The human fossil record is one of the most complete for any mammal. A basal ancestral species, Australopithecus afarensis, exhibits a well-preserved postcranium that permits reconstruction of important events in the evolution of our locomotor skeleton. When compared to those of living apes and humans, it provides insights into the origin and design of the modern human frame. Evolutionary aspects of the human knee are reviewed, including its highly specialized design with respect to upright walking and running. Design elements include increased tibial cartilage contact derived by both genomic and epigenetic mechanisms, valgus stance angulation, a mechanism for patellar retention, and a somewhat increased patellar moment arm. The history of these features in early hominids and their fundamental differences from their counterparts in apes are discussed.