Meanings of heart?

Question

why and when did all those sayings start that heart is not just a organ and people use that for love and that it is the most important thing in life

Answer 1

you answered the question yourself. you could not live without your heart and we could not live without love

Answer 2

in the East it was common for ritual suicides to be performed by slitting ones stomach open, for them the belly had the relatively same meaning as we give the heart.