The extent to which the earliest anatomically modern humans in Africa exhibited behavioral and cognitive traits typical of Homo sapiens sapiens is controversial. In eastern Zaire, archaeological sites with bone points have yielded dates older than 89(-15)+22 thousand years ago by several techniques. These include electron spin resonance, thermoluminescence, optically stimulated luminescence, uranium series, and amino acid racemization. Faunal and stratigraphic data are consistent with this age.

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This is the fourteenth in a series of Ceramic Educational Council Invited Review Papers.* Even with thermodynamically reversible electrodes, slow processes inhibiting the discharge of ions at the interface between a solid ionic conductor and the electrode can give rise to overvoltages when current is passed or to an interface impedance in ac measurements. Some of the more important of the mechanisms responsible for this inhibition, i.e. boundary layers, slow charge-transfer processes with double-layer formation, product storage in the electrodes with slow mass transport to the ultimate thermodynamic reservoirs, and rough or porous surfaces, can be understood in a nonrigorous physical way with the help of simple models. The properties of such systems can often be expressed in terms of equivalent circuits, and these are then very useful in analyzing the behavior of specific systems. Examples from studies of materials such as CaO-stabilized ZrO, (oxygen-ion conductor) and ,B-Al,O, (sodium-ion conductor) illustrate these effects.