This volume, the fifth in the important Koobi Fora series on human origins, reports archaeological finds from excavations at East Turkana in northern Kenya from 1969-1979. It concentrates on the evidence from the period between 1.9 and 0.7 million years ago for reconstructing the behavior of early human ancestors. During this research study, new interdisciplinary methods of survey, mapping, excavation, experimentation, and analysis were developed. The study investigated the geology, stratigraphy, site formation processes, technology of the stone assemblages, and associated fauna of the region. This book is a unique record for this time period in Kenya, and this work is a benchmark in the field of human evolution.