The challenging part of researching these early pies is most of us rely on translators of original texts.
These can vary according to scholarly proficiency and educated interpretation.
A piecrust used less flour than bread and did not require anything as complicated as a brick oven for baking.
More important, though, was how pies could stretch even the most meager provisions into sustaining a few more hungry mouths...
Cooking methods (baked or fried in ancient hearths, portable colonial/pioneer Dutch ovens, modern ovens), pastry composition (flat bread, flour/fat/water crusts, puff paste, milles feuilles), and cultural preference (pita, pizza, quiche, shepherd's, lemon meringue, classic apple, chocolate pudding).
 Experimenting with this formula, we have adhered to the instructions as closely as possible, using regular pie dough to envelope the parboiled meat.195) ---Around the Roman Table: Food and Feasting in Ancient Rome, Patrick Faas [Palgrave Mac Millan: New York] 2003 ? Bakers who specialized in this treat were known as the episat mersi, so mersu-making was probably an involved and respected process." ---Cooking in Ancient Civilizations, Cathy K. 254) American pies "As a favored dish of the English, pies were baked in America as soon as the early settlers set up housekeeping on dry land.Beyond mere preference, howevers, there was a practical reason for making pies, especially in the harsh and primitive conditions endured by the first colonists.254) "If the basic concept of 'a pie' is taken to mean a mixture of ingredients encased and cooked in pastry, then proto-pies were made in the classical world and pies certainly figured in early Arab cookery.But those were flat affairs, since olive oil was used as the fat in the pastry and will not produce upstanding pies; pastry made with olive oil is 'weak' and readily slumps." ---Oxford Companion to Food, Alan Davidson, 2nd edition, Tom Jaine editor [Oxford University Press: Oxford] 2006 (p.
The derivation of the word may be from magpie, shortened to pie.