Classification As said above, Columbiformes , Pterocliformes and  the recently individualized Mesitornithiformes have a common genetic ancestor.

The pigeon family is a natural and homogeneous assemblage of about 316 species of readily defined birds, unique in producing, for feeding their young, a nutritive secretion from the crop wall. Pigeon's milk is similar in composition to mammalian milk and is also induced by the secretion of the hormone prolactin from the pituitary gland. Pigeons also are distinctive in their unusual manner of drinking, in which water is sucked in as a continuous draft, the process being assisted by muscular contractions of the esophagus, whereas other birds take a sip of water and then tip back the head to swallow. The earliest known pigeon is Gerandia calcaria from the early Miocene of France (about 23 million years ago).

Nesting colonies of the Rock dove (Columba livia) were farmed by Neolithic husbandmen for food, and gradually the process of rearing young in confinement led to the production of domesticated strains. Evidence for domestication extends back to 4500 BC in ancient Iraq. From the domesticated pigeons have been derived the various fancy breeds including racing pigeons. Belgium, at the top of the international league table, has about 60,000 pigeon fanciers. Carrier pigeons were used to relay news of the conquest of Gaul to Rome, brought news of Napoleon's defeat at Waterloo to England, and were used extensively for message carrying in the two World Wars.

Hackett SJ et al (2008): Phylogenomic Study of Birds

http://en.wikipedia.org/wiki/Columbidae

columbiform. (2011).  Encyclopædia Britannica Ultimate Reference Suite.  Chicago: Encyclopædia Britannica.

Austin Jr, O.L. (1961): Birds of the World, Golden Press, New York, p 140.