The long-favored paradigm for the development of continental crust is one of progressive growth beginning at similar to 4 billion years ago (Ga). To test this hypothesis, we measured initial Hf-176/(177) Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. epsilon(Hf) (deviations of Hf-176/Hf-177 from bulk Earth in parts per 104) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf approximate to 0.01), perhaps as early as 4.5 Ga. Positive epsilon(Hf) deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.