Climate During Early Human Evolution: A Rollercoaster of Ice and Warmth

The climate experienced by early hominins (2 million–100,000 years ago) was anything but stable. It oscillated violently between glacial and interglacial cycles, forcing adaptation, migration, and innovation.

The Glacial-Interglacial Pulse

Driven by Milankovitch cycles (orbital eccentricity, axial tilt, precession), Earth cycled every 41,000 years (early Pleistocene) then ~100,000 years (late Pleistocene) between:

Glacial Periods (Ice Ages)

• Global temps 4–8°C colder
• Massive ice sheets: Laurentide (N. America), Fennoscandian (Europe), Himalayan
• Sea levels ~120 m lower — land bridges exposed (Beringia, Sundaland, Sahul)
• Arid, dusty, low CO₂ (180 ppm) — deserts expanded, forests retreated

Interglacial Periods

• Warmer, wetter, CO₂ ~280 ppm
• Forests expanded, sea levels rose
• Similar to early Holocene (pre-industrial)
• Lasted 10,000–30,000 years

Regional Impacts on Hominins

Africa (Cradle of Humanity)

Not uniformly wet/dry. Orbital precession drove 20,000-year "Green Sahara" cycles: lush savannas with lakes ↔ hyper-arid desert. East African Rift valleys buffered extremes, offering mosaic habitats (gallery forests, grasslands, wetlands) where hominins thrived.

Eurasia

Glacials pushed populations south into refugia (Iberia, Balkans, Caucasus, Southeast Asia). Interglacials allowed northward expansions. Neanderthals adapted to cold; H. sapiens used cultural buffering (clothing, shelter, fire).

Ecological Consequences

• Megafauna extinctions: Mammoths, woolly rhinos, giant ground sloths vanished at glacial-interglacial transitions—hunting + climate synergy
• Vegetation shifts: C₄ grasslands expanded in glacials (favoring grazers); forests returned in interglacials
• Water stress: Droughts forced movement to permanent springs, rivers, lakes

Human Responses

These pressures selected for: behavioral flexibility, complex toolkits, long-distance trade/exchange, symbolic culture, and ultimately, the cognitive capacity to modify environments rather than just adapt to them.