I believe the general consensus is that it was a Greek invention (Aristophanes mentions pastries in The Clouds, so 5th Century BC at least), but the pastry was used to carry and preserve the contents, rather than being designed to be one edible article. The convention of eating the crust came far later during late medieval times in England where the poor would eat the undesirable (and practically inedible) crust. Americans "refined" the pie down to sweet pastries only for some odd reason (perhaps a lack of meat available meant filling pastry with fruit was more cost-efficient?).

Here is a long article that covers the history of the pie (not in a very well structured order, unfortunately) which uses sources such as;

• Oxford English Dictionary, Volume III, 1982
• Pie: A Global History, by Janet Clarkson
• The White House Cookbook by Mrs. F.L. Gillette, 1887

http://www.everythingpies.com/history-of-pie/

As a Brit, I have to say that everyone should eat a steak pie at least once. There's nothing better than hot steak pie, chips and gravy.