You raise a good question, I guess it (the foot) is still missing in the layers.
Just one of many:
OH 8, found in 1960, consisted of a set of foot bones, complete except for the back of the heel and the tips of the toes. Estimated age is about 1.8 million years. The foot bones had most of the adaptations to bipedality possessed by modern humans. There is a well-developed arch, and the big toe is alongside the other toes instead of diverging, as is the case with apes and monkeys. Source.