Nice, but I believe the question was about the upper handguard, not about the lower forearm. If you're using the upper handguard as a place to grip your rifle while firing, you have an interesting stance indeed, my friend!
As to the upper handguard's use as a heat shield, it is pretty readily apparent from the time of its introduction. Early single-shot breechloading military rifles like the Martini, Springfield, Mauser Gew.71, and Mle.74 Gras do not have upper handguards, as the slow-firing black powder rifles rarely heated their barrel enough to matter except under the most unusual of circumstances. (Roarke's Drift, f'rinstance.)
It wasn't until the development of smaller-bore smokeless magazine rifles that the barrels began being enveloped to protect the rifleman's hand. The early designs, such as the Belgian Mausers and the German Commission Rifle, sometimes had tubular sheet steel barrel shrouds, but enveloping wooden handguards soon replaced those as being lighter and easier to produce.