12:30 a.m. on April 2, 2009 (EDT)
"Oh, no, that's all wrong--here's the real story. Blah, blah, blah...."
LOL! Pretty close, though, Perry.
Two posts have said that external frame packs are heavier than internal. Yet my Kelty Backpacker (bought in 1960 from Dick Kelty in his garage when he was selling the packs and other gear directly) and Barb's father's Kelty Mountaineer (bought about the same time, now in our possession) are about 2.5 pounds (the top bar only added a few ounces, not the half pound mentioned above). The lightest internal frame pack in our household (the count is close to 20 packs) is a Kelty Cloud at 3 pounds with its pockets and stays, with the Osprey Aether 60 at 4.5 pounds. Yes, my Kelty Sherpa (huge expedition-capacity external frame) is 7.5 pounds, on a par with my Dana Terraplane at 8.5 pounds (huge internal frame expedition-capacity pack).
The big advantages of an external frame (yes, I know, I have posted this same babble a half-dozen or more times before on Trailspace) are weight for capacity (some more recent versions with extra straps, pockets, etc are closer to internals), coolness, capacity (you can strap all sorts of stuff to the top, bottom, and outside), ability to carry awkwardly shaped and sharp-cornered objects without poking yourself (reason the re-supply people for the AMC huts use them, and ability to arrange weird loads to balance properly. The imbalance Perry mentions can be dealt with by properly arranging "weight and balance" (to use the aeronautical process that every beginning pilot learns). I have carried far heavier loads comfortably with an external than with any internal (and I have some of the best of each type). The advantages of an external show best on reasonably good trails. The disadvantages are rigidity (especially when accidentally hooking a branch or travelling off-trail - most externals are wider than most internals), and awkwardness when climbing technical stuff or skiing or generally travelling off-trail.
The big advantages of internals are conforming to your shape, and narrowness. The big disadvantages are the aforementioned heat (they can be intolerable on hot humid days), lower capacity for the same weight of pack, more difficulty in arranging loads (in part due to the narrower configuration), and huge discomfort when carrying rigid or awkwardly shaped objects (just try to arrange a large watermellon or a wooden box of re-supply for a hut comfortably in an internal). Tying stuff on the outside of an internal often results in losing stuff along the trail, much more frequently than with an external, though both are about equal when fastening something to the top. The internal works significantly better when off trail, skiing, or doing technical climbing. They aren't as comfortable for long days on good trails.
As for history, actually both types have been around for centuries (as have soft packs which are definitely not internals - they have no frame at all). The Trapper Nelson packboard and old traditional New England pack basket are ancient examples of external frame packs. The WWII vintage Bergans ski pack is an example of a hybrid internal/external (Bergans still makes excellent packs, though they are not often seen in this country - their designs are as modern as anyone else's and their workmanship excellent). Barb and I have a couple of internal frame packs we got in the mid-1960s which were well-designed for ski touring - much lighter than the Bergans Ski Pack (especially the US Army surplus variety).
Bottom line is that each style of pack has its proper place in the universe of carrying loads. None are inherently superior than any other as a general pack that does everything. As Perry noted, when backpacking became a big sport in the postWWII era, externals were popular for their comfortable load-carrying capability. Somewhere in the 1970s, the internal frame packs became more popular, in large part because the lighter gear made for lighter pack weights, which internals work quite well with. But both internals and externals grew heavy (empty weight) because pack manufacturers insisted in answering the calls for 50-way adjustments, hundreds of pockets and compartments, and a few thousand extra straps, daisy chains, gear loops and other fastening points, along with a semiinfinite range of detachable pockets, daypacks, and fanny packs (all requiring a 200 page manual to explain everything. Ok I exaggerate a bit, but why do 90% of backpackers need 2 ice ax loops, especially for hiking in the Smokies, summertime 'Dacks, Ozarks, etc? A lot of backpackers took to cutting off the extra straps and pockets (amazing how much weight that takes off).