Simplify the following text for a Grade 7 native English-speaking person.
Original: In 1929 , numbers were first added on the backs of baseball uniforms by the New York Yankees and the Cleveland Indians baseball teams .
Simplified: ( Okkonen , p. 36 , p. 120 ) In 1929 , numbers were first added on the backs of uniforms by the New York Yankees and the Cleveland Indians . By 1932 , all major league baseball teams had numbers on their players ' uniforms .