Around 2.6 million years ago, at the Pliocene-Pleistocene boundary, Earth's climate system entered a new and persistent regime: the Quaternary Glaciation. Ice sheets periodically covered North America and Eurasia; sea levels fell by hundreds of metres; bands of flora and fauna redistributed with every cycle. This prolonged pressure shaped not only geography but the evolving human lineage.

The driver of glaciation is Milankovitch forcing: the eccentricity of Earth's orbit (~100,000 years), the tilt of its axis (~41,000 years), and orbital precession (~23,000 years) periodically alter the seasonal distribution of solar radiation. In the early Pleistocene the dominant rhythm was 41,000 years; around 1 million years ago the "Mid-Pleistocene Transition" shifted the dominant cycle toward 100,000 years. The cause of this shift is still not fully understood.

The most recent glacial maximum (Last Glacial Maximum, LGM) occurred around 20,000 years ago: the Fennoscandian and Laurentide ice sheets coalesced; all of Scandinavia, most of Britain, the northern half of North America, and Siberia lay under ice. Sea level stood roughly 120 metres below today's; Beringia — the land bridge connecting Asia and the Americas — was open, and along it the first humans crossed into the New World.

Glacial cycles interweave with human evolution. The emergence of Homo erectus (~1.8 Ma), control of fire (~1 Ma), the spread of Homo heidelbergensis, the divergence of Neanderthal and modern human lineages — all occurred within the glacial-interglacial oscillations of the Pleistocene. The savanna hypothesis holds that as shifting climate opened forested areas in Africa, our ancestors were pressed toward new food sources and bipedality. Increases in brain volume, stone tool traditions, and fire use are all correlated with glacial pressure.

The Pleistocene ended around 11,700 years ago; the Holocene that followed is the stable warm interval in which we still live. All of civilisation — agriculture, cities, writing — emerged within this brief and unusually steady climatic window.