When NASA started recruiting for its first astronaut class in 1959, it had no idea what it was looking for because it didn't know what was going to happen to the human body in space. Basically the agency needed people used to flying dangerous missions in untested vehicles while giving real-time feedback to engineers... which pretty much narrowed the pool to test pilots.
President Eisenhower stipulated that due to the potential top-secret nature of this endeavour, they should be in the military. And this led to the basic qualifications for the first batch of astronauts. Military pilots who'd graduated from a test pilot school holding a bachelor's of engineering with at least 1,500 hours flying jets under their belts. This was basically the way things went until 1964 when NASA shifted to reward academic merits, looking at PhDs instead of flight time.
NASA's needs were changing. With long-duration lunar missions and even longer space station flights in the works, the agency was starting to need real scientists in space. During the Apollo-era, untested and new was the name game and the crews were just three men so each astronaut had to do everything -- be a test pilot, an engineer, a scientist... But with the pending beginning of the shuttle program, crews were larger, missions were longer, and the technology was designed to be reused so it would become familiar over time.
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