The Quaternary Period is the current chapter in Earth’s geologic history, beginning approximately 2.6 million years ago. It is the time in which recognizable human ancestors appeared and evolved into modern humans. This period is defined by two primary features: a pattern of cyclical climate shifts that produced a series of ice ages, and the evolution and global spread of Homo sapiens.
A World of Ice and Change
The defining characteristic of the Quaternary is its cyclical climate, which has alternated between long, cold glacial periods and shorter, warmer interglacial ones. These cycles are driven by variations in Earth’s orbit, known as Milankovitch cycles, which alter the solar radiation reaching the planet. Over the last 740,000 years, there have been eight distinct glacial cycles, with the planet currently in an interglacial period.
During colder glacial periods, continental ice sheets formed and expanded from the poles. At their peak, these glaciers covered vast portions of North America, Europe, and Asia, reaching thicknesses of up to 4 kilometers (2.5 miles). So much water was locked away in glaciers that global sea levels dropped by as much as 120 meters (about 400 feet), exposing land that is now submerged.
This drop in sea level created land bridges, such as the Bering Land Bridge connecting Asia and North America, facilitating the migration of animals and people. The weight and movement of the ice also reshaped the land. Glaciers carved deep valleys, created mountain ridges, and gouged out basins that later became vast lake systems, including the Great Lakes.
The Age of Megafauna and Humans
The fluctuating climates of the Quaternary Period fostered the evolution of giant mammals known as Pleistocene megafauna. These animals were well-adapted to the colder conditions of the ice ages and included species such as the woolly mammoth, saber-toothed cat (Smilodon), giant ground sloth, and cave bear. These large creatures roamed the expanded grasslands and forests of the era.
Toward the end of the last glacial period, roughly 12,000 years ago, an extinction event occurred, wiping out a significant portion of these large animals. The reasons for this mass extinction, often called the Quaternary Extinction Event, are debated. Leading scientific theories point to a combination of factors, including rapid climate warming at the end of the Pleistocene that altered habitats too quickly for many species to adapt.
This environmental shift coincided with the expanding presence and increasing sophistication of human hunters. The “overkill hypothesis” suggests that early humans, who had co-evolved with megafauna in Africa, arrived on other continents as highly effective predators encountering animals with no innate fear of them. Evidence suggests that the combination of hunting pressure and rapid environmental change proved overwhelming for many species, leading to their collapse across multiple continents.
It was during this period of climatic upheaval that modern humans, Homo sapiens, evolved and began their global migration. Originating in Africa, our species spread across the globe, demonstrating a capacity to adapt to the diverse environments of the Quaternary. They developed complex tools, social structures, and cultural practices that allowed them to survive, eventually populating every continent except Antarctica.
Epochs of the Recent Past
Geologists divide the Quaternary Period into two epochs, with a third under consideration. The first and longest is the Pleistocene Epoch, which spans from 2.6 million years ago to about 11,700 years ago. The Pleistocene is synonymous with the ice ages and the evolution of megafauna and archaic humans. Its end is marked by the conclusion of the last glacial period and the megafaunal extinction event.
Following the Pleistocene is the Holocene Epoch, which began approximately 11,700 years ago and continues to the present day. The Holocene is the current interglacial period, characterized by a relatively warm and stable climate. This climatic stability has been instrumental in the development of human civilization, allowing for the invention of agriculture, the growth of permanent settlements, and the rise of complex societies around the world.
In recent years, some scientists have proposed the formal designation of a new epoch: the Anthropocene. This proposed epoch would signify the point at which human activities became the dominant influence on the planet’s geology and ecosystems. Proponents suggest it began around the mid-20th century, coinciding with the “Great Acceleration” of population growth, industrial activity, and the first appearance of globally distributed radioactive elements from nuclear weapons testing. While the term is widely used, the proposal to formally recognize the Anthropocene as a new geological epoch was rejected in 2024 by the governing body of geological time.