Although I have collected a few rappelling and ascending devices, I have not done a serious academic study of the history of rappelling devices. What I present here is certainly incomplete and it has an American perspective, but perhaps still of some interest.
Before I start, keep in mind that I am primarily a caver and climbing is a distant second. Cavers use rappelling as a primary means of transportation to go down in a cave, while climbers use rappelling as an incidental aid to their main objective: going up. Cavers do moderate (150-500 foot) and long (500-1500 foot) rappels, with the occasional very long rappel (3000 feet and up) done on convenient cliffs for no reason at all. Climbers rarely do anything more than a short rappel of 150 or 200 feet. Cavers rappel in wet and muddy conditions in the dark, climbers prefer clean ropes on nice days in the sunlight - but it does not always work out that way. Cavers can justify carrying heavier gear than climbers can.
Canyoneers are another group. Their rappels are usually short (under 150 feet) and often wet. Then there are the industrial rappellers (window washers, etc.); much of our equipment does not meet OSHA specs, their equipment does and is often laughable from our perspective (e.g., its weight). The military commandos also rappel, using techniques appropriate when the concept of acceptable losses applies. Then there are people that call themselves “sport” rappellers, a concept that I do not understand as anything more than a way to waste time. To each his or her own, there is no one size fits all.
Now for the evolution of descenders. There are rappel devices that date back to the 1500s-1800s, but they never became popular and dropped out of production. I would guess that around the mid-1900s, the duflersitz or body rappel was the preferred means. Bill Cuddington introduced U.S. cavers to rappelling and prusiking in 1952. Until then, cavers here used cable ladders (we still do) or block & tackle.
Around 1955, Dean Abbott developed the idea of the rappel log - a wooden spool - and in 1956, Bob Geil made the first working prototype out of the limb of a catalpa tree. Locust soon replaced catalpa as the preferred log, although bowling pins also became popular sources for the wood. Dwight Deal, who documented the history of the rappel log, recently donated his (vintage 1957) to my collection - it will appear on my web site someday. Rappel logs were not suitable for nylon rope (which melted), but once “burned in,” work quite well on manila rope. Preston Haupt (1959) reported one instance where a caver soaked a log in linseed oil before rappelling into Catawba Murder Hole. The heat ignited the oiled log, which burned with a 3-inch flame. By the 1960s, rappel spools were metal; I patterned my first spool after Bill Cuddington's spool shown in Bill Halliday's Depths of the Earth (1966).
Not everyone upgraded to the rappel log. I do not know when, but by the time I was born, a carabiner on the seat sling had replaced the body rappel's loop under the thigh. Gaston Rébuffat recommended it in “Starlight and Storm” (1956),.and Dan Bloxsom & C. Link illustrate it in a 1956 Technicalia (Cumberland Grotto) article. All of this was in the 3/4” manila rope days, and the carabiner method was especially destructive to manila rope. The next step was probably to twist the rope around the carabiner; much like the military did when I took ROTC (and maybe still does). Remember, acceptable losses. The next obvious step was to use three carabiners (Bill Plummer, 1959) to spread the twist out; almost no one remembers how to rig this one (I do, it's handy)
Meanwhile, the European climbers invented a few things. Pierre Allain made his first prototype in 1947 and started selling his second a few years later. The earliest American reference to this descender I have found is in the 1964 Windy City Speleonews, and that shows the later of the two versions in my collection. Trevor Peck developed a descender as well. These are quite rare, and it took me decades to find one.
The figure eight was invented by Dr. Max Pfrimmer on October 1, 1943. Pfrimmer offered his idea to Schuster's sports equipment store in Munich, which commercialized their eight in the early 1960s. The next commercial figure eight that I know of was the Fisher eight advertised in the June 1963 issue of Mountain Craft. Alan Blackshaw's book Mountaineering, from Hill Walking to Alpine Climbing (1965) shows a later-model Fisher 8, a second-generation Pierre Allain trident, and a Peck Descender. I also have a noncommercial figure eight developed independently in the USA by Gary Kirk in 1965.
The earliest ads for commercial figure eights in the USA that I have seen are from the early 1970s (CMI ads appeared in 1974), but Bill Craig reported seeing an eight (probably Schusters') for sale in the Adirondacks in 1965. By 1968, Keith Liken wrote that eights had been “used for years.” He also created a new eight design because existing eights were too difficult to girth hitch. His idea never caught on.
I am not sure when people invented the brake bar, or when they figured out that they could use a carabiner as a brake bar, but the ideas are old. I have a letter from Warren Lewis saying that he learned of it from the Denver Mountain Club in 1933. Other brake-bar-like ideas have been around a while, e.g., Sam Geffner's 1961 idea. Dick Sanford did a detailed mathematical analysis of the single brake bar method in 1964, and Bill Plummer described the double brake bar method in the same year. I learned to use them at the Philmont Boy Scout ranch in 1971.
In 1966, John Cole invented the rappel rack (as did Warren Lewis, working independently). The standard open-frame rack is, from some perspectives (e.g., all-around versatility) still the best rappelling device on the market. The first rappel racks used commercial (Gerry) brake bars. Most people do not know how to use a rack properly, and my observation is that most people who do not like it for reasons other than size, weight, or cost fall into that group.
The bobbin is an old idea as well. As early as 1882, Davis patented a Fire Escape where the rope path followed the typical bobbin “S.” The first modern U.S. reference to bobbins that I know of was in the February 1968 NSS News.
Before 1956, people suffered with body rappels. In the next decade, many new ideas appeared. Since 1966, there has been no real progress in rappelling. Small improvements, yes, and introduction of better materials and minor variations, yes, but nothing to match the jumps made in the decade mentioned. Similar statements apply to ascending systems, but the end of revolutionary development was somewhat later.