Although O.C. Marsh and Charles Darwin lived on separate continents, their discoveries, which would forever shape the field of modern science, intersected in a most unlikely place: the American West.

Since the founding of the United States, the West had offered unlimited opportunities for those who dared explore it. By the second half of the 19th century, the completion of the transcontinental railroad had made the West more accessible to everyday Americans. For E.D. Cope and O.C. Marsh, acclaimed paleontologists searching to discover new dinosaur bones, the West was virgin territory, rich with fossils yet to be uncovered. The region's arid climate preserved bones buried in the earth, and its diverse geological formations dated back hundreds of millions of years. This "new and strange" landscape, as Marsh described it, would become critical to the field of paleontology.

Charles Darwin was an English naturalist who published his own theory on evolution by natural selection. Born on February 12, 1809, Darwin studied natural sciences at Cambridge before taking his famous voyage on the HMS Beagle to South America. While aboard, Darwin wrote extensively about the diverse wildlife and geology he encountered on his voyage; when published in 1859, On the Origin of Species established Darwin as a controversial but widely read author. His theory of natural selection argued that certain traits became more or less common in a population due to their effects on survival and reproduction. Though backed up by hundreds of observations from his travels, Darwin's theories were hotly debated in the scientific community and among the general public.

On the Origin of Species was published during Marsh's senior year at Yale University, and Darwin was famous by the time Marsh began unearthing his first fossils. By 1874, Marsh had discovered the bones of 13 extinct species — ancient versions of camels, pigs, and turtles. 

Most importantly, in one of his earliest discoveries, in the mounds of Antelope Station, Nebraska in 1868, Marsh extricated bones of a tiny horse, dubbed Equus parvulus. He described the animal as "scarcely a yard in height, each of his slender legs was terminated by three toes."

According to Marsh, the tiny horse was a predecessor of the modern horse — its three toes an earlier model of the hoof. For years, the fossil record had too many gaps, too many targets for critics, to produce a convincing argument for evolution. But with the discovery of Equus parvulus and other fossils, including a bird with teeth, there was sufficient evidence to support Darwin's theory of evolution. After learning of Marsh's discoveries, evolutionary theorist Henry Huxley visited New York and gave a series of lectures defending Darwin's theories using Marsh's skeletons.

Scientific debate about evolution raged throughout the final years of the 19th century, including commentary from Marsh's competitor E.D. Cope. A staunch neo-Lamarckian, Cope believed that adaptation was passed on from generation to generation in a more gradual design, a theory in keeping with conventional religious beliefs. Many other scientists, including Harvard University's Louis Agassiz, concurred with Cope's view that evolution was not "multifarious or promiscuous, but in definite directions." Ultimately, however, Marsh's extensive collection of ancient horse bones proved too convincing; he filled every gap in the fossil record and in doing so, validated Darwin's theory.