Infancy in 1973? Backpacking has roots going way earlier, easily into the 1930s, at least in the Sierra Nevada. I think backpacking and trekking got a significant push from the mounds of army surplus post WWII, and the development of staples like the Kelty packframe and significantly light, warm down bags.

By the 1970s, companies like Patagonia, North Face, and REI were beginning to amass significant sales, although tiny compared to what they have become today.