Multicellular? Most organisms on earth are unicellular[1] even now. My impression is that multicellular organisms evolved because the 'easy' evolutionary niches were already occupied by single-celled species, and it took new tricks to compete for new niches.
It's not known exactly how multicellular life came to be. It's happened only once in the history of the earth. Nuclear dna is from Archaea and Mitochondrial dna is from bateria. Lynn Margulis was the first to propose and substantiate symbiogenesis for this.
You need more than heat for life to evolve. You also need a source of useful energy. Probably a long-lasting chemical gradient like you see in undersea vents but there are a number of possibilities. Life has evolved on Earth to use other sources of energy too, most commonly sunlight, but the machinery to use those is much more complex.
1:https://www.quora.com/What-is-the-biomass-of-bacteria-What-i...