Dr. Frank Brown and Dr. Richard Klein served together for many years as co-chairs of The Leakey Foundation’s Scientific Executive Committee. Frank Brown passed away on September 30, 2017.
By Dr. Richard Klein, Stanford University
Frank Brown’s study of the Omo-Turkana Basin over a 50-year period (1966-2016) provides the basis for a detailed chronology of human evolution. By using the chemical composition of volcanic ashes, he has been able to put in order more than 150 ash layers in the basin deposited over the past 4.24 million years; with colleagues he has dated over 35 ashes using potassium-argon dating methods, and dated another half-dozen by orbital tuning to deep-sea ashes; in addition, he has used paleomagnetism to provide yet more time horizons through this stratigraphic record of human evolution. Many of these ashes are found in the Awash Basin in Ethiopia, where they provide context for hominin fossils recovered there. With his detailed mapping and chemical analysis, he has provided the temporal context for the hundreds of hominin fossils discovered in the Omo-Turkana Basin and elsewhere in Eastern Africa.
The Omo-Turkana Basin of northern Kenya and southern Ethiopia is renowned for its nearly continuous record of alluvial and lacustrine sedimentation between the Pliocene and the present. Sedimentary rocks exposed on the eastern and western shores of Lake Turkana and in the valley of the Omo River, which feeds the closed basin of Lake Turkana from the north, have provided tens of thousands of mammalian fossils, including more than 500 human fossils that illuminate the nature of the australopiths and early members of the genus Homo. The same sedimentary rocks have also provided thousands of artifacts that document the early development of human behavior and ecology. The fossils and artifacts, however, would lose much of their meaning if they could not be ordered in time. Frank Brown devoted his career to developing the necessary geochronologic framework, and it is no exaggeration to say that the hard-won result underlies much of what we now know about early human evolution.
The Omo-Turkana Basin covers approximately 40,000 square kilometers, which is more than the total for all other east African early human fossil localities combined. Volcanic ashes provide the main basis for dating and site correlation, and in Brown’s extensive fieldwork, mostly conducted in lonely, difficult, even dangerous conditions, he identified more than 150 distinct ash beds. He refined methods for characterizing these geochemically, so that they can be readily identified at widely scattered find-spots, and such “fingerprinting” has not only provided highly reliable correlations among sites within the Omo-Turkana Basin, it has also allowed external correlations to important early human fossil sites in the Middle Awash Basin, Afar Depression, 500 km to the north, the Baringo Basin, 200 km to the south, and the Albertine Rift, 300 km to the west.
Beyond dating, Brown’s mapping and sedimentological studies have provided fresh and previously unexpected insights into the paleogeography of the Omo-Turkana Basin. He also has demonstrated that the Kulal volcano, which “plugged” the basin in the late Pliocene, diverted Omo River flow from the Indian Ocean to the Mediterranean, which not only helps to explain the sapropel link, but also shows how fauna and flora from northeastern Africa managed to spread far to the south in times past.
In sum, specialists and interested lay observers will surely agree that the Omo-Turkana Basin has provided much of what we know about human and related mammalian evolution between the Pliocene and the present, and they will also recognize that the Turkana fossil and archaeological sites would mean little if they could not be ordered in time and related to the past geography of the Basin.
Paleoanthropologists will also know that it is primarily Frank Brown who has provided the stratigraphic and paleogeographic framework. What they may not appreciate is the colossal effort involved, spread over five decades and thousands of square kilometers of rugged, desolate countryside. Some aspects of the fossils and artifacts may remain debatable, but Brown’s stratigraphic and geochronologic framework is firmly established, and it will remain a lynchpin for future researchers who seek to enlarge and interpret the fossil and archaeological samples.
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