Since antiquity, human civilisation has invented various vehicles for going to other places faster than by running or walking. The motivation behind such developments has been a combination of necessity and the entertainment value of moving around.
The bicycle can be defined as a two-wheeled contraption for a human to travel on without any other assistance save that of gravity and muscle power. The earliest forms simply allowed the proto-cyclist to sit on something and run with it, occasionally lifting their legs off the ground to coast on its wheels.
Through the improvement of this vehicle, the aptly named boneshaker, the early cyclist would still suffer a bumpy ride on the crude roads and paths which were common before the introduction of the motorised vehicle. The invention of the rubber tyre and the tarmacadam road, and refinements such as pedals, gearing and brakes, made the early 20th Century bicycle a more or less perfect machine for propelling oneself along a smooth road.
The war in Europe produced further innovation. The bicycle, by this time in common use by the military, became less useful to soldiers when ridden across the typical terrain of a battlefield. Also any nice smooth roads, if they had existed, would probably be roughed up by recent application of high explosives. To counter this, an Italian gentleman invented a bicycle with suspension to ease the bumps and make it easier to navigate rough terrain.
However, the developments continuing through most of the 20th Century refined the bicycle for the road. The bicycle was made lighter and more aerodynamic. Improved gearing and drive mechanisms were added, along with better brakes, to allow the cyclist to negotiate all variety of road inclines, both for necessity and pleasure. This development was encouraged by the popularisation of bicycling as a professional sport, introducing such activities as Le Tour de France - where cyclists go around France as fast as possible; or pursuit cycling - pedalling around a track indoors in a big circle as fast as possible.
But still the bicycle relied upon the existence of smooth roads. The sleek, efficient, but frail, road bike simply could not stand the strain of off-road cycling for long.
In Europe many cyclists enjoyed racing their bikes as much as they could in off-road cyclocross competitions, using a similar machine to the road racer. In America, shortly following the Age of Aquarius, some were finding ways to continue subjugating nature by laying down more roads on the land, while a few folks were having fun riding their bikes in the hills of California. To improve the lifetime of a bicycle off the road, these creative enthusiasts introduced stiffer frames, fatter tyres and, to allow the rider to get anywhere, very low gearing. Soon the mountain bike was born.
During the 1980s many cyclists came to realise how much fun it is to propel yourself through tricky natural terrain, free from cars and pollution. Recreational and competitive off-road cycling became very popular.
Many people find speed exciting. Adding the challenge of moving fast over varying rocky, muddy and steep terrain makes going downhill fast tricky. To improve control and to make the ride more comfortable, suspension for the front wheel was reinvented. Soon, in the pursuit of speed, more suspension was added to both wheels, allowing the cyclist to hurtle faster down the side of a mountain. To improve control, better brakes were added. The cyclist - in real danger of getting seriously injured - would now wear body armour.
All of this added significant weight - creating the downhill specific bicycle - that is, a bike that is too heavy to do much else except hurtle down a mountain. Some other form of transport would be necessary to take the cyclist up the mountain in the first place.
As a side-industry, this sport introduced improved protective armour to reduce the likelihood of bones snapping or injury to the nervous system, and also creative new insurance policies. Improvement in the technology has allowed cyclists to negotiate the most difficult terrain, even on snow and hurtling down an ice covered mountainside.