The dromedary camel, commonly known as the one-humped camel, is one of the species in the genus Camelus , which also contains the Bactrian, two-humped camel. Camels are in the family Camelidae. Dromedaries account for approximately 90% of all the camels in the modern world (http://news.softpedia.com/news/10-Amazing-Facts-About-Camels-68843.shtml).
Scientists have traced the origins of the modern camel to the Protylopus a genus containing several closely related species, which would have been found about 50 million years ago during the Eocene period in North America, perhaps around the then heavily forested area that is modern day South Dakota (Origins). Protylopus was a very small animal, about the size of a modern rabbit. They had no hump, were no more than two feet tall, about two feet long, and weighed less than 50 pounds (http://animaladay.blogspot.com/2012/06/protylopus.html). This animal had a simple arrangement of low-crowned teeth along the jaw. The tooth and jaw structure of Protylopus indicates that it’s diet consisted of soft, leafy vegetation. The front limbs were shorter than the back limbs, and each limb contained four toes. This early ancestor of camels seemed to have narrow hooves and not broad pads like the modern camel. It is likely that Protylopus had a tendency to stand on their hind legs to feed on vegetation. This is clear given that their front legs are shorter and that they carried their weight on their 3rd and 4th toes (http://animaladay.blogspot.com/2012/06/protylopus.html). Although Protylopus does not appear to be a direct ancestor of the modern camel, it does most likely resemble the early ancestors of the modern camel (http://www.sciences360.com/index.php/extinct-species-protylopus-camel-224/).

As the dense woodlands that was once the habitat of Protylopus began to open up and become a dry savanna, it evolved into a creature that more closely resembled the modern camel (Sciences 360). This animal was the next step in the evolution of the Camelidae family, and was known as the Poebrotherium. This early camelid lived around 35 million years ago during the Oligocene epoch, and was found in the plains region of North America (http://news.softpedia.com/news/10-Amazing-Facts-About-Camels-68843.shtml). The Poebrotherium was larger than the Protylopus, standing about three feet tall and being sized similar to a modern goat. Poebrotherium resembled a modern day llama, with a narrow snout and long neck. Their front teeth were angled forward for the purpose of shearing vegetation (https://www.dmr.nd.gov/ndfossil/poster/PDF/Poebrotherium.pdf). The leg and toe structure of the Poebrotherium had evolved in a way that gave it greater speed than Protylopus, with it’s elongated hind legs, hoofed toes, and more slender overall build. The hoofed toes now bore weight centrally, and were beginning to develop into two separate hoofs. Poebrotherium was likely the ancestor of several different camelids. (Sciences 360).

After Poebrotherium, camelids evolved into Procamelus, which lived through the Oligocene to the Miocene epoch from about 20 to 5 million years ago (http://fossilworks.org/bridge.pl?action=taxonInfo&taxon_no=42549&is_real_user=1.). Similar to Probrotherium, it had long legs built for agility, and would have stood at about 4 feet tall. Procamelus had a small pair of incisor teeth on the upper jaw, and large teeth that would have been easily able to eat tough vegetation (http://en.wikipedia.org/wiki/Procamelus). Procamelus evolved into Camelus, the genus that the modern camel belongs to (Softpedia). However, another genus existing during the Pliocene through the Pleistocene was the Paracamelus, which is believed to be another ancestor of the modern camel (http://en.wikipedia.org/wiki/Paracamelus).
It appears that camelids, likely Procamelus or Paracamelus at this point, migrated out of North America some time during the Pliocene epoch, with some moving across the Bering Strait into Asia, and others moving into South America to eventually become the modern domesticated llama and alpaca (Origins). The ancestors of the modern camel appear to be those camelids that traveled through the arctic regions into Asia. A fossil recently discovered on Ellesmere Island in the Arctic region of Canada may explain the evolutionary process from the early Poebrotherium to the modern camel. The fossil indicates that there was a creature that was closely related to Paracamelus that lived around 5 million years ago during the middle of the Pliocene epoch, a time at which it is believed early camelids were migrating out of North America into Asia. The fossil seems to indicate that this camelid was about twice the size of a modern dromedary camel, and it’s location and time period may explain some of the adaptations found in modern camels. Dr. Natalia Rybczynski, a paleontologist with the Canadian Museum of Nature, has speculated that some of these adaptations including “their wide flat feet, large eyes, and humps for fat” (http://www.wired.co.uk/news/archive/2013-03/06/giant-arctic-camel), may have developed given the animal’s need to survive in an arctic climate.
These recent findings shed light on the evolution of the Procamelus into the modern Camelus. It is generally believed that the modern one-humped dromedary evolved from the two-humped bactrian species (Camelus bactrianus), which likely developed the way it did as a result of the arctic climate, with its thick coat that can withstand temperature of -20 degrees Fahrenheit and that falls away when temperatures reach 100 degrees Fahrenheit (http://animals.nationalgeographic.com/animals/mammals/bactrian-camel/). Around 2 million years ago, the bactrian, looking very similar to the modern bactrain, standing about 7 feet tall and weighing an average of 1,800 pounds (Nat Geo), had evolved from the Procamelus or perhap the Paracamelus camelids that had traveled across the Bering Strait into the arctic regions of Asia (http://en.wikipedia.org/wiki/Bactrian_camel#Evolutionary_history). The bactrian is thought to have evolved into the dromedary because modern dromedary camels foetus,during prenatal development, have two humps, and also a vestigial hind hump in adult dromedaries. Given this evidence, it has been speculated (Williamson and Payne 1978) that modern dromedaries evolved in the hotter, drier climates in western Asia (Origins).
During a period about 10,000 to 12,000 years ago in which most megafuna was wiped out in North America, the original camelid, Prolypotus, became extinct. However, evolution had already taken it’s course, and around 6,000 to 3,500 years ago began the first domestication of dromedary camels (Softpedia).