Period· The columns of time

Neogene

23 Ma → 2.6 Ma

Grasslands spread across the continents, the Mediterranean dries out and refills, and a branch of African apes begins to walk upright.

What the evidence shows

The world cools and dries, and the vegetation changes with it. Tropical grasses adopt a different way of running photosynthesis, one that works better when carbon dioxide is scarce and the air is hot and dry, and between roughly eight and three million years ago these C4 grasslands rise to dominance across whole continents, taking the place of woodland. Open country favours animals that can run and see far, and the teeth of the grazers change with the grass. Africa holds the fossils that concern us most: a branch of apes separates and begins to walk upright, and the specimens that record it are split between the two epochs, a crushed skull from Chad in the Miocene and, in the Pliocene, footprints, stone tools and the oldest jaw anyone assigns to Homo.

Bounds and conventions

Its bounds are golden spikes. The C4 expansion is dated by carbon isotopes: in fossil teeth, which record what the animals ate, and in ancient soils, which record what grew above them. A change of diet is therefore read more directly than a change of landscape, and what tipped the grasses into dominance is not settled. The hominin fossils are each contested in their own way, and their dating is discussed on the epochs that hold them.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Edwards et al. (2010), Science · The origins of C4 grasslands: integrating evolutionary and ecosystem science
3. García-Castellanos et al. (2009), Nature · Catastrophic flood of the Mediterranean after the Messinian salinity crisis
4. Brunet et al. (2002), Nature · A new hominid from the Upper Miocene of Chad, Central Africa
5. Villmoare et al. (2015), Science · Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia
6. Tada et al. (2016), Progress in Earth and Planetary Science · Evolution and variability of the Asian monsoon and its potential linkage with uplift of the Himalaya and Tibetan Plateau