Short answers, pulled from the story.
Dromedaries may have first been domesticated in Somalia or South Arabia during the third millennium BC. A 2016 genomic study pointed to the southeast Arabian Peninsula as the most likely origin. The Bactrian camel was domesticated around 2,500 BC, with evidence from Shar-i Sokhta in Iran.
Camels have a range of physiological adaptations that reduce water loss and tolerate dehydration. A dromedary can go up to 10 days without water under hot conditions and withstand losing up to 25% of its body weight in water, far beyond the 12-14% that causes cardiac failure in most mammals. Their oval red blood cells resist rupturing during rapid rehydration, and a rete mirabile keeps the brain cool without excessive sweating.
Camels originated in North America, where the earliest known ancestor, Protylopus, lived 40 to 50 million years ago in what is now South Dakota. The ancestor of modern camels, Paracamelus, crossed into Asia via the Bering land bridge between 7.5 and 6.5 million years ago. Camels went extinct in North America around 13,000-11,000 years ago.
Dromedaries have one hump and make up 94% of the world's camel population; Bactrian camels have two humps and account for the remaining 6%. Bactrian camels are also larger, standing up to a foot taller, and weigh up to 1,000 kilograms compared to the dromedary's maximum of 600 kilograms. The two species diverged about 1 million years ago according to the fossil record.
Camel humps are reservoirs of fatty tissue, not water. When metabolized, the fat releases energy and produces water as a byproduct. However, the breathing required by this metabolic process also evaporates water from the lungs, resulting in a net decrease in water in dry climates.
The first documented use of camel cavalry occurred at the Battle of Qarqar in 853 BC. Camels were used militarily because horses fear their scent, a tactic exploited by the Achaemenid Persians at the Battle of Thymbra in 547 BC. Major 20th-century camel corps included France's méhariste corps formed in 1912, Britain's Imperial Camel Corps formed in 1916 and disbanded in 1919, and the U.S. Camel Corps established in the 19th century and ended by the American Civil War.