Event 34

3 to 2 million years ago

Climate Cooling Graph
Graph showing the gradual lowering of world temperatures over the last 65 million years. Between 3 and 2 million years ago, the Earth experienced a cooling and drying period, leading to the spread of grasslands in places like Africa. Credit: Illustration is presented here courtesy of the National Park Service.

Between three and two million years ago, the African continent and the earth in general experienced a gradual climatic shift to cooler, drier conditions. In many places, tropical forests became woodlands, and woodland became grasslands over time. This cooling phase is correlated with the earliest known stone tools and the emergence of the larger-brained hominin, the genus Homo, as well as the so-called “robust australopithecines.” It is also a time of a major “faunal turnover,” a time when many animal lineages went extinct, while others evolve into new species over time. Prehistoric evidence suggests that hominins became more adapted to more open woodlands and grasslands during this period, an adaptation which became characteristic of the human lineage in Africa for much of its subsequent prehistory.

It is believed that a major reason for this cooling was the geological uplift that created the Isthmus of Panama, now connecting North America and South America by a land bridge, and disrupting the flow of ocean currents. Around three million years ago glacial ice sheets began to form in the northern hemisphere, and many subsequent phases of glacial advances and retreats of this new Ice Age have been documented after this time. When much of the earth’s water is locked up in frozen ice sheets, there is a strong tendency for conditions to become more arid, often with the spread of grasslands and deserts.

Badlands National Park, South Dakota
Researchers study ancient soil layers for clues about past climates.
Credit: Photo is presented here courtesy of the National Park Service.


Evidence of climate change includes oxygen isotope ratios in deep sea cores, carbon isotope ratios in soils in animal bones, pollen analysis, and analysis of the types of animals that were present at a given time.


This global cooling and drying trend characterized the last three million years of human evolution, and heralded the beginning of major glacial phases during the last two million years. Our ancestors had to adapt to changing environmental conditions, and evolved into new species over time.







This is a concise description of the Pliocene epoch, a cooling period after the warmer Miocene period.

NASA’s website contains a series of pages that deal with the Pliocene global warming period, ending in the late Pliocene with global cooling, and entering the glaciation period of the Pleistocene.

This webpage is a Pliocene “quick guide”.

This website also provides information on the Pliocene.





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