Wings evolved over many, many millions of years.  Scientists have extensively studied the wings of modern-day birds and have identified many important anatomical details. Especially important are those not found on the "arms" of non-birds.  Major efforts have gone into finding early occurrences of such details in the fossil record, but unfortunately it is far from complete.  Wings generally have thin, hollow bones that do not fossilize well and it appears that few of the animals that had them died in environments suitable for fossilization.  The lack of fossils makes it difficult to establish the time that each feature first appeared.  The times listed below are documented, but do not reflect an even progression from one step of deveopment to the next.