Definition
The Miocene-Pliocene boundary is the geological transition that separates the Miocene epoch (23.0 to 5.3 mya) from the Pliocene (5.3 to 2.6 mya). Formally defined by a geological reference section (a GSSP) at Eraclea Minoa, Sicily, the boundary sits at 5.33 million years ago. It marks the end of the Messinian salinity crisis — when the Mediterranean repeatedly dried out — and the beginning of cooler, more variable global climates.
For paleoanthropology, the boundary lies near the inferred chimp-human last common ancestor and within the span of the earliest hominin fossils. Sahelanthropus (~7 mya) is late Miocene; Orrorin (~6 mya) is also Miocene; Ardipithecus kadabba straddles the boundary; Ardipithecus ramidus and all later hominins are Pliocene or younger.
Why it matters
How it works
Geologically, the boundary is defined by changes in marine microfossil assemblages preserved in Mediterranean sections. The end of the Messinian salinity crisis, when normal marine conditions returned to the Mediterranean, provides a sharp event horizon. This event is dated radiometrically and magnetostratigraphically to ~5.33 mya, with very low uncertainty.
In African terrestrial sections, the boundary is not directly visible but is interpolated using biostratigraphy, volcanic ash chronologies, and paleomagnetic reversals. The result is a workable framework that lets paleoanthropologists place every African fossil within the global epoch sequence.