KNM ER 3733 is a fossilized hominid cranium of the extinct hominid Homo ergaster (or H. erectus) It was discovered in Koobi Fora, Kenya by Bernard Ngeneo in 1975.
KNM ER 3733 is one of the oldest Homo ergaster skulls in the world. In a 1989 publication in the American Journal of Physical Anthropology, Dr. Craig Feibel, now at Rutgers University Geology Department (U.S.A), and his co-workers estimated the age of KNM-ER 3733 at ~1.8 million years old. However, more recent research using magnetostratigraphy has determined the age of KNM-ER 3733 to be about 1.7 million years old (http://adsabs.harvard.edu/abs/2010E&PSL.290..362L)