I am old enough to remember when just about all bike frames were made of steel tubes.
Actually, that statement is a bit misleading. Long before I started cycling--or was even born--bikes were being made of all sorts of materials. Aluminum bicycles may have become popular during the 1980's (largely due to Cannondale) and titanium during the '90's, but bicycles were built from those materials--and others-- nearly a century earlier. Even carbon fiber appeared on the scene further back in cycling history than most people realize.
The problem with aluminum, titanium and carbon fiber was that until about 25 years ago, nobody (in the bike industry, anyway) knew how to use them. So frame tubes of those materials were of the wrong diameters and were overheated or otherwise improperly joined. That is why the Lu-Mi-Num bicycle of the 1890's, and the Speedwell and Exxon/Grafton frames of the 1970's, are displayed in museum halls and on collectors' walls but are not ridden on the roads.
There also have been bicycles made from bamboo and other woods, and a myriad of other materials. Now, a Brazilian artist/inventor is making bicycle frames from recycled materials, including shampoo and beverage bottles.
And then there is this bike:
After the cops took down their barricades, this guy rode off into the sunset, I'm sure.