instruction: From the text below about the history of peanut butter, where can the first reference be traced from?
context: The earliest references to peanut butter can be traced to Aztec and Inca civilizations, who ground roasted peanuts into a paste.[clarification needed] However, several people can be credited with the invention of modern peanut butter and the processes involved in making it.[according to whom?]
  
  The US National Peanut Board credits three modern inventors with the earliest patents related to the production of modern peanut butter. Marcellus Gilmore Edson of Montreal, Quebec, Canada, obtained the first patent for a method of producing peanut butter from roasted peanuts using heated surfaces in 1884. Edson's cooled product had "a consistency like that of butter, lard, or ointment" according to his patent application which described a process of milling roasted peanuts until the peanuts reached "a fluid or semi-fluid state". He mixed sugar into the paste to harden its consistency.[citation needed]
response: The earliest reference to peanut butter can be traced to Aztec and Inca civilizations.