The icons of the African and Arabian deserts are American in origin.
Like horses and dogs, camels evolved in the grasslands of America twenty million years ago. In those days, they were more like giraffes or gazelles than the humped beasts of burden we know and love. It wasn't until about four million years ago that they crossed the Bering Land bridge into Asia. (Maybe they passed the Native Americans on their way...)
They became extinct in North America during the last Ice Age and, unlike dogs and horses, haven't made it back.
It is not clear why the north American camel species died out. Climate change is the obvious culprit. More specifically it was have been due to the silica content of grass. As the North American climate got cooler and drier, silica levels in grass tripled. This new super tough grass wore out the teeth of even the toughest rooted grazers and the horses and camels gradually died of starvation, as a result of being unable to chew.
There is also some evidence that these already weakened species, their escape route to Asia blocked by the disappearance of the Bering Land bridge ten thousand years ago, were finished off by human hunters.