On April 9, 1959, NASA introduced the Mercury Seven, the first American astronauts. After exhaustive tests that some called "torture," the seven were selected from a pool of more than 500. The criteria for selection were extremely specific, including everything from educational achievement to the ability to withstand sitting in a 130-degree chamber for hours. They were all military test pilots, and they stood 5'11" or shorter, because NASA intended to build small spacecraft.
The Mercury Seven in pressure suits, circa 1960. Back: Alan Shepard, Gus Grissom, Gordo Cooper. Front: Wally Schirra, Deke Slayton, John Glenn, Scott Carpenter. Note the shoes. Slayton and Glenn are wearing boots that have been spray-painted. Photo courtesy of NASA (Public Domain)
At their first press conference, the astronauts wore civilian clothes, despite all holding military ranks. They seemed like normal guys, and NASA was somewhat dismayed when the men turned into overnight celebrities. (See The Right Stuff for an account of their, ahem, adventures.) The Mercury Seven were: Scott Carpenter, L. Gordon "Gordo" Cooper Jr., John H. Glenn Jr., Virgil "Gus" Grissom, Walter "Wally" Schirra Jr., Alan Shepard Jr., and Donald "Deke" Slayton.
All seven of the men ended up serving on missions, though John Glenn was clearly the most famous, as the first American in orbit and the sole Marine in the seven. He was also the longest-lived member of the group, passing away on December 8, 2016. (Gus Grissom died tragically in the Apollo 1 fire on January 27, 1967.)
Here's the press conference featuring the seven, dressed in snappy suits. Note the ashtrays.
In this last bit of the conference, note John Glenn's answer at 4:15. He was asked what the toughest part of the testing was. Sample quote: "We had some pretty good tests... If you figure how many openings there are on the human body and how far you can go into any one of 'em.... Now you answer which one would be the toughest for you, and [shrugs]."