Symantec Security Response has discovered new intelligence on the earliest known version of Stuxnet. In operation as early as 2007, this newly uncovered version featured an entirely different attack mechanism than its successors. Rather than affecting the speed of uranium enrichment centrifuges, Stuxnet 0.5 was designed to close crucial valves that feed uranium hexafluoride gas into the centrifuges, causing serious damage to the centrifuges and the uranium enrichment system as a whole.
In addition, hints in this early version indicate work on the Stuxnet project as a whole could date back to 2005 or earlier.
Source: Symantec Blog