Where did the phrase 'Thank you from the bottom of my heart' originate?

It goes back to the ancient greeks. The famouse philosipher, Archimedes, hypothesised that the brain chanelled the blood and that the heart did all the thinking and the feeling. Therefore if you wanted to say thank you in a very deep and meaningful way, you would say "thank you from the bottom of my heart", as that would be where all the most meaningful feelings were supposed to be. That's alos why we have hearts as a symbol of love today. Nobody bothered to change it, I guess.