It is widely believed that the archaeal ancestor was hyperthermophilic, but during archaeal evolution, several lineages - including haloarchaea and their sister methanogens, the Thaumarchaeota, and the uncultured Marine Group II and Marine Group III Euryarchaeota (MGII/III) - independently adapted to lower temperatures. Recent phylogenomic studies suggest that the ancestors of these lineages were recipients of massive horizontal gene transfer from bacteria. Many of the acquired genes, which are often involved in metabolism and cell envelope biogenesis, were convergently acquired by distant mesophilic archaea. In this Opinion article, we explore the intriguing hypothesis that the import of these bacterial genes was crucial for the adaptation of archaea to mesophilic lifestyles.