages help us understand the changes that happened over this long period.

Deposits

During the Pleistocene, scientists find many layers of soil and rocks that tell us about the past. These layers, called sediments, are mostly found in places where rivers used to flow, in old lakebeds, and in areas where wind blew lots of dust, called loess. Glaciers also moved huge amounts of earth and stone.

We can also find these old layers near the ocean, usually not far from where the shoreline is today. In some places that move a lot, like the coast of Southern California, these layers can even be found high up on hills.

Paleogeography and climate

The continents were mostly in the same places during the Pleistocene as they are today, having moved only a little since the start of this time period. During cold periods, called glacials, sea levels dropped by up to 120 metres, making more land visible.

The climate of the Pleistocene was like a never-ending warm rainy season in some places, with winds and weather patterns that were different from today.

Glacial features

The Pleistocene had many cold periods where huge sheets of ice covered large parts of the Earth. About 30% of the planet was covered in ice at its peak. These icy areas were coldest near their edges, where temperatures were just below freezing.

When these ice sheets grew, they locked up so much water that sea levels dropped around the world. Between these cold periods, during warmer times called interglacials, coastlines would flood as the ice melted.

Ice covered many high mountains and even parts of Africa and South America. In places like North America and Europe, several ice sheets joined together, while in other areas smaller glaciers formed scattered groups.

Major events

Further information: Timeline of glaciation

Over the 2.5 million years of the Pleistocene, Earth went through many cold periods, called glacials, and warmer periods, called interglacials. These cold periods happened roughly every 40,000 to 100,000 years, and the warmer periods lasted about 10,000 to 15,000 years each. The most recent cold period ended around 10,000 years ago.

Scientists have found evidence of over 11 major cold periods, along with many smaller ones. These events happened in slightly different ways depending on where you look on Earth, but they generally matched up across the globe.

Palaeocycles

The repeated advances and retreats of ice during the Pleistocene were driven by natural, repeating patterns in Earth’s orbit and tilt. These patterns, called Milankovitch cycles, changed how much sunlight reached different parts of the planet over thousands of years.

These cycles helped create the pattern of cold and warm periods we see in the fossil and ice records. Scientists study things like the ratio of different oxygen atoms in ancient ocean creatures to understand past temperatures and climate changes.

The pattern of temperature changes over time looks like waves, with cold periods and warm periods repeating in a predictable way. These waves help scientists divide Earth’s climate history into distinct stages.

Fauna

See also: Quaternary extinction event

Both land and sea animals during this time were similar to those we know today, but there were many more large land animals. These included mammoths, mastodons, Diprotodons, Smilodons, tigers, lions, aurochs, short-faced bears, giant sloths, and species within Gigantopithecus. Some places, like Australia, Madagascar, New Zealand, and Pacific islands, had large birds and reptiles such as the elephant bird, moa, Haast's eagle, Quinkana, Megalania, and Meiolania.

The big climate changes during this icy time greatly affected animals and plants. As glaciers moved forward, many animals had to move south to find food and shelter. This caused a lot of stress for them. By the end of this period, many large animals, such as mammoths, mastodons, saber-toothed cats, glyptodons, the woolly rhinoceros, and ground sloths, went extinct. Smaller animals like wood mice and whitetail deer became more common.

The loss of large animals was strongest in North America, where native horses and camels disappeared.

• Asian land mammal ages (ALMA) include Zhoukoudianian, Nihewanian, and Yushean.
• European land mammal ages (ELMA) include the Villafranchian, Galerian, and Aurelian
• North American land mammal ages (NALMA) include Blancan (4.75–1.8), Irvingtonian (1.8–0.24) and Rancholabrean (0.24–0.01) in millions of years. The Blancan extends significantly back into the Pliocene.
• South American land mammal ages (SALMA) include Uquian (2.5–1.5), Ensenadan (1.5–0.3) and Lujanian (0.3–0.01) in millions of years. The Uquian previously extended significantly back into the Pliocene, although the new definition places it entirely within the Pleistocene.

In July 2018, a team of Russian scientists working with Princeton University reported that they had brought two female nematodes frozen in permafrost from around 42,000 years ago back to life. These were, at the time, the oldest known living animals on Earth.

Pleistocene of [Northern Spain](/wiki/Northern_Spain), including [woolly mammoth](/wiki/Woolly_mammoth), cave lions eating a [reindeer](/wiki/Reindeer), horses, and [woolly rhinoceros](/wiki/Woolly_rhinoceros)

Pleistocene of [South America](/wiki/South_America), including _[Megatherium](/wiki/Megatherium)_ and two _[Glyptodon](/wiki/Glyptodon)_

Humans

Main articles: Human evolution, Paleolithic, and Settlement of the Americas

The development of humans as we know them today happened during the Pleistocene. At the start, there were other human-like species, but by the time this period was ending, only our ancestors remained. Tools made by early humans became more advanced over time.

Modern humans, known as Homo sapiens, appeared around 300,000 years ago. They moved out of Africa and spread around the world during this icy time.