Camelids are a family of even-toed ungulates which separated from their closest relatives, ruminates (cattle, antelopes, goats, etc.), and suines (pigs, peccaries, and hippos) about 47 million years ago, during the middle Eocene. Though they are not true ruminates (they do not chew a cud), camelids have a three-chambered stomach (rather than the ruminates' four-chambered stomach) and unlike suines, they are strictly herbivorous. Camelids include camels, dromedaries, llamas, alpacas, vicunas, and gaunacos.
As some of the earliest mammals to evolve multi-chambered stomachs, camelids represent the evolutionary trend towards adaptation to grasslands and away from woodlands. An event contemporaneous with the evolution of camelids, the Azolla event, caused worldwide carbon dioxide levels to increase rapidly, which initiated global cooling and glaciation at the Poles. This, in turn, killed off most of the tropical and subtropical forests that had dominated the planet during the Mesozoic, and ushered in an "Age of Grasses" that has continued to the present day. Camelids evolved their multi-chambered stomachs to better process nutrient-poor grasses.
Camelids initially evolved in North America, which at the time was an island continent. All camelid species that lived in North America are now extinct, the last of them dying out only 10,000 years ago, probably due to hunting and competition with newly arrived human settlers. Camelids lived exclusively in North America for tens of millions of years, only spreading into Asia and South America around 2-3 million years ago, when North America became connected to both continents via land bridges.
Three main groups of camelids survive today: the Dromedary of northern Africa and the Middle East, the Bactrian Camel of eastern Asia, and the Llamas, Alpacas, Vicunas, and Gaunacos of South America. Dromedaries and Bactrian camels are famous for their humps -- dromedaries have one, and Bactrian have two, sometimes causing them to be called one-humped and two-humped camels respectively. The most famous camel is the Dromedary, used widely throughout northern Africa and the Arabian peninsula. Both Dromedary and Bactrian camels have extensive adaptations to survive in nearly waterless environments, such as the Arabian Desert and Gobi Desert. In ancient times, camel caravans brought frankincense from Yemen and Oman across the Arabian Desert, but today, the desert has become so harsh that no one dares cross it.