Emeritus Professor John Speth is quoted in this NPR article about the origins of soup:

"The power of the expanding steam cracks the rocks, a distinct characteristic that first shows up in the archaeological record around 25,000 years ago in Western Europe, says archaeologist John Speth, an emeritus professor of anthropology at the University of Michigan in Ann Arbor."

Stone Age Stew? Soup Making May Be Older Than We'd Thought: http://www.npr.org/blogs/thesalt/2013/02/06/171104410/stone-age-stew-soup-making-may-be-older-than-wed-thought