I heard it evolved from Bertrand Russell, who used a notation such as \hat{x}.(x+1) to mean similar in Principia Mathematica. Church and maybe others in the logic community originally used the same notation but Church's publisher, not having latex, was unable to print it. So they wanted to move the hat in front of the letter, such as ^x.(x+1). The hat symbol either wasn't available or the instruction got mangled during production into the similar-looking lambda.