Where did the earliest forms of life come from?

The Descent of Man by Charles Darwin one a landmark work in what became the science of evolutionary biology. In this work, Darwin noticed the correspondences among various species. He also was aware of how selective breeding worked. Farmers had for millennia been breeding animals for desired traits to develop horses that ran faster or could haul heavier weights, dogs that were efficient hunting partners, or food animals that were hardy, placid, and able to efficiently convert fodder to meat. By combining these types of knowledge with his encounters with primitive peoples, he argued that humans, and in fact, all animals, were the product of evolution.
The key mechanisms he used to explain evolution were random mutation and natural selection. Random mutation is something we can all observe when we see, for example, polydactyl cats or other such minor variations in species. Natural selection ensures that if a mutation helps the species survive, it will propagate through the species, eventually creating a distinct subspecies or even a new species. Thus Darwin argues that humans evolved from non-human primates through precisely this mechanism of random mutation and natural selection.
The very earliest forms of life, of course, were not human but were unicellular organisms that formed nearly four billion years ago, probably from chemical reactions.