Plants have a remarkable capacity to acclimate to their environment. Acclimation is enabled to a large degree by phenotypic plasticity, the extent of which confers a selective advantage, especially in natural habitats. Certain key events in evolution triggered adaptive bursts necessary to cope with drastic environmental changes. One such event was the colonization of land 400–500 million years ago. Compared with most aquatic habitats, fluctuations in abiotic parameters became more pronounced, generating significant selection pressure. To endure these harsh conditions, plants needed to adapt their physiology and morphology and to increase the range of phenotypic plasticity. In addition to drought stress and high light, high temperatures and fluctuations thereof were among the biggest challenges faced by terrestrial plants. Thermomorphogenesis research has emerged as a new sub-discipline of the plant sciences and aims to understand how plants acclimate to elevated ambient temperatures through changes in architecture. While we have begun to understand how angiosperms sense and respond to elevated ambient temperature, very little is known about thermomorphogenesis in plant lineages with less complex body plans. It is unclear when thermomorphogenesis initially evolved and how this depended on morphological complexity. In this review, we take an evolutionary–physiological perspective and generate hypotheses about the emergence of thermomorphogenesis.