I tried to do a research but couldn't find any info. I know humans started using fire widely approximately 125,000 years ago. But to be able to boil water, I guess, they needed the metals? Because it is unlikely that they gathered rocks with large empty volume inside.
And when they were able to boil water widely, what impacts it had on humanity? Cooking perhaps? Or when did they first notice, for example, it makes certain waters 'drinkable'? What are your thoughts?
Firstly, clay pottery and soft, shapable stone (the Viking-age Norse used soapstone) mean that metal was not required - which is fortunate, as we've only been working metal for c.8000 years.
Secondly, there is ample evidence for the use of heated rocks thrown into cisterns of water (of varying size and construction). This seems to be a comprehensive, solidly-sourced article.