From about this time in the mid-1970s I began to be asked to review manuscripts that used the K-Ar and ⁴⁰Ar-³⁹Ar techniques to date rocks within sedimentary sequences in which hominid fossils had been found, especially in East Africa. At the time there was a good deal of controversy surrounding the numerical age of spectacular new hominid finds made by Richard Leakey and his team from the National Museums of Kenya in the sediments exposed in the Koobi Fora area, lying adjacent to the eastern shores of Lake Turkana in northern Kenya. In particular, the cranium ER 1470, often assigned to Homo habilis, was collected from sediments lying just below the KBS Tuff, an important marker horizon produced by a contemporary explosive volcanic eruption. Dating of this tuff by the K-Ar and ⁴⁰Ar-³⁹Ar methods had given a wide range of numerical ages from about 1.6 to 2.6 Ma, leading to much debate, particularly as the older age would have meant that ER 1470 was the oldest hominid known at that time. The co-leaders of the Koobi Fora Research Expedition, Richard Leakey and Glynn Isaac, invited me to participate in further isotopic dating of the tuffs, a number of which were known to occur in the sedimentary sequence. In principle, isotopic dating of alkali feldspar crystals found within pumice clasts in a tuff would yield an age for the igneous eruption. As deposition of a tuff within a sedimentary basin generally would have occurred within a very short interval after the eruption, days to perhaps tens of years, extending to hundreds of years at most, the age obtained on a tuff also provides a close estimate of the age of deposition. Thus, in 1978 I was able to undertake field work at Koobi Fora, collecting pumice clasts and tuffs from a number of horizons throughout the sequence, under the guidance of the geologist of the expedition at that time, Ian Findlater. Subsequently, our initial K-Ar and ⁴⁰Ar-³⁹Ar dating results on the KBS Tuff were published in Nature in 1980 and 1981, showing that it had a quite reproducible age of 1.88 ± 0.02 Ma. These results were seen as resolving the controversy on the numerical age of the KBS Tuff and thus of ER 1470 recovered from sediments just below the tuff (e.g. see Hay, R. L., Nature 284: 401, 1980).

We also made age measurements on other tuffs in the sequence, demonstrating that the sequence at Koobi Fora was laid down between about 4.0 and 0.7 Ma ago. The results were consistent with the stratigraphic order, providing much confidence in the age measurements, and yielding a reliable numerical time frame for the deposition of the sediments and their contained fossils. This work was published in 1985 (McDougall, I., Bulletin of the Geological Society of America 96: 159-175), and further amplified in 1989 when the stratigraphy, geochronology, and the stratigraphic position and estimated age of the many hominid fossils from the Koobi Fora area and elsewhere in the Turkana Basin were reviewed (Feibel, Brown, and McDougall, American Journal of Physical Anthropology 78: 595-622). Co-authors Frank Brown and Craig Feibel were both then at the University of Utah. They have worked closely with the paleoanthropologists of the National Museums of Kenya to provide the stratigraphic context for the fossils, as well as developing an understanding of the depositional history in the Turkana Basin, of which the sediments of the Koobi Fora area are part. Their work, together with that of other colleagues (e.g., Thure Cerling), has been crucial in providing a comprehensive, overall synthesis of the basin’s history. This has been accomplished through identification and correlation of the many tuffaceous horizons throughout the Turkana Basin using traditional stratigraphic mapping methods, together with chemical analyses of the volcanic glass comprising the tuffs and pumice clasts, which they showed to commonly have distinctive compositions.

As a result of the success of our geochronological work in the Koobi Fora sedimentary sequence in the late 1970s and early 1980s, I continued to collaborate with the paleoanthropologists of the National Museums of Kenya, initially with Richard Leakey, but later through his wife and professional colleague Meave Leakey. All my work was undertaken in close association with Frank Brown and Craig Feibel, as they studied the stratigraphy of the related sediments exposed adjacent to the western shores of Lake Turkana in the 1980s and 1990s. This was done in conjunction with the search for new hominid fossils, with the latter the major driving force behind the expeditions.

With the continuing discovery of new hominid fossils in Kenya and Ethiopia, the geochronology measurements on associated volcanic rocks became increasingly important in constraining the age of the fossils. The greater sensitivity of the analytical instruments used in the measurements and the higher precision attainable, together with the demonstration by several laboratories that it was possible to measure precise ⁴⁰Ar-³⁹Ar ages on single crystals of alkali feldspar of 1 mm or less in size, led to this latter technique becoming the main one used in documenting the age of new fossil discoveries.

The three papers of particular note published over the last decade, recognized in the Social Sciences field of ISI Essential Science Indicators, were all multiauthored, with Meave Leakey as first author, as the focus was new paleoanthropological results in each case, arising from expeditions organized and run by Meave. One of the papers (Leakey et al., 1995) assigned hominid fossils from Kanapoi and Allia Bay in the Turkana Basin sequences to a new species Australopithecus anamensis, shown to be bipedal, found in sediments deposited between 4.17 and 4.07 Ma ago. The 2001 Leakey et al. paper described a new hominid genus and species, Kenyanthropus platyops, of age 3.5 Ma. That several authors were involved in each paper serves to emphasize the multidisciplinary approach necessary to provide the appropriate context for the hominid fossils. Thus, in each case, the essential geological information and interpretation was provided by a stratigrapher (Craig Feibel or Frank Brown), with myself undertaking the geochronological measurements to constrain the age of the fossils. For each study, I spent several weeks in the field with the stratigrapher collecting material suitable for the isotopic dating work. Single crystal ⁴⁰Ar-³⁹Ar dating of alkali feldspars from quite small pumice clasts yielded very precise results. The actual time spent in the field by me was small in comparison with the time the stratigraphers and the fossil hunters worked in the hot, near-desertic environment that typifies the Turkana region. Much more of my time was devoted to preparing samples for analysis and making the measurements, although it should be emphasized that the field collecting is always a most essential and critical part of the overall research program.

This work is ongoing within the Turkana Basin, in close collaboration with Frank Brown of the University of Utah. We now have precise age measurements on more than 20 tuffaceous horizons within the Turkana Basin stratigraphic sequence. Thus, when new hominid or other fossils of interest are found, it is commonly possible to give an age estimate to better than 0.1 Ma for the stratigraphic level from which the fossil or fossils were derived, provided that the stratigraphic level can be determined relative to one or preferably more than one of the known tuffaceous beds. However, when additional tuffaceous horizons are found within the stratigraphic sequence in association with important fossils, there will continue to be an interest in and need to undertake further age measurements if suitable materials for dating are available, in order to further improve the time framework and to refine our understanding of evolutionary paths and processes. This kind of research into the origin and evolution of our own species obviously will continue in the future, no doubt with many unexpected discoveries of hominid fossils on the way that will shake our current views, but at the same time provide an increasingly well-documented history. As further hominid fossils are found and their age and provenance determined, I’m confident that we will improve our understanding of the evolutionary processes that have lead to modern humans, and extend our knowledge as to when the lineage leading to modern humans bifurcated from that of the great apes. In this context, the general interest in hominid evolution will also clearly continue, especially in relation to Africa, which is widely accepted as the cradle of mankind.