The ages assigned to these fossils have been obtained through radiometric dating of volcanic rocks interbedded with the fossiliferous sediments.
During the latter half of this century anthropological surveys in East Africa have made significant contributions to understanding how the human species has evolved.
In the past two decades, particularly, discoveries of our fossil ancestors have been made in unprecedented numbers and diversity.
Detailed studies of these fossils provide new insights into human evolution, such as the origin of locomotion and cultural activity, and the evolution of the brain, among many other complex features that have come to define humanity.
Even during the time this manuscript was written, new hominid discoveries in Ethiopia and Kenya were announced that trace our earliest ancestors further back into the Pliocene.
If during K-Ar analyses these detrital grains are not recognized and eliminated then they can cause the measured ages to be systematically too old.
Recent advances in K-Ar geochronology, specifically the Ar dating has been a major factor in this success.
This grain-discrete method now permits precise and accurate ages to be measured on single grains and, thus, contaminating grains can be eliminated.
Yet even this seminal K-Ar dating study was plagued by the seemingly insurmountable problem of contamination.
The principal materials for dating East Africa hominid sites are volcanic ashes, yet many of these ashes are not deposited as primary air fall (Greek for ash).
Rather, most are reworked by stream action and are redeposited into the sedimentary environment.
In the process of reworking, these ashes can pick up pre-existing detrital grains that, by definition, are older than the juvenile ash.