It is easy to think of camels and their close relatives as highly specialised animals that can only exist in the most extreme environments, but the camels that survive in the modern world are mere remnants of a formerly widespread and diverse group. They first evolved in the Late Eocene, about 40 million years ago, not in Asia or Africa but in North America, and reached their peak in the Late Miocene some 10 million years ago. It was not until five million years ago that camels migrated into Eurasia and Africa. At around the same time, the ancestors of modern llamas made their first appearance in South America.
By the beginning of the Pleistocene epoch, otherwise known as the Ice Ages some two million years ago, the camelids ranged all over North America, from Florida to Alaska. They had successfully spread southward to South America across the Panama land bridge and eastward into Asia across the Bering land bridge.
Like some other animal groups that evolved chiefly in North America, such as the horses, the camelids survive today only on other continents; they became extinct in North America at the hands of the first human settlers toward the end of Pleistocene times, some 12,000 years ago.
Today, there are two living species of camel, the two-humped Bactrian camel and the one-humped Arabian camel or dromedary. Wild camels still inhabit the Gobi Desert, but the camels of Africa were introduced there by humans as recently as 2,500 years ago. Camels that are now feral in Australia were brought there just over 100 years ago. In South America, the llama, alpaca, vicuna and guanaco are the only representatives of the second remaining group.
As with most groups of ungulates, and indeed most surviving mammals, the first camelids were small; in their case they were rabbit-sized animals. Protylopus, which evolved in the Eocene epoch, possessed simple, low-crowned teeth, which were arranged along the jaw without any beaks, a primitive feature and one that indicates that the animal’s diet were the soft leaves of the forest vegetation.
The forelimbs, which were shorter than the hind limbs, had four toes, all of which touched the ground. The hind limbs also had four toes, but only the third and fourth were used to carry the animal’s weight; the second and fifth toes were present as vestigial "dew-claws." The functional toes were pointed, which indicates that the early camels had narrow hooves rather than broad pads.
It is unlikely that Protylopus was a direct ancestor of later camels, but it probably closely resembled, and was contemporary with, the earliest camels.
By 35 million years ago (the Oligocene epoch), the dense forests of Dakota that had served as Protylopus’ habitat had given way to more open woodlands. As a result, Protylopus evolved into something that began to resemble modern camels. It grew to about the size of a sheep, developed a distinctive narrow snout, similar to a llama, and even possibly evolved llama-like ears, too.
This creature known as Poebrotherium evolved slightly longer hind legs than forelegs, and hoofed toes, yet its legs were clearly adapted for greater speed. These were relatively longer and more slender than those of Protylopus. They had lost their lateral toes, and the two central weight-bearing digits were beginning to diverge. There had also been a change in the animal’s dentition, with spaces beginning to appear between the teeth. It’s likely that a number of different camelid lines radiated from this creature.