In school, I've been taught that stories like Beowulf were possibly based on a historical figure that became embellished over the centuries and finally entered mythical status. Even the story of Achilles being dipped into the river Styx was a later embellishment to the Achilles legends.
In other words, what are the earliest examples of the Greek heroes before they became popular myth?
If this question is too broad, or has been asked before (i checked the wiki, I swear!), please delete this post and accept my apologies!
The Styx incident is a later embellishment on the Iliad - in Homer there would have been no point to the story if Achilles couldn't die.
As to whether there was a historical character analogous to Achilles, well, probably not. The text of the Iliad wasn't written down until about 400 years after the events it represents, and Achilles-like characters show up in both Irish and Farsi myths. He's probably representative of an Indo-European hero myth, and if anyone like him existed it would have been in a completely different context and probably unrecoverable.
There is a lot of archaeological evidence for pre-archaic hero cult in Greece. Ian Morris' books come to mind immediately, but any handbook or manual on archaic or dark age Greece will point you in the right direction.