KNM-ER 3733, Homo erectus
Discovered by Bernard Ngeneo in 1975 at Koobi Fora in Kenya. Estimated age is 1.7 million years. This superb find consisted of an almost complete cranium. The brain size is about 850 cc, and the whole skull is similar to the Peking Man fossils. The discovery of this fossil in the same stratum as ER 406 (A. boisei) delivered the coup de grace to the single species hypothesis: the idea that there has never been more than one hominid species at any point in history.
A very similar skull, ER 3883, is even smaller, at 800 cc.
Credit: photo and (edited) text courtesy of Talk Origins.