THE EARLIEST HOMINIDS FROM SOUTH AFRICA: CHRONOLOGY AND ECOLOGICAL BACKGROUND

The first absolute dates for early hominids from Sterkfontein show that Australopithecus was widely dispersed across Africa from the early Pliocene. These new dates have, together with around a dozen magnetic reversals spanning both dated and undated parts of the long sedimentary sequence, permitted the stratigraphy at Sterkfontein to be reinterpreted with less reliance having to be placed on long-distance palaeontological comparisons for chronological control.

It is now clear that Australopithecus was present around Sterkfontein for about two million years, surviving there well after Homo appeared in the East African fossil record and continuing until shortly before the first appearance of Paranthropus. More than one species of Australopithecus is preserved in the Sterkfontein record, and those specimens dating to the Lower Pliocene remain contenders for membership of the lineage which gave rise to our earliest ancestors.

The wide geographical dispersal of Australopithecus and the longevity of this genus were associated with an interval during most of which the African tropics and subtropics enjoyed more mesic and generally warmer conditions than today. It was only in the Upper Pliocene that widespread cooling and aridification were experienced on a continental scale. However, major tectonic disturbances, including substantial uplift, imposed sequential changes of both a regional and local character along the entire eastern hinterland of Africa, spanning both hemispheres. The environmental consequences of these changes are evident within the Sterkfontein sedimentary record. Stable isotopes and plant fossils provide ancillary evidence. That the adaptive strategies evolved over time by Australopithecus in response to incessant and pervasive change were successful is confirmed by its relatively low demographic risk profile, which contrasts sharply with those of genera that followed it.