Epoch· The columns of time

Pliocene

5.3 Ma → 2.6 Ma

The Pliocene, from 5.3 to 2.6 million years ago, was the last warm epoch before the ice ages: footprints attest bipedalism, the oldest stone tools were knapped and the two Americas were joined.

Last warm epoch before the ice ages, in which bipedalism is attested by footprints, the oldest stone tools are knapped, and the two Americas are joined.

What the evidence shows

Footprints in the volcanic ash at Laetoli record a heel strike followed by a forward roll, as in human walking, and a complete fourth metatarsal attests the corresponding anatomy: an arched foot, adapted to walking and not to grasping. At 3.3 million years, the cores and flakes of Lomekwi, in Kenya, are the oldest known stone tools. Around 2.8 million years, the last seaway separating North from South America closes, and faunas cross in both directions after a long separation. At sea, baleen whales, of moderate size for most of their history, become gigantic in the last few million years, at the same time as seasonal cold-water upwellings become established and concentrate their food.

Bounds and conventions

Both limits are golden spikes; the discussion concerns the contents. The closure of the Isthmus of Panama is genuinely contested: a synthesis combining geology, fossils and molecular data retains about 2.8 million years, while other work argues for an older and more complex closure, a comment and a reply having been published on the point. The age of the Lomekwi tools has also been questioned, and the identity of their makers remains unknown.