The Earliest Crash Test Dummies Were Cadavers

Injuries are the most common cause of death if you're under 44, and about one-fifth are connected to car accidents.

Over the past few decades, the annual death rate in America has declined dramatically, but the leading causes of death have remained the same. Heart disease kills the most people, and then cancer. Strokes kill a smaller portion of us, but they're often up there, too. After that, often, it's accidents that kill people—especially younger people. According to the Centers for Disease Control and Prevention, "injuries are the leading cause of death for people ages 1-44." And the largest single cause of those injuries are car accidents.
But, really, it's incredible that more people don't die in motor vehicles (which are, essentially, giant chunks of metal hurtling through space, on the enormous strength of combustion). In 2010, the death rate from motor vehicle-related injuries was about two-fifths of what it was in the 1970s, when it peaked. In other words, safety standards worked. But making them work required a very specialized piece of technology—the "anthropomorphic test device," more popularly known as the crash-test dummy.
The first of these were designed for America's aerospace program, which wanted to test jet-ejection seats. The car industry, meanwhile, was testing its first safety devices on cadavers, which, as The New York Times later wrote—quite delicately!—"lacked the durability required for repeated trials." There were other problems with this method, besides the obvious ones: Cadavers do not come in standard shapes, and in crashes they don't behave like the bodies of living people. Pigs were also early test subjects.
Alderson Research Laboratories was one of the first companies to make dummies. Popular Science visited in 1956:

The skeleton, made of rugged steel bars and plates welded together, is covered with molded vinyl plastisols which simulate the soft tissues of human flesh. The standard model…can duplicate all the major motions of man. He stands six feet one inch high, weighs 200 pounds and comes dressed in blue cotton coveralls and heavy-duty oxford shoes. His skull is a large aluminum casting, and he can be instrumented with an "intelligence" consisting of accelerometer, rate gyro, telemetering equipment, recorders, and pressure devices.

In 1966, ARL created the VIP, the first dummy designed specifically to test cars. He weighed just 170 pounds, with an updated pelvis and elbows. He could sit in the car just like a driver. But, as BBC4 explains, he lacked "repeatability"—he wouldn't behave consistently from crash to crash, which meant manufacturers couldn't be sure of their results. The design, though, was part of the Hybrid dummy that GM designed at the beginning of the 1970s—and which, in various iterations, became the standard crash test dummy for decades to come, as cars became, well, not safe, but safer.