Epoch· The columns of time

Pliocene

5.3 Ma → 2.6 Ma

The last warm epoch before the ice ages: apes walk upright, strike the oldest stone tools yet found, and the two Americas join.

What the evidence shows

Footprints in volcanic ash at Laetoli record a heel striking first and the weight rolling forward, the way ours does, and a complete fourth metatarsal argues for the anatomy behind it: an arched foot, which is a walking foot and not a climbing one. At 3.3 million years, cores and flakes struck at Lomekwi in Kenya are the oldest stone tools known, made by hands that were learning how stone breaks. At about 2.8 million years the last seaway between North and South America closes, and animals cross in both directions after fifteen million years apart. In the sea, baleen whales that had stayed moderate in size for most of their history become gigantic in the last few million years, in step with the onset of seasonal upwelling that packs their food into dense patches.

Bounds and conventions

Both boundaries are held by golden spikes; the argument is inside them. The closing of the Panama isthmus is genuinely contested: a review that weighs geology, fossils and molecular data together settles on about 2.8 million years, while other work argues for a much earlier and more complex closure, and a comment and a reply have been published on the point. The age of the Lomekwi tools has also been questioned, and the identity of their makers is unknown.

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. Raichlen et al. (2010), PLoS ONE · Laetoli footprints preserve earliest direct evidence of human-like bipedal biomechanics
3. Harmand et al. (2015), Nature · 3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya
4. O'Dea et al. (2016), Science Advances · Formation of the Isthmus of Panama
5. Slater, Goldbogen & Pyenson (2017), Proceedings of the Royal Society B · Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics