Thermodynamics, if we mean anything resembling the modern science, is a relatively recent science. It's a child of the early industrial revolution, with influences from early modern chemistry. From those beginnings in the 17th century, we have modern thermodynamics largely growing in the 19th century.

What came between the 17th and the 19th century? In this time, thermodynamics was dominated by caloric theory (and similar), wherein "heat" is proposed to be a conserved fluid quantity that can be transferred from one body to another. Notably, wanting to go from hotter to colder bodies, and therefore a fluid that repelled itself. This explained (but incorrectly) a great deal about combustion and the change in energy during chemical reactions (thermochemistry), and pushed aside older ideas about heat being related to microscopic movement or vibration. Thus, while thermochemistry was successful (and even more so in the 19th century once thermodynamics was better understood), and caloric theory could predict useful things about the performance of heat engines (Sadi Carnot developed his ideal heat engine cycle, the Carnot cycle, using caloric theory), in terms of fundamental theory, caloric was a dead end.

As noted above, connections between heat and microscopic motion predated caloric. Robert Hooke wrote in Micrographia that "Heat being nothing else but a very brisk and vehement agitation of the parts of a body" (Hooke's italics) in his explanation of the differences between solids and fluids. The key worker using this idea at the time was Robert Boyle, in his study of the relationship between the pressure and volume of gasses. Boyle through that since friction (resulting from motion) produced heat, that heat and motion were related. He worked with Hooke and Denis Papin (most famous for inventing the pressure cooker and steam engine, but had worked with Christian Huygens in Huygens' attempts to build an internal combustion engine). The scientific success of caloric largely pushed ideas such as this aside, but the relationship between friction and heat kept the idea alive.

Papin's machine - the steam engine - drove a lot of development in thermodynamics (as already noted, much using caloric theory). Mechanical energy - potential and kinetic - was understood, and the relationship with heat was clarified. A major missing key was entropy. The key scientist who led us to our modern understanding of entropy in (macroscopic) classical thermodynamics was Rudolf Clausius, who did so in a series of steps in a little over a decade, from 1854 to 1865, from recognising the importance of dQ/T to the Third Law of Thermodynamics. From the overthrow of caloric to this point (entropy), the development of classical thermodynamics was rapid and fairly linear.

But at the same time that the concept of macroscopic entropy was entering science, the idea of heat as microscopic motion made a strong comeback - this began with the kinetic theory of gasses, followed by the extension of kinetic theory to other substances, and statistical thermodynamics. Significant early work was carried out by some scientists who made major contributions to both macroscopic thermodynamics and statistical mechanics, notably Clausius and Maxwell, and by others who are famed for their contributions to statistical mechanics. A notable late part of this work was by Stefan, Boltzmann, and Wien (leading to a Nobel Prize for Wien in 1911 (by which time Stefan and Boltzmann were dead)) on the role of radiation - the laws describing blackbody radiation and its interaction with matter.

This work on blackbody radiation led to a major revolution in physics: quantum mechanics, which has increasingly led to further development in thermodynamics in the 20th and 21st centuries.

There are plenty of fun questions left to answer in thermodynamics, even without getting into quantum thermodynamics and modern condensed matter physics (or relativistic thermodynamics), such as the role of entropy in non-equilibrium systems and few-particle systems.

Further reading:

On caloric vs mechanical theories of heat, and the development of kinetic theory and statistical mechanics: M. S. Longair, Theoretical Concepts in Physics: An Alternative View of Theoretical Reasoning in Physics, Cambridge University Press, 2003. Warning: while this book uses the history of physics to illustrate theoretical reasoning in physics, it isn't a history book. It's a physics book.