Trained as a lawyer and medical doctor, Hutton found himself attracted to the nascent science of geology. While working as a "gentleman farmer" in Berwickshire during his thirties and forties, he hit on a variety of ideas to explain the rock formations he saw around him. Moving to Edinburgh, then in the throes of the Scottish Enlightenment, he fell in with several first-class minds in the sciences including John Playfair and Joseph Black.
His new theories placed him into opposition with the then-popular Neptunist theories of Abraham Werner, that all rocks had precipitated out of a single enormous flood. He noted, for example, that many layers of sedimentary rocks butted up against other layers at unusual angles, which suggested that one had been layed down, then tilted, then another layer deposited. He proposed that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood-oriented theory.
As well as combatting the Neptunists, he also opened up the concept of deep time for scientific purposes, in opposition to Catastrophism. Rather than accepting that the earth was no more than a few thousand years old, he maintained that the Earth must be much older (indeed he went rather overboard and asserted that the Earth was infinitely old). His main line of argument was that the tremendous displacements and changes he was seeing did not happen in a short period of time by means of catastrophe, but that the processes happening on the Earth in the present day had caused them. As these processes were very gradual, the Earth needed to be ancient, in order to allow time for the changes. Before long, scientific inquiries provoked by his claims had pushed back the age of the earth into the millions of years -- still too short when compared with what is known in the 21st century, but a distinct improvement. Hutton also advocated uniformitarianism for living creatures too -- evolution, in a sense --and even suggested natural selection as a possible mechanism affecting them:
"...if an organised body is not in the situation and circumstances best adapted to its sustenance and propagation, then, in conceiving an indefinite variety among the individuals of that species, we must be assured, that, on the one hand, those which depart most from the best adapted constitution, will be the most liable to perish, while, on the other hand, those organised bodies, which most approach to the best constitution for the present circumstances, will be best adapted to continue, in preserving themselves and multiplying the individuals of their race." -- The Theory of the Earth, volume 2
Alas, Hutton's theory was published in his The Theory of the Earth, a dense and borderline unreadable work that essentially prevented anyone from following up on this thought. It remained for Charles Darwin, influenced by Hutton disciple Sir Charles Lyell to bring the idea to the forefront of public consciousness, propose the process as the origin of species, and provide the voluminous evidence necessary to win over the scientific community to the theory.
The prose of The Theory of the Earth was so obscure, in fact, that it also impeded the acceptance of Hutton's geological theories. Restatements of his ideas by John Playfair in 1802 and then Lyell in the 1830s removed this hinderance. If anything, Hutton's ideas were eventually accepted too well. At least some of the initial resistance to modern scientific ideas like plate tectonics and asteroid strikes causing mass extinctions can be attributed to too-strict adherence to uniformitarianism.