I think people had always understood that sun exposure can lead to burns and tanning -- which wasn't considered attractive the way it is today. The link between sun exposure and skin cancer wasn't properly understood until sometime in the 20th century, however.

The first instance of a skin treatment that may have been used for sun protection was discovered in an archeological dig in London, and was made from tallow mixed with tin oxide. Since today tin oxide and zinc oxides are still used for "physical block" sunscreens, it's not too much of stretch that some high-class lady was putting this on her face to prevent to prevent wrinkles and tans.

Also, when did humans first start to realize that overexposure to the sun was harmful?